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Old 7th March 2021, 05:21 AM   #161
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Originally Posted by Roboramma View Post
SpaceX plans to get Starship to Mars.

Starship can't just launch from the earth to Mars. It would need to refuel in orbit six times.

That seems to be exactly what you're talking about. There's not much difference between launching six different small payloads and assembling them in orbit and launching a ship and refueling it six times in orbit: the payload you send to Mars is still much larger than a single launch from the Earth's surface can allow.

So it seems like SpaceX trying to develop exactly the capability that you say they aren't developing.

I like your idea of an orbital shipyard, and mostly agree about robotic vs. manned spaceflight (mostly, not entirely), but at least to an extent SpaceX is working on what you think they should be working on.
You're absolutely right. Thank you for the correction.
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Old 7th March 2021, 05:41 AM   #162
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Originally Posted by smartcooky View Post
Do you have any idea what you are saying?

Have you seen what it takes to assemble and attach the rover to the skycrane, and if so, do you really think it would be possible to do that in space?
If you want us to be an interplanetary species then we need to start learning to do this type of engineering out of a gravity well.
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Old 7th March 2021, 11:32 AM   #163
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Originally Posted by Darat View Post
If you want us to be an interplanetary species then we need to start learning to do this type of engineering out of a gravity well.
True, but why would you want to do this for a task that can be completed without doing it?
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Old 7th March 2021, 01:10 PM   #164
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They don't need clean rooms and armies of technicians to assemble and integrate ISS modules. Stop thinking about doing small things the hard way. Start thinking about making the small things easier, and doing big things.

Also I'd much rather put humans to work in the relative safety of LEO, where a rescue flight (or an evacuation drop) is just one launch and a couple hours away. Smartcooky's proposal is for humans to build an entire launch facility and vehicle on Mars, as a matter of pure survival, with no hope of help or rescue if anything goes wrong.

Imagine how many robots you could send to Mars, even under our current limitations, to start doing science almost immediately, just with the launches sending consumables to keep the astronauts alive long enough to finish building their lifeboat.
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Old 7th March 2021, 02:36 PM   #165
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Originally Posted by theprestige View Post
They don't need clean rooms and armies of technicians to assemble and integrate ISS modules. Stop thinking about doing small things the hard way. Start thinking about making the small things easier, and doing big things.
Have you seen what is involved in assembling the rover/skycrane package?

This is a 2 min excerpt from "Voyage of Curiosity"

https://www.youtube.com/watch?v=k4qAfMJNcCQ

but you should try to watch the whole thing. Its 54min long, but it will give you some idea just how difficult it would be to do what you are suggesting.

https://www.youtube.com/watch?v=qaUhLXolGaI


Originally Posted by theprestige View Post
Also I'd much rather put humans to work in the relative safety of LEO, where a rescue flight (or an evacuation drop) is just one launch and a couple hours away. Smartcooky's proposal is for humans to build an entire launch facility and vehicle on Mars, as a matter of pure survival, with no hope of help or rescue if anything goes wrong.
1. I proposed no such thing.

2. Do you actually know what IS proposed? I mean, are you just making it up as you go? Have you done any actual research on what the proposals are for the human exploration of Mars?

Its obvious that you haven't, so here is a primer for you to get you started

https://www.nasa.gov/mission_pages/m...iew/index.html

Originally Posted by theprestige View Post
Imagine how many robots you could send to Mars, even under our current limitations, to start doing science almost immediately, just with the launches sending consumables to keep the astronauts alive long enough to finish building their lifeboat.
Why can't we do both?
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Old 7th March 2021, 04:11 PM   #166
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Originally Posted by smartcooky View Post
Have you seen what is involved in assembling the rover/skycrane package?

but you should try to watch the whole thing. Its 54min long, but it will give you some idea just how difficult it would be to do what you are suggesting.
They do it that way because they have no choice but to assemble everything into one payload. If they had options, they'd come up with different ways to do it.

Even if they sent up the rover/lander as one package, and the Mars transfer vehicle as a separate package, that would still result in more payload to Mars than the current solution of lifting rover/lander and MTV from Earth's surface as a single package.

Even if the rover package includes some sort of interface device so that it can be easily clamped to the lander outside of a clean room, that's still more rover on Mars than the current solution.

Quote:
1. I proposed no such thing.
It's obviously implicit in your proposal to send humans to Mars.

You can't send them to Mars until you have a good way to get them home again. What's your good way to get them home again?

Obviously it must include some kind of launch vehicle to get them from the Martian surface to Mars orbit. Also obviously it must include some kind of launch facility for that vehicle. Either built into the vehicle, in which case you need to get that entire vehicle to Mars somehow. Or as a separate assembly. In which case you're back to astronauts assembling things in space (or the Martian surface, which is effectively the same as space) without a clean room and without a squadron of trained technicians.

Quote:
2. Do you actually know what IS proposed? I mean, are you just making it up as you go? Have you done any actual research on what the proposals are for the human exploration of Mars?
None of the actual research I've seen addresses the objections I've raised. If you have research that does, please show it to me.

Quote:
Its obvious that you haven't, so here is a primer for you to get you started

https://www.nasa.gov/mission_pages/m...iew/index.html
None of that says anything about the costs and practicality of returning humans safely from Mars.

Quote:
Why can't we do both?
We can certainly do both. But one would be an unjustifiable waste of resources and risk to human life. The question isn't why can't we do both. It's why we should do either.
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Old 7th March 2021, 04:20 PM   #167
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You can't send them to Mars until you have a good way to get them home again.
Who made that rule?
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Old 7th March 2021, 04:28 PM   #168
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Originally Posted by smartcooky View Post
Have you seen what is involved in assembling the rover/skycrane package?
Nobody has told them to figure out how to do it in orbit.

When the proposal came to assemble ISS modules in orbit, nobody said, we need a clean room and an army of technicians in a shirtsleeve environment, can't be done otherwise. Instead, they figured out how to launch complete modules that could be hooked up to an existing structure by the crew already up there.
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Old 7th March 2021, 07:49 PM   #169
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Originally Posted by theprestige View Post
Imagine how many robots you could send to Mars, even under our current limitations, to start doing science almost immediately, just with the launches sending consumables to keep the astronauts alive long enough to finish building their lifeboat.
That is very easy to answer, none, the return vehicle would be ready on Mars before the astronauts even launched from Earth

If you read up on it all the current ideas for Mars exploration (other than the ones that don't involve brining the astronauts back at all) use variations on the Mars Direct plan, the first part of which is sending the return vehicle so it has time to generate the fuel for lifting off from Mars and not sending astronauts until it is ready
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Old 7th March 2021, 09:51 PM   #170
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Originally Posted by theprestige View Post
Nobody has told them to figure out how to do it in orbit.

When the proposal came to assemble ISS modules in orbit, nobody said, we need a clean room and an army of technicians in a shirtsleeve environment, can't be done otherwise. Instead, they figured out how to launch complete modules that could be hooked up to an existing structure by the crew already up there.
You still don't understand the issue.

Did you watch either of the videos I posted? Apparently not! If you had, you would understand why the engineers assembling the payload are wearing white clean-room overalls and masks. Its because they are in a ******* clean room. The whole thing is build in a clean room with really stringently enforced standards regarding what comes in what goes out.

Assembling pre-fab ISS modules in space is mere ducksoup compared with what you are suggesting.
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Old 7th March 2021, 10:23 PM   #171
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Originally Posted by theprestige View Post
They do it that way because they have no choice but to assemble everything into one payload. If they had options, they'd come up with different ways to do it.
Why would they when they have a way of doing it now?

Originally Posted by theprestige View Post
Even if they sent up the rover/lander as one package, and the Mars transfer vehicle as a separate package, that would still result in more payload to Mars than the current solution of lifting rover/lander and MTV from Earth's surface as a single package.
Nope... the skycrane system (not the actual skycrane itself but the concept) cannot be scaled up any more. Its near its maximum, building a bigger skycrane to lower a bigger rover simply is not an option.

One of the first things I learned as an aeronautical engineer is that not all technologies will scale up - its called the “Square/Cube law”. If you take a piece of machinery such as an aeroplane or rocket, and increase its size by a factor of two, its area will increase by a factor of four, and its volume (and therefore its weight) will increase by a factor of eight. Make skycrane twice as big, and it will he eight times heavier. That's a law of engineering there is simply no way around - all the ribs and struts and superstructure need to be eight times stronger and heavier to cope with the loads.

Originally Posted by theprestige View Post
Even if the rover package includes some sort of interface device so that it can be easily clamped to the lander outside of a clean room, that's still more rover on Mars than the current solution.
Nope... for the reasons stated above


Originally Posted by theprestige View Post
It's obviously implicit in your proposal to send humans to Mars.

You can't send them to Mars until you have a good way to get them home again. What's your good way to get them home again?

Obviously it must include some kind of launch vehicle to get them from the Martian surface to Mars orbit. Also obviously it must include some kind of launch facility for that vehicle. Either built into the vehicle, in which case you need to get that entire vehicle to Mars somehow. Or as a separate assembly. In which case you're back to astronauts assembling things in space (or the Martian surface, which is effectively the same as space) without a clean room and without a squadron of trained technicians.


None of the actual research I've seen addresses the objections I've raised. If you have research that does, please show it to me.


None of that says anything about the costs and practicality of returning humans safely from Mars.
Answered by Grashtel in post 169

Try actually reading and learning about the subject before making stiff up on-the-fly about what you imagine is "implicit in people's proposals"

Originally Posted by theprestige View Post
We can certainly do both. But one would be an unjustifiable waste of resources and risk to human life. The question isn't why can't we do both. It's why we should do either.
Yeah, I get it. You have a bee in your bonnet about manned spaceflight. Fortunately however, the people actually involved in manned spaceflight don't have your saturnine attitude.
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Old 8th March 2021, 10:38 AM   #172
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Originally Posted by smartcooky View Post
You still don't understand the issue.

Did you watch either of the videos I posted? Apparently not! If you had, you would understand why the engineers assembling the payload are wearing white clean-room overalls and masks. Its because they are in a ******* clean room. The whole thing is build in a clean room with really stringently enforced standards regarding what comes in what goes out.

Assembling pre-fab ISS modules in space is mere ducksoup compared with what you are suggesting.

The present Mars lander system was designed from the outset to be assembled in a clean room, because it had to be launched as a single payload anyhow so assembling it in that way was easily possible and put no additional constraints on it... except that it all had to be small enough to be launched as a single payload.

ISS modules are designed from the outset to be assembled in orbit. This puts constraints on their design and has costs (I'm sure a given module would be cheaper or more capable for a given weight if it could be carefully wired into place in a clean-room environment by teams of technicians instead of assembled in orbit by astronauts). But this allows the ISS to be larger and more capable than it would be if it had to be launched in a single payload.

There are electronic devices manufactured that require essentially clean room conditions to replace a component or change a battery... and other ones with very similar functions for which anyone can swap modules and batteries in and out in the field with no tools. Those characteristics aren't determined by the functionality of the device, but by engineering choices including size, cost, longevity, and materials trade-offs. Of course the current Mars lander systems aren't designed to be modular and assembled in orbit, but that doesn't mean they couldn't possibly be designed that way.

(For what it's worth, LEO is cleaner than a clean room.)


Originally Posted by smartcooky View Post
Nope... the skycrane system (not the actual skycrane itself but the concept) cannot be scaled up any more. Its near its maximum, building a bigger skycrane to lower a bigger rover simply is not an option.

One of the first things I learned as an aeronautical engineer is that not all technologies will scale up - its called the “Square/Cube law”. If you take a piece of machinery such as an aeroplane or rocket, and increase its size by a factor of two, its area will increase by a factor of four, and its volume (and therefore its weight) will increase by a factor of eight. Make skycrane twice as big, and it will he eight times heavier. That's a law of engineering there is simply no way around - all the ribs and struts and superstructure need to be eight times stronger and heavier to cope with the loads.

I don't see any fundamental reason why a larger skycrane would have to be scaled up equally in all three dimensions.

Imagine if you will two skycranes of roughly the present design, flying independently but in tight synchrony, sharing the load of a single payload. If you could keep their operations synchronized it would work. That would be difficult, though, and in any case it would be really awkward to try to get them into that configuration after the earlier atmospheric entry phases. So instead, bolt them together side by side (or rather, make one larger skycrane of those dimensions).
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Old 8th March 2021, 12:30 PM   #173
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Originally Posted by Myriad View Post
The present Mars lander system was designed from the outset to be assembled in a clean room, because it had to be launched as a single payload anyhow so assembling it in that way was easily possible and put no additional constraints on it... except that it all had to be small enough to be launched as a single payload.

ISS modules are designed from the outset to be assembled in orbit. This puts constraints on their design and has costs (I'm sure a given module would be cheaper or more capable for a given weight if it could be carefully wired into place in a clean-room environment by teams of technicians instead of assembled in orbit by astronauts). But this allows the ISS to be larger and more capable than it would be if it had to be launched in a single payload.

There are electronic devices manufactured that require essentially clean room conditions to replace a component or change a battery... and other ones with very similar functions for which anyone can swap modules and batteries in and out in the field with no tools. Those characteristics aren't determined by the functionality of the device, but by engineering choices including size, cost, longevity, and materials trade-offs. Of course the current Mars lander systems aren't designed to be modular and assembled in orbit, but that doesn't mean they couldn't possibly be designed that way.
Interesting, but ultimately, irrelevant.

The assembly of things like satellites and spacecraft is precise work requiring fine touch and specialist tools. Currently, astronauts in spacesuits struggle to use anything finer that a wrench. Hubble repair astronaut Michael Massimo took almost two hours to.... remove a stuck bolt. Every small bolt, part and component that had to be removed had to be taped down to prevent it floating away - and that was just to remove a handrail to get at the part that needed replacing. Operating precision tools while wearing a spacesuit would be like trying to remove a splinter, holding a pair of tweezers with boxing gloves.

Any engineering task, ANY engineering task, that can be done on the ground is many, many times more difficult (and therefore many, many times more expensive) to accomplish in LEO.

Originally Posted by Myriad View Post
(For what it's worth, LEO is cleaner than a clean room.)
Its worth nothing, and is also irrelevant.

Originally Posted by Myriad View Post
I don't see any fundamental reason why a larger skycrane would have to be scaled up equally in all three dimensions.

Imagine if you will two skycranes of roughly the present design, flying independently but in tight synchrony, sharing the load of a single payload. If you could keep their operations synchronized it would work. That would be difficult, though, and in any case it would be really awkward to try to get them into that configuration after the earlier atmospheric entry phases.
The skycrane lowering the rover is already an extremely complicated technical and engineering achievement, and you want to make it many times more complicated?

Originally Posted by Myriad View Post
So instead, bolt them together side by side (or rather, make one larger skycrane of those dimensions).
I hope this is a poe, I really do, because you can't really be serious here.

For starters you would have to have a very strong mechanical interface - that means extra weight which neither of your two skycranes would have needed to carry individually. Also the superstructure of the vehicle would need to be strengthened to take the additional stresses and forces involved at the attachment points (so more weight). This means additional thrust will be required to carry that extra weight, so another couple of rocket engines will be needed (so more weight), and more fuel to for those rockets (so more weight), and that means a higher payload penalty for the launch vehicle (so more weight).

No, trying to work around the square/cube law by bolting two existing things together does not work as well as you think.

I must write to Adam Steltzner at JPL and tell him about your idea. I'm sure he and his team would enjoy a good laugh.
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Old 8th March 2021, 02:36 PM   #174
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Every small bolt, part and component that had to be removed had to be taped down to prevent it floating away -
Was that before zip-lock bags were invented?
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Old 8th March 2021, 02:59 PM   #175
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Originally Posted by Skeptical Greg View Post
Who made that rule?
I did. If smartcooky has a different rule, I'd like to see him put it in writing.

I suspect that a lot of the anticipated efficiency gains from sending humans to do science will not be realized if the scientists are being sent on a suicide mission, or otherwise treated as expendable mission components.
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Old 8th March 2021, 03:01 PM   #176
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Originally Posted by Grashtel View Post
That is very easy to answer, none, the return vehicle would be ready on Mars before the astronauts even launched from Earth

If you read up on it all the current ideas for Mars exploration (other than the ones that don't involve brining the astronauts back at all) use variations on the Mars Direct plan, the first part of which is sending the return vehicle so it has time to generate the fuel for lifting off from Mars and not sending astronauts until it is ready
That's a tolerable mission profile to me, but honestly I'd still rather invest those resources in sending more and better robots, and leaving humans with all their consumables and safety margins out of the equation.
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Old 8th March 2021, 03:12 PM   #177
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Originally Posted by smartcooky View Post
Hubble repair astronaut Michael Massimo took almost two hours to.... remove a stuck bolt.
IRRC, Hubble wasn't actually designed to be repaired in orbit. Massimo was struggling because nothing had been designed with his task in mind. That he accomplished it at all is pretty amazing, IMO.

And also let's note that he carried out these repairs outside of a clean room.

Here, have an analogy: When I started my career in systems administration, servers were big clunky things that were a pain in the ass to put together. All the components - RAM, CPUs, NICs, were shipped separately. I'd spend many a night shift shucking computer parts out of cardboard boxes, and sticking them into server cases. And those cases were a miserable assortment of awkward screws and hard edges. I'd come with scraped knuckles and bruised fingertips from getting everything properly installed.

But within a few years, manufacturers were making servers with easy assembly and reconfiguration in mind. What had previously been a bloody business of fiddling with screws in cramped spaces became a simple matter of flipping an ergonomic catch here and there, and snapping in a card that could only go in the right way. A 20-minute task became a 2-minute task. All that changed was people taking the time to study the problem and optimize the required steps.

If you cannot even conceive of the possibility that painstaking efforts in a cleanroom may not be the only way to do things, then we're fundamentally at odds. So it's probably best we leave it here.

In any case, you asked me my opinion. Now you have it. Hopefully the next time I voice it, you'll remember this conversation and how it ended, and not have to ask me again for the full details.
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Old 8th March 2021, 05:33 PM   #178
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Originally Posted by smartcooky View Post
I hope this is a poe, I really do, because you can't really be serious here.

For starters you would have to have a very strong mechanical interface - that means extra weight which neither of your two skycranes would have needed to carry individually. Also the superstructure of the vehicle would need to be strengthened to take the additional stresses and forces involved at the attachment points (so more weight). This means additional thrust will be required to carry that extra weight, so another couple of rocket engines will be needed (so more weight), and more fuel to for those rockets (so more weight), and that means a higher payload penalty for the launch vehicle (so more weight).

No, trying to work around the square/cube law by bolting two existing things together does not work as well as you think.

I must write to Adam Steltzner at JPL and tell him about your idea. I'm sure he and his team would enjoy a good laugh.

You're implying a universality that you're not justifying. Can two locomotives be hitched together to pull a longer train? Of course not, they'd have to be twice as wide and twice as tall as well, and then the square cube law would make them fail! (Plus they wouldn't fit the width of the rails or the clearance under bridges.) If you can build a 30 by 60 foot house out of two by fours, could you build one 60 by 60 feet? Absolutely not, it would have to be twice as tall as well, to deal with the higher lateral forces, so you'd need a stronger framing system. This is nonsense, of course.

The sky crane must balance the lift of its thrusters against the load on its cables plus its own weight. If it's doing that by transferring large fractions of those forces laterally across its width, then it's very poorly designed indeed. I do agree that if you put all the thrusters at one end, and all the cable attachments at the other, you'd need a highly strengthened superstructure (but it still wouldn't work very well).
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Old 8th March 2021, 05:45 PM   #179
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Originally Posted by Myriad View Post
Can two locomotives be hitched together to pull a longer train? Of course not, they'd have to be twice as wide and twice as tall as well, and then the square cube law would make them fail! (Plus they wouldn't fit the width of the rails or the clearance under bridges.) If you can build a 30 by 60 foot house out of two by fours, could you build one 60 by 60 feet? Absolutely not, it would have to be twice as tall as well, to deal with the higher lateral forces, so you'd need a stronger framing system. This is nonsense, of course.
https://en.wikipedia.org/wiki/Multiple_unit

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Old 8th March 2021, 05:50 PM   #180
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Originally Posted by Skeptical Greg View Post
Was that before zip-lock bags were invented?
Try opening and closing a zip lock bag with a pair of inflated spacesuit gloves, and good luck with that.
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Old 8th March 2021, 06:12 PM   #181
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Originally Posted by Myriad View Post
... Can two locomotives be hitched together to pull a longer train? Of course not, ....
I'm not really involved in the rest of this discussion but this is done all the time.

This one has 5 engines in front and two in back. Note they don't all face the same way which is normal. And according to Wiki they are under the control of one engineer.

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Old 8th March 2021, 06:12 PM   #182
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All that hopeful talk about how human space travel will inspire a vast wealth of technological advances. But it seems perhaps the ergonomics of working in space has hit an insurmountable barrier to innovation. However good it is now, that's the most we can possibly hope for.

At least the robot probes and telescopes keep getting better and better at doing stuff.
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Old 8th March 2021, 06:17 PM   #183
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Originally Posted by theprestige View Post
All that hopeful talk about how human space travel will inspire a vast wealth of technological advances. But it seems perhaps the ergonomics of working in space has hit an insurmountable barrier to innovation. However good it is now, that's the most we can possibly hope for.

At least the robot probes and telescopes keep getting better and better at doing stuff.
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Old 8th March 2021, 06:26 PM   #184
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Originally Posted by Myriad View Post
You're implying a universality that you're not justifying. Can two locomotives be hitched together to pull a longer train?
Not twice the number of cars, nor twice the weight.

I look forward to the day when we have airliners with 400 metre wing spans carrying 5000 passengers.

Originally Posted by Myriad View Post
The sky crane must balance the lift of its thrusters against the load on its cables plus its own weight. If it's doing that by transferring large fractions of those forces laterally across its width, then it's very poorly designed indeed. I do agree that if you put all the thrusters at one end, and all the cable attachments at the other, you'd need a highly strengthened superstructure (but it still wouldn't work very well).
Now you're just being plain ridiculous. Of course it would never be designed like that, but you could not just drill a few holes in the frame, bolt two of them together, and hey presto, your new double header skycrane can lower two bolted together rovers onto the surface of Mars.
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Old 8th March 2021, 06:34 PM   #185
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Originally Posted by Skeptic Ginger View Post
I'm not really involved in the rest of this discussion but this is done all the time.

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That's Myriad's point.

Sometimes scaling factors differ. Other times they don't.

If I double the diameter of my pizza I've quadrupled it's area. If I double the length of a two by four I've only double'd it's area.

I think the discussion of the sky crane is a bit of a distraction though. Let's say that you can't scale up the sky crane method beyond current payloads. As has been mentioned it's always possible to do a powered descent to the Martian surface. That will be more expensive than other methods that can bleed off more delta v in the atmosphere, but there's no fundamental limit to the payload that can be delivered to the martian surface.

Are there efficiencies to be gained by sending larger payloads to Mars than can be launched as a single payload from the earth's surface? I'm pretty sure that there are. As I mentioned earlier there's a reason that SpaceX is planning to refuel Starship in orbit rather than building a rocket that's six times the size (and has twelve stages).
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Old 8th March 2021, 06:35 PM   #186
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Originally Posted by arthwollipot View Post
"I think there is a world market for maybe five computers."

Thomas Watson, president of IBM, 1943
And for the computers being built at the time, he was absolutely right. I think we can forgive him for not predicting the transistor and its implications. I'm not sure we can forgive someone today, for not even considering the possibility that we might actually get better at working in space.

Seriously the biggest investment we could make in human space travel would be to commoditize LEO operations.
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Old 8th March 2021, 06:37 PM   #187
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Originally Posted by smartcooky View Post
Now you're just being plain ridiculous. Of course it would never be designed like that, but you could not just drill a few holes in the frame, bolt two of them together, and hey presto, your new double header skycrane can lower two bolted together rovers onto the surface of Mars.
Why phrase it like this when you could just say "build a bigger skycrane"?
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Old 8th March 2021, 06:42 PM   #188
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I propose a four-stack: Mars transfer vehicle, high altitude rocket brake, mid altitude skycrane, low altitude skycrane, rover. Each segment of the stack stages off, leaving less weight for the next stage to handle. Like launching a satellite, but in reverse.
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Old 8th March 2021, 06:52 PM   #189
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Originally Posted by smartcooky View Post
Now you're just being plain ridiculous. Of course it would never be designed like that, but you could not just drill a few holes in the frame, bolt two of them together, and hey presto, your new double header skycrane can lower two bolted together rovers onto the surface of Mars.
Myriad was making a pretty clear illustration that it seems like the scaling of the payload will scale linearly with the sky crane. He's not literally saying that making the sky crane twice the size would be as easy as building two of them and bolting them together. What is it about the sky crane that you are saying shouldn't scale linearly with it's payload size?

I've heard it said from NASA that the sky crane can't actually be made larger, and had generally assumed that this was due to scaling factors as you mention. But it's not obvious, and just saying that things don't always scale linearly doesn't make it clear.
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Old 8th March 2021, 06:57 PM   #190
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Originally Posted by Roboramma View Post
That's Myriad's point....
I see. I thought he was saying you can't use more than one engine to pull a bigger train.

Never mind then.
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Old 8th March 2021, 07:02 PM   #191
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Originally Posted by Skeptic Ginger View Post
I see. I thought he was saying you can't use more than one engine to pull a bigger train.

Never mind then.
Sarcasm doesn't always come across so well in text.
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Old 8th March 2021, 07:16 PM   #192
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Originally Posted by Roboramma View Post
Myriad was making a pretty clear illustration that it seems like the scaling of the payload will scale linearly with the sky crane. He's not literally saying that making the sky crane twice the size would be as easy as building two of them and bolting them together. What is it about the sky crane that you are saying shouldn't scale linearly with it's payload size?

I've heard it said from NASA that the sky crane can't actually be made larger, and had generally assumed that this was due to scaling factors as you mention. But it's not obvious, and just saying that things don't always scale linearly doesn't make it clear.
EVEN IF there's a scaling limit, that doesn't mean we've reached it. There's probably a scaling limit to the size of cargo ships, but it's not defined by the width of the Panama Canal.
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Old 8th March 2021, 07:33 PM   #193
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Originally Posted by theprestige View Post
EVEN IF there's a scaling limit, that doesn't mean we've reached it. There's probably a scaling limit to the size of cargo ships, but it's not defined by the width of the Panama Canal.
That's a good point. It would be quite a coincidence if the max payload that could be delivered to the Martian surface with the sky crane method was equal to the max payload that could be delivered to Mars with current rockets launched from the Earth's surface.
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Old 8th March 2021, 07:43 PM   #194
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Originally Posted by theprestige View Post
And for the computers being built at the time, he was absolutely right. I think we can forgive him for not predicting the transistor and its implications. I'm not sure we can forgive someone today, for not even considering the possibility that we might actually get better at working in space.
Because there are no physical limits to working in space

Originally Posted by theprestige View Post
Seriously the biggest investment we could make in human space travel would be to commoditize LEO operations.
This is the first thing you have said that makes any sense to me.
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Old 8th March 2021, 07:55 PM   #195
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Originally Posted by smartcooky View Post
Not twice the number of cars, nor twice the weight.

Not quite. Yet as others have confirmed, it's done quite routinely anyhow.

Quote:
I look forward to the day when we have airliners with 400 metre wing spans carrying 5000 passengers.

You'd need to come up with a business model justifying those capabilities and the development cost. Which is pretty unlikely because such a craft would be incompatible with the world's air travel infrastructure.

In principle, though, it would be quite achievable. The key is not to just mindlessly scale up a 747 or whatever. Use several parallel fuselages linked by a grid of wings. Tricky? Sure, especially a dynamic control system for actively managing stresses instead of relying on brute rigidity. But if for some reason you really needed to do it, I don't see anything that would make it physically impossible with existing materials.

Quote:
Now you're just being plain ridiculous. Of course it would never be designed like that, but you could not just drill a few holes in the frame, bolt two of them together, and hey presto, your new double header skycrane can lower two bolted together rovers onto the surface of Mars.

The idea is to be able to lower one rover that's larger or at least heavier (perhaps better instrumented, or more robustly powered, or equipped with spinning blades to fend off other Battlebots) than Perseverance. If doubling the mass of the skycrane only allows increasing the rover mass by 1.7, that's not a show stopper, and it's certainly no proof that a larger rover than Perseverance is impossible.

I'm sure there are complexities inherent in a larger skycrane with more (not larger) thrusters and cables/winches, some I probably haven't thought of. Obviously literally attaching two of the old design together without modification, which they were never designed for, wouldn't actually work. But your claim is that no skycrane design could work with a rover payload significantly larger than Perseverance, and your gesturing vaguely toward the square-cube law doesn't seem sufficient evidence for that claim. Nor do your continued attempts to depict all adaptations to resolve the problem as ridiculous, comical, or Poes help your case. On the contrary, it's your objections that are starting to come across as ridiculous or, more to the point, unfounded.
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Old 9th March 2021, 12:23 AM   #196
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Originally Posted by smartcooky View Post
…snippp


This is the first thing you have said that makes any sense to me.
He’s been saying that all along and has done so in previous threads.
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Old 9th March 2021, 01:37 AM   #197
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Originally Posted by Myriad View Post
The idea is to be able to lower one rover that's larger or at least heavier (perhaps better instrumented, or more robustly powered, or equipped with spinning blades to fend off other Battlebots) than Perseverance. If doubling the mass of the skycrane only allows increasing the rover mass by 1.7, that's not a show stopper, and it's certainly no proof that a larger rover than Perseverance is impossible.
Here is the problem with that

Let's suppose that this fantasy of theprestige's of building a twice-as-big rover, and a bigger, more powerful sky-crane, and to then launch them separately into orbit and somehow assemble them in LEO, is all feasible (I think its impossible, at least at our, and any realistically foreseeable, level of technology).

OK, so you have now built your bigger, heavier rover, which required you to build a bigger, heavier and more powerful sky crane, capable of lowering the greater weight. That means your sky crane has more rocket engines to provide the additional required thrust, and more fuel, which means bigger fuel tanks to carry the extra fuel adding more weight. Now your your whole spacecraft (cruise stage + back-shield + sky-crane + rover + heat-shield) is in LEO orbiting at 28,000 kph. This velocity has to be increased using a TMI burn so that it can gets its ass to Mars.

Oh, hang on! That TMI burn now has to provide a bigger kick to the spacecraft's orbital velocity than it would have for the Curiosity or Perseverance setups, because our spacecraft is much heavier. There are only two ways that bigger kick can be achieved -

1. Bigger, more powerful, and therefore heavier rockets to deliver more thrust during the TMI burn... more weight.

2. A longer TMI burn, which will require more fuel, and therefore, higher capacity tanks, which results in.. more weight.

Looks like you will be needing a 3rd launch just to put the boost stage into orbit!

So far, we're not winning - two single sized rovers require two single launches, but now our sooper dooper, double-sized rover needs three launches. This is beginning to look like a bad idea, and it doesn't stop there because we haven't even come up against the real show stopper yet.... Mars atmosphere EDL - Entry, Descent and Landing.

In order to kick our spacecraft out of LEO, we had to boost its velocity. Pretty much any TMI burn to Mars during a Hohmann transfer window is going to result in a Mars atmosphere interface velocity of somewhere between 16 and 28 thousand kph (the slowest on record was Viking at 16K and the highest was Mars Pathfinder at 27K). However, our spacecraft is much heavier than anything we have ever previously sent to land on Mars so its going to take much longer to slow down in the atmosphere, and that means it will get hotter, so it will need a bigger, more robust heat shield (oops) which adds more weight. And then there is the parachute.... it will it have to arrest a heavier load (so it has to be bigger, and so will its housing... yet more weight)


But wait.... there's more

Originally Posted by Myriad View Post
I'm sure there are complexities inherent in a larger skycrane with more (not larger) thrusters and cables/winches, some I probably haven't thought of. Obviously literally attaching two of the old design together without modification, which they were never designed for, wouldn't actually work. But your claim is that no skycrane design could work with a rover payload significantly larger than Perseverance, and your gesturing vaguely toward the square-cube law doesn't seem sufficient evidence for that claim.
This document talks about the plan after Curiosity and before Perseverance to land a Curiosity-like rover in 2018, and while it talks mostly about the seasonal atmospheric pressure issues with regard to landing zone altitude limits and landing ellipse accuracy, it does briefly address the mass issue.

Warning: This is a 3.9MB pdf
https://www.nap.edu/resource/13117/A...-Sky-Crane.pdf
2.4 Mass Delivery Capabilities of the MSL-Derived Sky Crane
The delivery capabilities of an MSL-derived EDL/Sky Crane system*1 for the 2018 opportunity have been estimated by applying the current understanding of the system’s sensitivities and adjusting for conditions expected in the 2018 arrival season. Additionally, the performance impact of several options for improving landed precision has been estimated based on landing precision studies commissioned by the Mars Program in fiscal year (FY) 2009. The options and their impact on performance are described below.

The 2018 opportunity would afford a more favorable part of the seasonal pressure cycle for landed mass than MSL’s 2011 opportunity; however, the increased risk of exposure to dust events would partially offset the increased mass capability and also negatively impact landing precision. As shown in Table 2-2, after accounting for the positive and negative atmosphere impacts, the delivery mass capability is estimated to be 1,050 kg, roughly 100 kg more than MSL.*2 Dust impact on wind fields would cause additional variability in the amount and direction of wind drift experienced while on parachute. This would result in an increase to the landing ellipse diameter to 30 km versus MSL’s 25 km ellipse.


NOTE: MOLA - Mars Orbiter Laser Altimeter, a sort of sea-level reference for Mars

Several options for improving landing precision have been considered. Improving navigated state knowledge at entry, through improved attitude initialization knowledge and through either spacecraft to spacecraft navigation or optical navigation, would improve expected landing precision. The improved state knowledge would enable guided entry to be more accurate in controlling range to target; this would result in an estimated 22 km landing ellipse.

Adding a range-based parachute deployment trigger, rather than a simple velocity trigger, to the enhanced state knowledge approach could further increase landing precision. Often dubbed “smart chute,” this option would deploy the supersonic parachute when the desired range flown has been met,
resulting in significant precision improvement. Unfortunately, this option would come at the expense of landing elevation capability, because parachute deployment must be delayed from the maximum allowable inflation Mach to allow flexibility to deploy using range information.*3 The landing elevation expense would depend on the atmospheric conditions and correlations for a given landing site, but could be estimated at approximately 2 km for most cases. The improved ellipse for the 2018 opportunity is estimated at 15 km in diameter
*1 "MSL-derived EDL/Sky Crane system" - any system using a parachute, retro-rockets and a tether to lower the payload.

*2The mass capability of the the above system and any system derived from it is about 1050 kg.

*3the lack of atmospheric density means that even the target landing zone altitude is critically affected by how long the spacecraft takes to slow down in the atmosphere. Too long, and the skycrane is too low to for the ranging system to find the landing zone and navigate to it.

We have now reached the limits of what is possible. This is not the limits of those two sky-cranes, it is the limits of this system as a whole - about 1050 kg is the most you can land on Mars using the system as described. Both Curiosity (899kg) and Perseverance (1,025 kg) used precision steering, a huge supersonic parachute, retro-rockets and a sky-crane. As Adam Stelzner has said, this technique skirts the very edge of what parachutes and propulsive landing systems can do in terms of braking. Curiosity was pushing the limits of the parachute/sky-crane system... Perseverance pushed almost to the end of those limits. The atmosphere of Mars is thick enough to be a nuisance, but not thick enough to be really useful. We might be able to build a supersized rover but we are not going to be able to simply use a scaled up version of this technique to land it on Mars, i.e. building a bigger sky-crane will not work, it will actually make the situation worse. If we try we will almost certainly end up with with a smoking pile of scrap-metal and electronic parts on the Martian surface.
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Old 9th March 2021, 03:13 AM   #198
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I don't understand your first issue with respect to requiring more fuel to get to Mars. You seem to be saying that the fuel cost to get to Mars doesn't scale linearly with payload size.

You talk about putting separately the components into orbit and assembling them there. Let's say that both were the same size as Perserverence was when launched to LEO. So, can we then get the assembly to be twice the size of Perseverance? It seems like we should. When we launched Perseverence into LEO it didn't just carry the rover/skycrane: it also carried to engines and fuel tanks to get it to Mars. Sure, if only one of our two components carried that same amount of fuel, it wouldn't be enough to get the assembly to Mars. But we've got two components, each with enough fuel to get itself to Mars.

If anything, I'd expect that we could be getting savings here, given that you only need one engine to burn the fuel.
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Old 9th March 2021, 10:22 AM   #199
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Originally Posted by smartcooky View Post
Here is the problem with that

Let's suppose that this fantasy of theprestige's of building a twice-as-big rover, and a bigger, more powerful sky-crane, and to then launch them separately into orbit and somehow assemble them in LEO, is all feasible (I think its impossible, at least at our, and any realistically foreseeable, level of technology).

OK, so you have now built your bigger, heavier rover, which required you to build a bigger, heavier and more powerful sky crane, capable of lowering the greater weight. That means your sky crane has more rocket engines to provide the additional required thrust, and more fuel, which means bigger fuel tanks to carry the extra fuel adding more weight. Now your your whole spacecraft (cruise stage + back-shield + sky-crane + rover + heat-shield) is in LEO orbiting at 28,000 kph. This velocity has to be increased using a TMI burn so that it can gets its ass to Mars.

Oh, hang on! That TMI burn now has to provide a bigger kick to the spacecraft's orbital velocity than it would have for the Curiosity or Perseverance setups, because our spacecraft is much heavier. There are only two ways that bigger kick can be achieved -

1. Bigger, more powerful, and therefore heavier rockets to deliver more thrust during the TMI burn... more weight.

2. A longer TMI burn, which will require more fuel, and therefore, higher capacity tanks, which results in.. more weight.

Looks like you will be needing a 3rd launch just to put the boost stage into orbit!

Probably, but what's so bad about that? The Apollo lander (LEM) routinely docked with the modules containing the trans orbital injection engine and fuel (CSM) in Earth orbit. Fifty years ago! Those were launched in the same vehicle, but the docking maneuver wouldn't have been much different if they'd been launched separately.

Quote:
So far, we're not winning - two single sized rovers require two single launches, but now our sooper dooper, double-sized rover needs three launches. This is beginning to look like a bad idea, and it doesn't stop there because we haven't even come up against the real show stopper yet.... Mars atmosphere EDL - Entry, Descent and Landing.

In order to kick our spacecraft out of LEO, we had to boost its velocity. Pretty much any TMI burn to Mars during a Hohmann transfer window is going to result in a Mars atmosphere interface velocity of somewhere between 16 and 28 thousand kph (the slowest on record was Viking at 16K and the highest was Mars Pathfinder at 27K). However, our spacecraft is much heavier than anything we have ever previously sent to land on Mars so its going to take much longer to slow down in the atmosphere, and that means it will get hotter, so it will need a bigger, more robust heat shield (oops) which adds more weight. And then there is the parachute.... it will it have to arrest a heavier load (so it has to be bigger, and so will its housing... yet more weight)

We were talking about skycrane/lander configurations. Sure you need more hardware for the TMI and initial entry phases. But the skycrane doesn't have to land the TMI engine. The skycrane doesn't have to land the heat shield, or the parachute or its housing, etc. Your claim was that a bigger skycrane landing a bigger lander is fundamentally impossible, but now you're talking about all the other equipment needed to get it there. That's a different topic.


Quote:
This document talks about the plan after Curiosity and before Perseverance to land a Curiosity-like rover in 2018, and while it talks mostly about the seasonal atmospheric pressure issues with regard to landing zone altitude limits and landing ellipse accuracy, it does briefly address the mass issue.

Warning: This is a 3.9MB pdf
https://www.nap.edu/resource/13117/A...-Sky-Crane.pdf
2.4 Mass Delivery Capabilities of the MSL-Derived Sky Crane
The delivery capabilities of an MSL-derived EDL/Sky Crane system*1 for the 2018 opportunity have been estimated by applying the current understanding of the system’s sensitivities and adjusting for conditions expected in the 2018 arrival season. Additionally, the performance impact of several options for improving landed precision has been estimated based on landing precision studies commissioned by the Mars Program in fiscal year (FY) 2009. The options and their impact on performance are described below.

The 2018 opportunity would afford a more favorable part of the seasonal pressure cycle for landed mass than MSL’s 2011 opportunity; however, the increased risk of exposure to dust events would partially offset the increased mass capability and also negatively impact landing precision. As shown in Table 2-2, after accounting for the positive and negative atmosphere impacts, the delivery mass capability is estimated to be 1,050 kg, roughly 100 kg more than MSL.*2 Dust impact on wind fields would cause additional variability in the amount and direction of wind drift experienced while on parachute. This would result in an increase to the landing ellipse diameter to 30 km versus MSL’s 25 km ellipse.

https://www.dropbox.com/s/ze55lpm8ao...e2-2.jpg?raw=1
NOTE: MOLA - Mars Orbiter Laser Altimeter, a sort of sea-level reference for Mars

Several options for improving landing precision have been considered. Improving navigated state knowledge at entry, through improved attitude initialization knowledge and through either spacecraft to spacecraft navigation or optical navigation, would improve expected landing precision. The improved state knowledge would enable guided entry to be more accurate in controlling range to target; this would result in an estimated 22 km landing ellipse.

Adding a range-based parachute deployment trigger, rather than a simple velocity trigger, to the enhanced state knowledge approach could further increase landing precision. Often dubbed “smart chute,” this option would deploy the supersonic parachute when the desired range flown has been met,
resulting in significant precision improvement. Unfortunately, this option would come at the expense of landing elevation capability, because parachute deployment must be delayed from the maximum allowable inflation Mach to allow flexibility to deploy using range information.*3 The landing elevation expense would depend on the atmospheric conditions and correlations for a given landing site, but could be estimated at approximately 2 km for most cases. The improved ellipse for the 2018 opportunity is estimated at 15 km in diameter
*1 "MSL-derived EDL/Sky Crane system" - any system using a parachute, retro-rockets and a tether to lower the payload.

*2The mass capability of the the above system and any system derived from it is about 1050 kg.

*3the lack of atmospheric density means that even the target landing zone altitude is critically affected by how long the spacecraft takes to slow down in the atmosphere. Too long, and the skycrane is too low to for the ranging system to find the landing zone and navigate to it.

We have now reached the limits of what is possible. This is not the limits of those two sky-cranes, it is the limits of this system as a whole - about 1050 kg is the most you can land on Mars using the system as described. Both Curiosity (899kg) and Perseverance (1,025 kg) used precision steering, a huge supersonic parachute, retro-rockets and a sky-crane. As Adam Stelzner has said, this technique skirts the very edge of what parachutes and propulsive landing systems can do in terms of braking. Curiosity was pushing the limits of the parachute/sky-crane system... Perseverance pushed almost to the end of those limits. The atmosphere of Mars is thick enough to be a nuisance, but not thick enough to be really useful. We might be able to build a supersized rover but we are not going to be able to simply use a scaled up version of this technique to land it on Mars, i.e. building a bigger sky-crane will not work, it will actually make the situation worse. If we try we will almost certainly end up with with a smoking pile of scrap-metal and electronic parts on the Martian surface.

All that is clearly talking about the limits of what is achievable within the overall envelope of a single launch vehicle and the already-designed mission framework that fits within that envelope. That's why they're talking about things like optimizing the timing and navigational precision, rather than building anything besides the lander any bigger.

They're saying, you can't add fifty more passenger seats (or even 2 more) in a Honda Odyssey, no matter how much you redesign the interior to eliminate unneeded ashtray space. Fine, no argument there. But that doesn't mean designing and building a bus is impossible. Not if we can put modules together in Earth orbit, which we obviously can if they're designed as modules to begin with, and have done since the Gemini program.
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Old 9th March 2021, 10:59 AM   #200
smartcooky
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Originally Posted by Roboramma View Post
I don't understand your first issue with respect to requiring more fuel to get to Mars. You seem to be saying that the fuel cost to get to Mars doesn't scale linearly with payload size.

You talk about putting separately the components into orbit and assembling them there. Let's say that both were the same size as Perserverence was when launched to LEO. So, can we then get the assembly to be twice the size of Perseverance? It seems like we should. When we launched Perseverence into LEO it didn't just carry the rover/skycrane: it also carried to engines and fuel tanks to get it to Mars. Sure, if only one of our two components carried that same amount of fuel, it wouldn't be enough to get the assembly to Mars. But we've got two components, each with enough fuel to get itself to Mars.

If anything, I'd expect that we could be getting savings here, given that you only need one engine to burn the fuel.
The Tyranny of the Rocket Equation
"'the rocket' is taken to mean 'the rocket and all of its unburned propellant'.
"...there is a limit to the amount of payload that the rocket can carry, as higher amounts of propellant increment the overall weight, and thus also increase the fuel consumption."
"The rocket equation can be applied to orbital maneuvers in order to determine how much propellant is needed to change to a particular new orbit, or to find the new orbit as the result of a particular propellant burn. When applying to orbital maneuvers, one assumes an impulsive maneuver, in which the propellant is discharged and delta-v applied instantaneously."


Asad's answer from a few years back on Stack Exchange pretty much sums it up

https://physics.stackexchange.com/qu...ig/88163#88163

"Upper limit on payload:

A very interesting thing happens near the inflection point of the rocket mass - fuel mass curve. Before the inflection point, adding more fuel allowed us to hoist a larger payload to the desired velocity.

However, somewhere around 4⋅106 kg of fuel mass (for our selected parameter values) we discover that adding more fuel starts to decrease the payload that can be hoisted! What is happening here is that the cost of the additional fuel having to fight against gravity begins to win out against the benefit of having a high fuel to payload mass ratio.

This shows there is a theoretical upper limit to the payload that can be hoisted on Earth using the propellant technology we have available. It is not possible to simply keep increasing the payload and fuel masses in equal proportion in order to lift arbitrarily large loads, as would be suggested by using the Tsiolkovsky equation with no extra terms for gravity."


If the payload weighs X, you need Y amount of fuel to get it to LEO.

From there, if you need to go anywhere else MEO, GEO, SSO, the Moon, Mars or another planet, value of Y (the amount of fuel you need) is greater, so you need bigger fuel tanks to carry the additional fuel into orbit. This increases the weight of of the rocket so you need even more fuel.

The same applies to boosting an object from LEO to another orbit, including a transfer orbit to another body... the greater the mass of the object, the more inertia you have to overcome to move it to its new orbit and that means more fuel and therefore, more weight. You have to get that extra fuel into LEO somehow, so one or both of your two launches that you used to put the components of your bigger Mars rover and EDL into LEO also has to carry this extra fuel. That either means your initial rocket has to be bigger, or you will need a third launch to bring that extra fuel into orbit.
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