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Old 21st November 2021, 09:17 AM   #121
RecoveringYuppy
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Some numbers for the Saturn V for reference. Gross mass in kilograms from Wikipedia:
Code:
Stage I    2,290,000    73% of total mass.
Stage II     496,200    17% of total mass.
Stage III    271,000     9% of total mass.
Payload      140,000     5% of total mass.
Note that the combined mass of the 2nd and 3rd stages is only 5.2 times the mass of the payload. Assuming the same ratio for the 200 kg Spinlaunch payload you'd expect a bit over a 1,000 kg, not 11,000 kg. I think someone slipped a decimal point.

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Old 21st November 2021, 11:34 AM   #122
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Originally Posted by RecoveringYuppy View Post
Some numbers for the Saturn V for reference. Gross mass in kilograms from Wikipedia:
Code:
Stage I    2,290,000    73% of total mass.
Stage II     496,200    17% of total mass.
Stage III    271,000     9% of total mass.
Payload      140,000     5% of total mass.
Note that the combined mass of the 2nd and 3rd stages is only 5.2 times the mass of the payload. Assuming the same ratio for the 200 kg Spinlaunch payload you'd expect a bit over a 1,000 kg, not 11,000 kg. I think someone slipped a decimal point.
I started to look at existing rockets to get an idea of what the "second stage" of the Spin Launch system might be comparable to. But, with a typical rocket each stage accelerates the later stages and there isn't much coasting between stages. In the case of the Spin Launch system, the rocket coasts for long time before ignition. I am not sure how fast it will still be moving when the rocket motor fires. It may be more comparable to a rockoon than a typical multistage rocket.
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Old 21st November 2021, 12:04 PM   #123
RecoveringYuppy
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Originally Posted by jadebox View Post
I started to look at existing rockets to get an idea of what the "second stage" of the Spin Launch system might be comparable to. But, with a typical rocket each stage accelerates the later stages and there isn't much coasting between stages. In the case of the Spin Launch system, the rocket coasts for long time before ignition. I am not sure how fast it will still be moving when the rocket motor fires. It may be more comparable to a rockoon than a typical multistage rocket.
Here is a profile for a shuttle launch.

There's a definite transition from "get up in the atmosphere" to "get up to orbital velocity". Given the benefit of not speeding up until you've left the atmosphere that is pretty typical. First stage is optimized for flying in the atmosphere and gaining altitude. Upper states for getting up to orbit speed.

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Old 21st November 2021, 01:39 PM   #124
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Originally Posted by RecoveringYuppy View Post
Yeah, the math isn't the problem here.

But do people realize how extremely expensive rocket launches are? If this completely obliterates a set of ball bearings every time you use it that might still be an incredible cost saving.
When I was designing shock and vibration isolation cases we quoted a cable trunk for NASA. During the construction of the international space station. It protected a bunch of cable connections on the lunch structure during launch of the space shuttle but got so severely damaged in the process that a new one was required for each launch. I expect it was from the rocket exhaust. I can't remember how much we charged them for each one but certainly less than even a drop in the bucket.
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Old 21st November 2021, 07:30 PM   #125
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Originally Posted by RecoveringYuppy View Post
Some numbers for the Saturn V for reference. Gross mass in kilograms from Wikipedia:
Code:
Stage I    2,290,000    73% of total mass.
Stage II     496,200    17% of total mass.
Stage III    271,000     9% of total mass.
Payload      140,000     5% of total mass.
Note that the combined mass of the 2nd and 3rd stages is only 5.2 times the mass of the payload. Assuming the same ratio for the 200 kg Spinlaunch payload you'd expect a bit over a 1,000 kg, not 11,000 kg. I think someone slipped a decimal point.
Yeah, this makes much more sense.
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Old 22nd November 2021, 12:21 AM   #126
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The problem I have with a launch mass as low as this is that it leads to an improbably low density for the launch vehicle. We can see a picture at 3:41 on Manley's video. The rocket diameter appears to be about 1 metre (judging by the two 60cm cube satellites it carries) and about 8 metres in length. If it has a mass of 1120 kg, it would have an average density of only 0.178g/cm3. Since a good portion of its interior is liquid fuel with (I presume) a density close to that of water, this seems impossibly low to me.

If it has a mass of 11200kg, then its average density would be a much more reasonable 1.78 g/cm3.

Happy to be pointed out where I have made a mistake, and one of those mistakes might be "you shouldn't take that glossy video too seriously".

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Old 22nd November 2021, 12:26 AM   #127
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I see the density of liquid hydrogen is a lot lower than I thought it might be at only 0.07 g cm3, so perhaps I've got that wrong.
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Old 22nd November 2021, 12:50 AM   #128
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Originally Posted by RecoveringYuppy View Post
They also retracted it I've been told.

ETA: I just looked back. You said they removed the numbers, right?
Yes. I don't think that sudden lack of transparency speaks in their favor.
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Old 22nd November 2021, 05:35 AM   #129
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Originally Posted by BillC View Post
The problem I have with a launch mass as low as this is that it leads to an improbably low density for the launch vehicle. We can see a picture at 3:41 on Manley's video. The rocket diameter appears to be about 1 metre (judging by the two 60cm cube satellites it carries) and about 8 metres in length. If it has a mass of 1120 kg, it would have an average density of only 0.178g/cm3. Since a good portion of its interior is liquid fuel with (I presume) a density close to that of water, this seems impossibly low to me.

If it has a mass of 11200kg, then its average density would be a much more reasonable 1.78 g/cm3.

Happy to be pointed out where I have made a mistake, and one of those mistakes might be "you shouldn't take that glossy video too seriously".
Yeah. There is certainly only so much that can be read in to or out of CGI mockups. I tried to make the same guestimates based on extrapolating from the size of the payload.

Note that the 1.78 you got it twice the density of Kerosene (I seem to recall that is their intended fuel but don't have a reference handy).

What density do you get for the Electron that I cited back in post 120?
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Old 22nd November 2021, 05:37 AM   #130
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Originally Posted by erlando View Post
Yes. I don't think that sudden lack of transparency speaks in their favor.
Well, you haven't yet told us how tall you are so I can't take you seriously.

Seriously? Correcting a possible mistake is a lack of transparency? And they are a private company.
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Old 22nd November 2021, 05:40 AM   #131
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Originally Posted by RecoveringYuppy View Post
Well, you haven't yet told us how tall you are so I can't take you seriously.

Seriously? Correcting a possible mistake is a lack of transparency? And they are a private company.
They didn't correct the claim. They removed it. There's a significant difference.
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Old 22nd November 2021, 06:07 AM   #132
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Thunderf00t's criticism of the test projectile possibly tumbling as it emerges from the chamber does point out one interesting technical issue. If the rocket is released from the centrifuge arm all at once, it will still have angular momentum from the centrifuge's spin, at the same angular velocity as the centrifuge itself (so, about 450 rpm in the full-scale scenario). That needs to be counteracted for the rocket to go straight. Perhaps by releasing the front/top of the rocket an instant before the back/base of it.
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Old 22nd November 2021, 06:45 AM   #133
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Originally Posted by Myriad View Post
Thunderf00t's criticism of the test projectile possibly tumbling as it emerges from the chamber does point out one interesting technical issue. If the rocket is released from the centrifuge arm all at once, it will still have angular momentum from the centrifuge's spin, at the same angular velocity as the centrifuge itself (so, about 450 rpm in the full-scale scenario). That needs to be counteracted for the rocket to go straight. Perhaps by releasing the front/top of the rocket an instant before the back/base of it.
An interesting solution, but wouldn't that require the rocket to pivot, to counter said angular momentum, while still being held if not driven by the base causing torsional stress on the rocket body? Or perhaps a pivoting support launch frame? Having built up that momentum slowly then countering in just the release window is certainly problematic.

ETA: Just doing some quick diagrams in my head depending on where the shortest radius from the center meets the rocket (base, nose cone, center or somewhere in between them) relates how fast one end will be moving in relation to the other and what direction the rocket will rotate upon release. Also related to the length of the rocket. By sliding the shortest arm to center (of the centrifuge) point from one end of the rocket to the other as released, again allowing it to pivot, might just do the trick. Or at least reduce that momentum enough such that attitude control systems (active and passive) can handle.

ETA2: The centrifuge at that point (release window) basically becomes an atlatl
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Old 22nd November 2021, 07:05 AM   #134
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Originally Posted by Myriad View Post
Perhaps by releasing the front/top of the rocket an instant before the back/base of it.
Just a quick thought ...

The rocket could be mounted on a gear driven by the arm so that it always points in the direction that it will be released.
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Old 22nd November 2021, 07:12 AM   #135
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Originally Posted by jadebox View Post
Just a quick thought ...

The rocket could be mounted on a gear driven by the arm so that it always points in the direction that it will be released.
That puts significant limits on how long it can be and how close to the wall it can get.
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Old 22nd November 2021, 07:30 AM   #136
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Originally Posted by RecoveringYuppy View Post
That puts significant limits on how long it can be and how close to the wall it can get.
And if the illustrations and photos I've seen are accurate, the rocket does seem to be fixed to the end of the arm until it is released.
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Old 22nd November 2021, 07:31 AM   #137
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Originally Posted by The Man View Post
Or perhaps a pivoting support launch frame?

Originally Posted by jadebox View Post
Just a quick thought ...

The rocket could be mounted on a gear driven by the arm so that it always points in the direction that it will be released.

Originally Posted by RecoveringYuppy View Post
That puts significant limits on how long it can be and how close to the wall it can get.

That's conceivable, and probably not too problematic mechanically, but besides the length/wall clearance issue, it would mean the 1000g centripetal acceleration on the rocket would be constantly and rapidly changing direction, which sounds even harder to manage structurally than the unidirectional (relative to the rocket) high-g acceleration in the basic scheme.

I'm not sure that releasing the nose end of the rocket first to cancel out its tumble would cause much torsional stress on the rocket. It's like letting go of one end of a heavy horizontal weight that's supported at two points, one toward either end. The weight pivots as the released end just "free falls" (in the rotating reference frame of the centrifuge). The remaining attachment would have to be designed to permit that pivoting, though.
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Old 22nd November 2021, 07:35 AM   #138
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Originally Posted by Myriad View Post
it would mean the 1000g centripetal acceleration on the rocket would be constantly and rapidly changing direction, which sounds even harder to manage structurally than the unidirectional (relative to the rocket) high-g acceleration in the basic scheme.
Good point. (BTW I think it's 10K g?) Constantly and rapidly changing the direction of the g stress would be killer.
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Old 22nd November 2021, 07:35 AM   #139
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Originally Posted by RecoveringYuppy View Post
Originally Posted by jadebox View Post
Just a quick thought ...

The rocket could be mounted on a gear driven by the arm so that it always points in the direction that it will be released.
That puts significant limits on how long it can be and how close to the wall it can get.
Not to mention reversing momentum on the rocket as it points in the release direction but is rotating away from it. Getting to supper sonic speed will probably require significant RPMs on the centrifuge increasing the frequency of the momentum reversal (from the rockets perspective) introducing considerable vibrational, resonance and harmonic structural considerations.
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Old 22nd November 2021, 07:40 AM   #140
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Originally Posted by Myriad View Post
That's conceivable, and probably not too problematic mechanically, but besides the length/wall clearance issue, it would mean the 1000g centripetal acceleration on the rocket would be constantly and rapidly changing direction, which sounds even harder to manage structurally than the unidirectional (relative to the rocket) high-g acceleration in the basic scheme.

I'm not sure that releasing the nose end of the rocket first to cancel out its tumble would cause much torsional stress on the rocket. It's like letting go of one end of a heavy horizontal weight that's supported at two points, one toward either end. The weight pivots as the released end just "free falls" (in the rotating reference frame of the centrifuge). The remaining attachment would have to be designed to permit that pivoting, though.
Not sure of the stress either also the pivoting support frame I mentioned would only pivot during the release window as a support for whatever torsional stress was resulting from the momentum transition.
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Old 22nd November 2021, 07:47 AM   #141
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Can't they just angle the rocket is relation to the trajectory ? Tip the nose outward rather than release it early?
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Old 22nd November 2021, 08:47 AM   #142
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Originally Posted by casebro View Post
Can't they just angle the rocket is relation to the trajectory ? Tip the nose outward rather than release it early?
That's what releasing the nose first and allowing it to pivot would do. Thing is the rocket isn't uniform throughout and generally is structurally engineered to be pushed from the bottom. A review of failed rocket launch videos can show how easily torsional stress can rip a rocket apart. With a pivoting support frame the rocket can rotate when needed and most of the stress can be confined to elements of the structure that can best handle it.
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Old 22nd November 2021, 07:08 PM   #143
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It's pretty easy to translate circular to ballistic motion. Just ask Trebuchet. It's even easier when your projectile is geometrically self-stabilising. It's not quite throwing a dart at a dartboard, but I think the concerns being discussed now aren't as significant as some people might think.
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Old 22nd November 2021, 10:35 PM   #144
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Originally Posted by Myriad View Post
If the rocket is released from the centrifuge arm all at once, it will still have angular momentum from the centrifuge's spin, at the same angular velocity as the centrifuge itself (so, about 450 rpm in the full-scale scenario)
Would it? I've been trying to get my head round this.

Imagine a projectile consisting of a series of point masses linked by inflexible joins:


|

|

|

|


The pivot is to the left and the throw arm is connected to the centre mass. At the instant of release the centre mass is moving upward. But at that same instant, what direction are the other masses moving in?

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Old 23rd November 2021, 07:13 AM   #145
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Originally Posted by BillC View Post
Would it? I've been trying to get my head round this.

Imagine a projectile consisting of a series of point masses linked by inflexible joins:


|

|

|

|


The pivot is to the left and the throw arm is connected to the centre mass. At the instant of release the centre mass is moving upward. But at that same instant, what direction are the other masses moving in?
Upwards, as they are all along a tangent line. The thing is that being along that tangent line some will have different distances from the center of rotation then others. Meaning they will be moving at different tangential velocities when released. With the center of the rocket moving slower than both its ends it will tend to tumble and have torsional stress. Also since the rocket is not two dimensional and has some none zero width, one side of the rocket (closer to the center of rotation) will be moving slower (tangentially) than the other side. Again putting a torque on the rocket causing it to tumble and experience torsional stress.


ETA: Sorry I forgot about the tangents for the other mass points on the rocket. As you move to the points up from the center the tangent line to a radial line from the center of rotation will tend to point increasingly left. while a tangent line will tend to point increasing right as you move to points below the center of the rocket. So in addition to having different velocities along the tangent line of the center the points on the rocket below the center will also have an increasingly right component while those above the center will have an increasing left component. Tending to rotate the rocket counterclockwise.

As noted by Myriad if the front of the rocket is released as its tangent line is parallel to the centers tangent line in your example and allowed to pivot at the bottom end it will tend to remain on that tangent line and then is released from the bottom when its tangent line is parallel. The different parts will still have different speeds but that will result in structural stress along that tangent line. The difference in speeds from one side of the rocket to the other, because it has width, will still have to be dealt with but perhaps in how it is allowed to pivot before release.
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Old 23rd November 2021, 06:41 PM   #146
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Originally Posted by Myriad View Post
Thunderf00t's criticism of the test projectile possibly tumbling as it
emerges from the chamber does point out one interesting technical issue.
If the rocket is released from the centrifuge arm all at once, it will still have
angular momentum from the centrifuge's spin, at the same angular velocity
as the centrifuge itself (so, about 450 rpm in the full-scale scenario).
That needs to be counteracted for the rocket to go straight. Perhaps
by releasing the front/top of the rocket an instant before the back/base of it.

What if you launched the rocket parallel to the rotor arm?

No. That wouldn't work.

The far end would rotate faster than the near end.

I know! Make the rocket curved like a Cheeto.


P. S. Second video on the subject.

YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
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Old 8th December 2021, 10:06 PM   #147
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Originally Posted by Susheel View Post
Yeah...I agree. Not a big thuderf00t fan myself, but I clicked on the link and wasn't disappointed. Also, not a very long video, so didn't take too much of my time.

When I saw the original launch video, I had two very serious doubts.
  • First how does that fragile membrane that the missile ruptures through retain the vacuum behind it? Even considering the possibility of an inside port that seals off the vertical channel until the split second the missile is released to go through it.
  • Second, wouldn't the balance of the armature be dangerously off the moment it disengages the missile?

Perhaps the other end of the armature could simultaneously release an equal weight of water? Maybe it would be less damaging and dangerous than a solid counterweight?

Go ahead and laugh I'm no scientist.
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Old 9th December 2021, 12:54 AM   #148
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Originally Posted by mgidm86 View Post
Perhaps the other end of the armature could simultaneously release an equal weight of water? Maybe it would be less damaging and dangerous than a solid counterweight?

Go ahead and laugh I'm no scientist.
Water going at 5 times the speed of sound might just as well be a solid.
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Old 9th December 2021, 05:45 AM   #149
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Originally Posted by erlando View Post
Water going at 5 times the speed of sound might just as well be a solid.
And you have to contain the water in a container sturdy enough to not only carry the needed amount of it, but also sturdy enough to do it, while undergoing the G forces during the spinning.

Let's just say, I would not want to be in the same building complex when that thing is slamming into the ground.
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Old 9th December 2021, 06:34 AM   #150
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I suggest as counterweight a container with half matter and antimatter, separated by a slider that is released at the same time as the rocket.
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Old 9th December 2021, 07:03 AM   #151
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Originally Posted by The Great Zaganza View Post
I suggest as counterweight a container with half matter and antimatter, separated by a slider that is released at the same time as the rocket.
That would work.

I retract my objection to not wanting to be in the same building complex.
The same continent though is another matter.

But one can't bake an omelet without cracking one or more continents.
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Old 9th December 2021, 07:49 AM   #152
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Originally Posted by The Great Zaganza View Post
I suggest as counterweight a container with half matter and antimatter, separated by a slider that is released at the same time as the rocket.
What is the container and separator made of?
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Old 9th December 2021, 09:23 AM   #153
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Originally Posted by Jack by the hedge View Post
What is the container and separator made of?
Unobtanium? But first we'll have to go to Pandora and negotiate for mining rights steal it.
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Old 9th December 2021, 09:32 AM   #154
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Originally Posted by Myriad View Post
"Several times" the speed of sound, with a radius (in the 1/3 scale system) of about 25m, translates to several thousand g's of angular acceleration. Payloads will have to be pretty sturdy.

Maybe that speed applies only to the full-scale version, which would make it around 1200 g's instead.
And "several times" the speed of sound hardly cuts it, since escape velocity as far as I recall is about 20 km/sec. Not only will the payload experience extreme G forces, but what will keep it from burning up from friction in the lower atmosphere?

Hans
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Old 9th December 2021, 10:21 AM   #155
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Originally Posted by MRC_Hans View Post
And "several times" the speed of sound hardly cuts it, since escape velocity as far as I recall is about 20 km/sec. Not only will the payload experience extreme G forces, but what will keep it from burning up from friction in the lower atmosphere?

Hans
Satellite orbits decay and the debris falls to earth all he rime. This is the reverse process of launching (orbital speed to zero vs zero to orbital speed). You ask a good question.

More Unobtanium?
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Old 9th December 2021, 11:00 AM   #156
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Originally Posted by Gord_in_Toronto View Post
Satellite orbits decay and the debris falls to earth all he rime. This is the reverse process of launching (orbital speed to zero vs zero to orbital speed). You ask a good question.

More Unobtanium?
Satellites are not designed to survive re-entry. The projectiles are designed to survive travelling through the air at high speed. For one thing, they are pointed and aerodynamic so the air flows around them instead of compressing into a hot plasma.
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Old 9th December 2021, 11:51 AM   #157
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Escape speed is 11 km/s.
Orbital speed is about 8 km/s.
Mach 5 is about 1.7 km/s.

Note that it's the last number that is relevant for talking about what happens to the rocket in the lower atmosphere. And at that speed it won't be in the lower atmosphere very long.

I don't remember the exact flight profile of this thing but I'm pretty sure they won't be attempting to exceed mach 5 until somewhere in the stratosphere.
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Old 9th December 2021, 11:58 AM   #158
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Originally Posted by RecoveringYuppy View Post
Escape speed is 11 km/s.
Orbital speed is about 8 km/s.
Mach 5 is about 1.7 km/s.

Note that it's the last number that is relevant for talking about what happens to the rocket in the lower atmosphere. And at that speed it won't be in the lower atmosphere very long.

I don't remember the exact flight profile of this thing but I'm pretty sure they won't be attempting to exceed mach 5 until somewhere in the stratosphere.
Yes. I have no idea about the numbers but deltaV saved at launch is worth an awful lot in terms of fuel required. I don't know how much fuel a Falcon 9 uses to get to Mach 5 but I bet it's a bucketload.
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Old 9th December 2021, 02:26 PM   #159
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Originally Posted by jadebox View Post
Satellites are not designed to survive re-entry. The projectiles are designed to survive travelling through the air at high speed. For one thing, they are pointed and aerodynamic so the air flows around them instead of compressing into a hot plasma.
True enough. So no cookie for me.
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Old 9th December 2021, 02:40 PM   #160
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Originally Posted by 3point14 View Post
Yes. I have no idea about the numbers but deltaV saved at launch is worth an awful lot in terms of fuel required. I don't know how much fuel a Falcon 9 uses to get to Mach 5 but I bet it's a bucketload.
And it'd be an exceptionally large bucket at that.
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