Author Topic: Abort options for Starship and Starship/SuperHeavy  (Read 135039 times)

Offline eriblo

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #200 on: 11/24/2022 11:16 pm »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?
Because it can be engineered (and tested) to do so.

At what mass penalty?

There's a reason ship hulls are 3-10 times (or more) thicker than SS's tanks.
Remember that Starship will ride on top of the water with less than 1 m of draft. Unless you divert into a storm I think it will be fairly resilient as long as it can maintain some pressurization. My quick estimate gives less than 1 bar to maintain a cylinder that is 50 m by 9 m and 300 t in tension even if supported only on the ends.

Offline Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #201 on: 11/25/2022 12:00 am »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?
Because it can be engineered (and tested) to do so.

At what mass penalty?

There's a reason ship hulls are 3-10 times (or more) thicker than SS's tanks.
Remember that Starship will ride on top of the water with less than 1 m of draft. Unless you divert into a storm I think it will be fairly resilient as long as it can maintain some pressurization. My quick estimate gives less than 1 bar to maintain a cylinder that is 50 m by 9 m and 300 t in tension even if supported only on the ends.

Remember, it will fall over.  That impact load will drive everything. And it will be huge.

Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #202 on: 11/25/2022 01:55 am »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?
Because it can be engineered (and tested) to do so.

At what mass penalty?

There's a reason ship hulls are 3-10 times (or more) thicker than SS's tanks.
Remember that Starship will ride on top of the water with less than 1 m of draft. Unless you divert into a storm I think it will be fairly resilient as long as it can maintain some pressurization. My quick estimate gives less than 1 bar to maintain a cylinder that is 50 m by 9 m and 300 t in tension even if supported only on the ends.

Remember, it will fall over.  That impact load will drive everything. And it will be huge.
Sounds concern-trolly.

Deployable airbags are one possibility. Thrusters specifically for this scenario is another. Either way, if Starship gets to the point where thatís actually an important factor, itíll be solved. With analysis and testing.


Itís worth pointing out that a Falcon 9 booster is about the same height and it HAS, on multiple occasions, survived such a scenario in spite of having a higher fineness ratio (less resistance to bending) and having no requirement to survive such a load.

https://twitter.com/elonmusk/status/1070399755526656000
« Last Edit: 11/25/2022 04:13 pm by Robotbeat »
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Online TheRadicalModerate

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #203 on: 11/25/2022 02:39 am »
Seems like this is a problem that doesn't have to be solved if you're going with an escape capsule.  We have plenty of experience making blunt bodies float, although this one would be larger and possibly top-heavier.  Same thing for rough-surface landings.

It's possible that there would be parachute limitations.  However, if you have a propulsive system, you can arbitrarily reduce the descent speed before popping the chutes, which should reduce peak loads to something manageable.

Online mikelepage

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #204 on: 11/25/2022 03:05 am »
Seems like this is a problem that doesn't have to be solved if you're going with an escape capsule.  We have plenty of experience making blunt bodies float, although this one would be larger and possibly top-heavier.  Same thing for rough-surface landings.

It's possible that there would be parachute limitations.  However, if you have a propulsive system, you can arbitrarily reduce the descent speed before popping the chutes, which should reduce peak loads to something manageable.

Not sure if you would have seen the video I did on point to point transport with Starship earlier this year?



The capsule escape system is depicted starting at 25:48, and while it's something of an afterthought to the main thrust of the video, it did make sense to me as something that would be easier if you have a standard capsule format. That is, you wouldn't start using capsules to enable an escape system (or even to do crewed flight), but if you decided to use capsules for fast cargo transit, then that could lead to crew-rating those capsules, and then this form of escape system would be possible.

Caveat: I realise few will watch the whole thing, but as I say earlier on in the video, this is meant to be "a buffet of ideas", not that I'm especially fixated on any specific one. My thing is trying to imagine the system as a whole, and how it might evolve as a result of trying to integrate into the world as it currently is.


 

Offline sebk

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #205 on: 11/25/2022 01:34 pm »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?

You shouldn't expect that even for perfectly calm sea in fact.

The tip of 50m tall vehicle toppling sideways would impact at about 38m/s. Good luck with thin shell staying intact after that.

After all the tip velocity of falling rigid stick is sqrt(3*g*l) where l is the length of the object, g is surface gravitational acceleration. This approximation (specific moment of intertia I = 1/3 * l^2) is good enough for Starship.

Offline DanClemmensen

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #206 on: 11/25/2022 02:00 pm »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?

You shouldn't expect that even for perfectly calm sea in fact.

The tip of 50m tall vehicle toppling sideways would impact at about 38m/s. Good luck with thin shell staying intact after that.

After all the tip velocity of falling rigid stick is sqrt(3*g*l) where l is the length of the object, g is surface gravitational acceleration. This approximation (specific moment of intertia I = 1/3 * l^2) is good enough for Starship.
Does the SS have RCS thrusters near the nose? If so, these could be used to control the tip-over. If not, then adding thrusters for tip-over may be cheaper, lighter, and more reliable than other abort systems. During a normal landing, SS transitions from "belly flop" to vertical. This is in effect the reverse tranistion.

Offline Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #207 on: 11/25/2022 02:20 pm »
0:15, breaks apart after tipping.
0:48, used thrusters on the top to control tipping.


Offline DanClemmensen

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #208 on: 11/25/2022 02:28 pm »
0:15, breaks apart after tipping.
0:48, used thrusters on the top to control tipping.

But F9 boosters are not designed to handle a 2g lateral force, while an SS must do this as part of a normal EDL. A video compilation airliner landing failures would be just as relevant.

Offline Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #209 on: 11/25/2022 02:39 pm »
0:15, breaks apart after tipping.
0:48, used thrusters on the top to control tipping.

But F9 boosters are not designed to handle a 2g lateral force, while an SS must do this as part of a normal EDL. A video compilation airliner landing failures would be just as relevant.

If SS is coming in at a couple hundred tons, you think it's going to have thrusters on the top that can generate 2g's of lateral force, like 400 tons?  Or maybe half that?

Offline DanClemmensen

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #210 on: 11/25/2022 02:45 pm »
0:15, breaks apart after tipping.
0:48, used thrusters on the top to control tipping.

But F9 boosters are not designed to handle a 2g lateral force, while an SS must do this as part of a normal EDL. A video compilation airliner landing failures would be just as relevant.
If SS is coming in at a couple hundred tons, you think it's going to have thrusters on the top that can generate 2g's of lateral force, like 400 tons?  Or maybe half that?
Of course not. during EDL, most control is from the elonerons. But if the SS manages to get to a vertical "landing" in the ocean at 0 veloticity, it can perhaps use RCS to topple itself to horizontal with its ventral side down without exceeding 2g when it hits the water.
Edited to fix the quoting
« Last Edit: 11/25/2022 03:58 pm by DanClemmensen »

Offline spacenut

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #211 on: 11/25/2022 03:17 pm »
Also, the Starship seems to be bottom heavy, not top heavy.  A Starship landing on a barge or back on earth will have an empty payload bay until passenger Starships start to operate.  The engines, bottom fins, and landing gear will be heavier than the fairly empty upper portion.  This would make for a lower center of gravity and once landed, can be secured, like the F9 booster, which is taller in proportion to width than Starship.  Maybe they need to install better landing gear than what was used on the test vehicles.

Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #212 on: 11/25/2022 03:56 pm »
0:15, breaks apart after tipping.
0:48, used thrusters on the top to control tipping.

But F9 boosters are not designed to handle a 2g lateral force, while an SS must do this as part of a normal EDL. A video compilation airliner landing failures would be just as relevant.

If SS is coming in at a couple hundred tons, you think it's going to have thrusters on the top that can generate 2g's of lateral force, like 400 tons?  Or maybe half that?
I am amazed (but not totally surprised) that you refuse to acknowledge the fact that falcon 9 has actually already survived this scenario in some cases (even a single case would prove this point) and so it would be totally reasonable to engineer a vehicle to *intentionally* survive it without needing to be some extremely heavy battleship design as you claimed.

No one ever admits theyíre wrong on the Internet, even when shown clear evidence.

(And I see no reason why thrusters couldnít scale with vehicle size.)
« Last Edit: 11/25/2022 04:09 pm by Robotbeat »
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Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #213 on: 11/25/2022 04:03 pm »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?

You shouldn't expect that even for perfectly calm sea in fact.

The tip of 50m tall vehicle toppling sideways would impact at about 38m/s. Good luck with thin shell staying intact after that.

After all the tip velocity of falling rigid stick is sqrt(3*g*l) where l is the length of the object, g is surface gravitational acceleration. This approximation (specific moment of intertia I = 1/3 * l^2) is good enough for Starship.
And yet Falcon 9, made out of thin aluminum alloy and very nearly as tall, actually did survive this on more than one occasion. When reality shows something actually happening even without intentional engineering, it’s no longer viable to claim it couldn’t be reasonably engineered to happen intentionally.
« Last Edit: 11/25/2022 04:06 pm by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline InterestedEngineer

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #214 on: 11/25/2022 04:27 pm »
I don't think letting a Starship fall over on its side in the water is viable, because there will always be wave conditions that will break it open.

Why would a pressurized Starship break open falling over its side into the water?

SN10 didn't even survive a hard landing on its bottom.  You really think you can tip over a 16 story building into the turbulent sea and expect it to survive?

You shouldn't expect that even for perfectly calm sea in fact.

The tip of 50m tall vehicle toppling sideways would impact at about 38m/s. Good luck with thin shell staying intact after that.

After all the tip velocity of falling rigid stick is sqrt(3*g*l) where l is the length of the object, g is surface gravitational acceleration. This approximation (specific moment of intertia I = 1/3 * l^2) is good enough for Starship.
And yet Falcon 9, made out of thin aluminum alloy and very nearly as tall, actually did survive this on more than one occasion. When reality shows something actually happening even without intentional engineering, itís no longer viable to claim it couldnít be reasonably engineered to happen intentionally.

When reality says X, and  math says Y, the math is probably wrong.

Let's see if we can figure out why.

(1) The thrusters are at most 1-2kn, not enough to really matter
(2) Aerodynamic drag at the front flaps at about half terminal velocity (70-80 m/s) is 1/4g.  The impact of the thrusters on the drag coefficient of about 5m of Starship length changes the drag force more than the thrust itself, from back of the envelope calculations.   But with all this it's not enough to subtract more than 1-4 m/sec off of the impact velocity and the lower the impact velocity, the less the drag.
(3) The center of mass of Starship isn't that of a pencil, it's probably that of a pyramid, or 1/4 of the way up
(4) The base is dug into the water about 2 meters, and water has 800x density that of air.  That base is going to resist rotation, by quite a bit, as that water has to be lifted up.
(5) As the body rotates more of the base contacts the water, at very low velocities, slowing the rotation down further and in a safe manner.

My guess is that (4) and (5) matter the most. Alas, calculating what the velocity is going to be in such a dynamic settling situation is beyond most of our analytic skills, and requires CFD.

Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #215 on: 11/25/2022 04:30 pm »
I agree definitely about the last two. And since weíre discussing a hypothetical legged starship with possible airbags, seems pretty likely to me you could maximize the water drag and soften the impact dramatically by a more gradual contact with the water by optimizing the legs and the airbags.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline InterestedEngineer

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #216 on: 11/25/2022 05:04 pm »
Here's the video of the water landing that falcon-9 survived without sploding:


Offline InterestedEngineer

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #217 on: 11/25/2022 07:28 pm »
The complexities of rotating while submerged in water and with horizontal movement of the base are beyond our meager analytic abilities, but we can calculate some more simpler sides of the problem.

How many joules need to be dissipated, and how much force does it to take to cancel out the 0.5MPa of internal pressure on the tanks where the water is being impacted, thus causing buckling?

With center of mass about (17-2) = 15m above the surface and a mass of 120t, mgh gives us 17.6MJ of potential energy that needs to be dissipated somehow.   The bottom is under water by 2m and that water has to be moved up, and 2m*120t*9.8 gives  2.4MJ.  So let's say net 15MJ of potential energy that has to go to zero.

The final displacement of water will be 120t, or 120m^3 of water.  This works out to 0.72m submerged height and an a water contact area of about 2.5 * 50 = 125m^2 ( per https://www.mathsisfun.com/geometry/cylinder-horizontal-volume.html )

0.5MPa with an area of 125m^2 gives a max force before buckling the pressurized tanks of 62.5MN.  Applied to F=ma with a mass of 120t yields a maximum acceleration of 520m/sec, or 53gs.

A rough estimate of the deceleration length in water is the final displacement of the water, which here is 0.72m, so plugging that into the kinematic equations with 0.72m length, 520m/sec acceleration, final velocity zero, and solving for initial velocity, yields 27.7m/sec, and the deceleration happens over 0.053 seconds.

27.7 m/sec is a free fall from 39m, which is well above the center of mass height.

Given the rotation is happening in water, which slows the rotation, and the impact on the water happens at the smaller radius first (you can see the progressive splash, it's about 1-2 frames from bottom to top of F9), or about the same length of time as the deceleration above, it seems entirely plausible that pressurized Starship fuel tanks can survive falling over with its tail in water.

Now, whether the header tanks or other internal plumbing can handle 53gs is another question.  They did on F9, apparently (which tells me that the max force is probably less than 53gs).

It's also easy to pad a precious human cargo area to handle 53gs, that kind of force is routine in car wrecks.

TL;DR - abort to water landing seems feasible.

Offline DanClemmensen

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #218 on: 11/25/2022 09:09 pm »
Another thought: we have been discussing the scenario where SS "lands" vertically a velocity zero at the surface. But there are trajectories where SS reaches the surface with velocity zero but at an angle, with the thrust of the raptors immediately prior to landing being vectored to oppose the torque imposed by gravity while also slowing the descent. I don't know how close to horizontal you can get, but If you can get all the way down to 30 degrees you reduce the energy of the final fall by a factor of four, right?

Offline InterestedEngineer

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #219 on: 11/25/2022 09:37 pm »
The complexities of rotating while submerged in water and with horizontal movement of the base are beyond our meager analytic abilities, but we can calculate some more simpler sides of the problem.

How many joules need to be dissipated, and how much force does it to take to cancel out the 0.5MPa of internal pressure on the tanks where the water is being impacted, thus causing buckling?

With center of mass about (17-2) = 15m above the surface and a mass of 120t, mgh gives us 17.6MJ of potential energy that needs to be dissipated somehow.   The bottom is under water by 2m and that water has to be moved up, and 2m*120t*9.8 gives  2.4MJ.  So let's say net 15MJ of potential energy that has to go to zero.

The final displacement of water will be 120t, or 120m^3 of water.  This works out to 0.72m submerged height and an a water contact area of about 2.5 * 50 = 125m^2 ( per https://www.mathsisfun.com/geometry/cylinder-horizontal-volume.html )

0.5MPa with an area of 125m^2 gives a max force before buckling the pressurized tanks of 62.5MN.  Applied to F=ma with a mass of 120t yields a maximum acceleration of 520m/sec, or 53gs.

A rough estimate of the deceleration length in water is the final displacement of the water, which here is 0.72m, so plugging that into the kinematic equations with 0.72m length, 520m/sec acceleration, final velocity zero, and solving for initial velocity, yields 27.7m/sec, and the deceleration happens over 0.053 seconds.

27.7 m/sec is a free fall from 39m, which is well above the center of mass height.

Given the rotation is happening in water, which slows the rotation, and the impact on the water happens at the smaller radius first (you can see the progressive splash, it's about 1-2 frames from bottom to top of F9), or about the same length of time as the deceleration above, it seems entirely plausible that pressurized Starship fuel tanks can survive falling over with its tail in water.

Now, whether the header tanks or other internal plumbing can handle 53gs is another question.  They did on F9, apparently (which tells me that the max force is probably less than 53gs).

It's also easy to pad a precious human cargo area to handle 53gs, that kind of force is routine in car wrecks.

TL;DR - abort to water landing seems feasible.

Doing a rough piece-wise radial deceleration in 10m sections along a thin cylinder into water, with a starting angular velocity of .75rad/sec, 50m cylinder, mass 120t  (that's a tip velocity of .75*50 = 37.5m/sec)

Moment of inertia is 100e6 kg-m^2 (1/3mL^2)

Change in angular velocity = torque / momentI = .75 = torque / 100e6

net torque must be 75e6 N-m to cancel all angular velocity.   By rough 10m sections hitting and displacing 0.72m of water each sequentially:

10: 7.5MN-m
20: 15MN-m
30: 22.5MN-m
40: 30MN-m

Thus each meter of length is exerting 0.75MN

Each 1m section is 2.5m^2 so 0.75MN/2.5 = 300kPA.  That's 3 bar, which is well within the 5 bar limit for buckling of the tanks.

By the time the last section hits the water the rotation is down to 0.3 radians/sec, which at 50m is 15m/sec.  Cross checking against linear acceleration with a length of 0.72m, that's 16gs on the tip of the nose.

The angle where things start to happen is sin-1(0.72/10) or 4 degrees, so the tip of the nose is 3.6 meters off the water when the the first 10m has sunk 0.72 into the water.  Not unreasonable.

This all assumes each 10m section settles in the water 0.72m before the next section hits.  Not completely unreasonable.   There are errors on both the pessimistic and optimistic side that could only be uncovered by numeric simulation

I feel like I got something wrong, be nice correcting ;)

TL;DR - the Starship stops its rotation not all at once, but starting near the base which is in the water.  This reduces the speed of the tip of the nosecone as it hits the water by more than half.
« Last Edit: 11/25/2022 10:00 pm by InterestedEngineer »

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