Author Topic: Abort options for Starship and Starship/SuperHeavy  (Read 292766 times)

Online InterestedEngineer

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #540 on: 12/11/2022 08:11 pm »
What is the cross range of SS? It's a lot more than a capsule, but not as good as an airplane.
You want to look at down range capacity as well, i.e. the total landing ellipse (or whatever shape it is)

I assume this is in the context of an abort from orbit, since this is the only case where unplanned cross range is needed.*  In that case you also need to look at down range options.  You are moving at 8km/s.  Even if you need to get on the ground NOW several hundred km of down range requires nothing but timing.  There are situations where ms count, but there are many more where a few minutes don't matter.


* An abort during launch should be preplanned, if necessary you arrange for suitable diversion landing points. If it's non-flat land send in the bulldozers months ahead of time.  If its non-land send in the drone ship. 

An abort during a planned landing is going to end up pretty much where you planned.  You might divert from LZ-1 to LZ-2 or the skid strip if someone parks a car on your destination (or some such idiocy), but this does not require much cross range and only minimal planning.

All take off & landing spots are going to be next to water.  Take off trajectory is over water.

This is why abort to water should be made to work.

 

Online Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #541 on: 12/11/2022 08:56 pm »
To be honest, I'm much more worried about the landing system.  The fall with nothing guiding you but drag fins, startup at the last second and use power to slow down and then guide to a landing ellipse on the chopsticks that's, what, maybe a meter by a few meters is inherently risky.  A good gust of wind and you miss and run out of fuel before there's anything you can do about it.

One time I was on a manlift at work.  It was a clear blue sky day without a cloud in the sky and I was in a t-shirt.  The wind was about 2m/s average, which you can barely feel.  Then a gust came by and the wind went from 2m/s to 25m/s (55mph) in 2 seconds.  I had a sonic anemometer on a tower right next to me so I had recorded data showing that.  The wind didn't slow down immediately.  It was still around 18m/s when I got down a couple of minutes later.

Unless I did the math wrong (and I might have - someone check me as this doesn't seem right), the drag on a hovering SS at sea level in 25m/s winds is about 5MN, or about 3 Raptors at full throttle pushing straight sideways.

Online tbellman

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #542 on: 12/11/2022 10:56 pm »
Unless I did the math wrong (and I might have - someone check me as this doesn't seem right), the drag on a hovering SS at sea level in 25m/s winds is about 5MN, or about 3 Raptors at full throttle pushing straight sideways.

FD = ρ * v2 * CD * A / 2

Approximating Starship as a cylinder, and plugging in:

          ρ = 1.2 kg/m3
          v = 25 m/s
          CD = 1.17
          A = 50 m * 9 m = 450 m2

I get FD ≈ 200 kN, or about 20 tonne force, which is a factor 25 lower than you got.  That sounds more reasonable.

(Edit: Considering the factor 25 difference, did you perhaps cube the air velocity instead of squaring it?)
« Last Edit: 12/11/2022 11:04 pm by tbellman »

Offline warp99

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #543 on: 12/11/2022 11:15 pm »
Unless I did the math wrong (and I might have - someone check me as this doesn't seem right), the drag on a hovering SS at sea level in 25m/s winds is about 5MN, or about 3 Raptors at full throttle pushing straight sideways.

FD = ρ * v2 * CD * A / 2

Approximating Starship as a cylinder, and plugging in:

          ρ = 1.2 kg/m3
          v = 25 m/s
          CD = 1.17
          A = 50 m * 9 m = 450 m2

I get FD ≈ 200 kN, or about 20 tonne force, which is a factor 25 lower than you got.  That sounds more reasonable.

(Edit: Considering the factor 25 difference, did you perhaps cube the air velocity instead of squaring it?)
As a backup to this estimate SpaceX were planning to develop methox gas-gas thrusters of about 10 tonnes force for the ship so several such thrusters would be able to hold against a random direction wind gust even after allowing for cosine losses.

Offline eriblo

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #544 on: 12/11/2022 11:46 pm »
Another backup is the belly-flop terminal velocity under <2MN of gravity...

Online Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #545 on: 12/12/2022 12:02 am »
Unless I did the math wrong (and I might have - someone check me as this doesn't seem right), the drag on a hovering SS at sea level in 25m/s winds is about 5MN, or about 3 Raptors at full throttle pushing straight sideways.

FD = ρ * v2 * CD * A / 2

Approximating Starship as a cylinder, and plugging in:

          ρ = 1.2 kg/m3
          v = 25 m/s
          CD = 1.17
          A = 50 m * 9 m = 450 m2

I get FD ≈ 200 kN, or about 20 tonne force, which is a factor 25 lower than you got.  That sounds more reasonable.

(Edit: Considering the factor 25 difference, did you perhaps cube the air velocity instead of squaring it?)

Probably!  I do wind energy, and the formula for that is 1/2 * rho * A * V^3 * Cp.  I probably hit "cube" out of habit!

Thanks for checking.  That's why I said it didn't seem right!

Online TheRadicalModerate

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #546 on: 12/12/2022 01:36 am »
I assume this is in the context of an abort from orbit, since this is the only case where unplanned cross range is needed.

Does anybody have a handle on how big a nav error one can have from either a translunar or interplanetary direct entry? 

If this is a problem, you can obviate it simply by not doing direct entries, relying instead on aerocaptures, post-capture orbit computation/refinement, and then doing an entry.  But that assumes a non-emergency situation.  If there's an emergency (an imminent ECLSS failure, a medical emergency, a dodgy structural or guidance issue that's more likely to succeed with one entry instead of two, etc.), direct entry is still a problem that would need to be bounded for how far off-target you're going.

A related issue:  Are there cases where your mid-course corrections are non-existent or unreliable for some reason but you can still hit a viable entry corridor, or is it the case that if your nav system has gone belly-up enough to not return reliable course corrections, you're going to burn up or miss anyway?  Can you eyeball an entry corridor without necessarily knowing the related landing ellipse?

Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #547 on: 12/12/2022 01:51 am »
Unless I did the math wrong (and I might have - someone check me as this doesn't seem right), the drag on a hovering SS at sea level in 25m/s winds is about 5MN, or about 3 Raptors at full throttle pushing straight sideways.

FD = ρ * v2 * CD * A / 2

Approximating Starship as a cylinder, and plugging in:

          ρ = 1.2 kg/m3
          v = 25 m/s
          CD = 1.17
          A = 50 m * 9 m = 450 m2

I get FD ≈ 200 kN, or about 20 tonne force, which is a factor 25 lower than you got.  That sounds more reasonable.

(Edit: Considering the factor 25 difference, did you perhaps cube the air velocity instead of squaring it?)

Probably!  I do wind energy, and the formula for that is 1/2 * rho * A * V^3 * Cp.  I probably hit "cube" out of habit!

Thanks for checking.  That's why I said it didn't seem right!
Side note, but what do you think about wind electricity to make hydrogen on-site?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline LouScheffer

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #548 on: 12/12/2022 01:54 am »
Can you eyeball an entry corridor without necessarily knowing the related landing ellipse?
Yes.  This was tested on the Apollo missions, to see if a successful landing could be made even if all communications were lost. (I believe the entry corridor corrections were sent from the ground, not computed on board).

Nowadays, at least for Earth, if you are close enough to re-enter you've likely got GPS (it works at least as high as GSO) and plenty of onboard compute power.

Online TheRadicalModerate

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #549 on: 12/12/2022 02:11 am »
Can you eyeball an entry corridor without necessarily knowing the related landing ellipse?
Yes.  This was tested on the Apollo missions, to see if a successful landing could be made even if all communications were lost. (I believe the entry corridor corrections were sent from the ground, not computed on board).

Nowadays, at least for Earth, if you are close enough to re-enter you've likely got GPS (it works at least as high as GSO) and plenty of onboard compute power.

Thanks.  So this means that you could be quite a bit off and still make a successful direct entry.  (You probably can't be so far off that the pre-eyeball impact parameter is much bigger than the diameter of the Earth, but at this granularity, the Earth is pretty big, even if you're coming from Mars.)

This is a contingency where your landing ellipse location is ¯\_(ツ)_/¯.

Online Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #550 on: 12/12/2022 02:46 am »
Unless I did the math wrong (and I might have - someone check me as this doesn't seem right), the drag on a hovering SS at sea level in 25m/s winds is about 5MN, or about 3 Raptors at full throttle pushing straight sideways.

FD = ρ * v2 * CD * A / 2

Approximating Starship as a cylinder, and plugging in:

          ρ = 1.2 kg/m3
          v = 25 m/s
          CD = 1.17
          A = 50 m * 9 m = 450 m2

I get FD ≈ 200 kN, or about 20 tonne force, which is a factor 25 lower than you got.  That sounds more reasonable.

(Edit: Considering the factor 25 difference, did you perhaps cube the air velocity instead of squaring it?)

Probably!  I do wind energy, and the formula for that is 1/2 * rho * A * V^3 * Cp.  I probably hit "cube" out of habit!

Thanks for checking.  That's why I said it didn't seem right!
Side note, but what do you think about wind electricity to make hydrogen on-site?

If it's a windy site, wind energy is denser than solar energy, so you can make a lot of energy in a small space.  It just takes energy to make hydrogen.  A lot of energy.

Offline sebk

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #551 on: 12/12/2022 06:05 pm »
And "reasonably flat land" isn't as common as you think.

and runways are all over the place.

Reasonably flat land is at least as common as runways

Runways are reasonably flat, if you can land a spaceplane you can land vertically. 

That assumes you can steer to reasonably flat land.  SS has very little cross range.  A space plane can steer to a runway because it has enormous cross range - in the 1000km range.

SS has about half of cross range of Shuttle, i.e. right with the other space planes (Shuttle had an extreme cross range even for a spaceplane because AOA for polar orbits requirement; its cross range was about 2500km).

I've no idea where did't you come with the notion that SS entry is ballistic (or anywhere close to that).

SpaceX has published entry profile, and max g-load is 2, while Shuttle's was 1.4. Ballistic leo entry is ~8g.
« Last Edit: 12/12/2022 06:11 pm by sebk »

Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #552 on: 12/12/2022 06:24 pm »
Starship Point to Point would actually probably have even higher L/D than Shuttle to get the lift needed for long distance in a single stage, and this comes from stuff Elon has said. So if we’re talking airline-like flight rates and reliability, it would look significantly different than the Starship we have at Starbase right now. If used as an orbital stage, it’d have much larger cross-range than OG Starship at Starbase, probably comparable or even greater than Shuttle.

The Air Force is interested in Starship for point to point logistics, which would require landing at places without an optimized landing pad.
« Last Edit: 12/12/2022 06:26 pm by Robotbeat »
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Offline sebk

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #553 on: 12/12/2022 06:32 pm »
I assume this is in the context of an abort from orbit, since this is the only case where unplanned cross range is needed.

Does anybody have a handle on how big a nav error one can have from either a translunar or interplanetary direct entry? 

If this is a problem, you can obviate it simply by not doing direct entries, relying instead on aerocaptures, post-capture orbit computation/refinement, and then doing an entry.  But that assumes a non-emergency situation.  If there's an emergency (an imminent ECLSS failure, a medical emergency, a dodgy structural or guidance issue that's more likely to succeed with one entry instead of two, etc.), direct entry is still a problem that would need to be bounded for how far off-target you're going.

Perseverance has landed few meters from the center of its designated landing point, directly from intetrplanetary path, without GPS (since there's none at Mars) and being tracked and course corrected from the Earth about 100M km away.

They allowed error of a few km, but they expected and got much less.

Of course nav was working. But this shows precision achievable from extreme remoteness of another planet.

On the Earth there should be multiple independent methods of precise location: GPS, Earth based tracking communicating position, automated navigation by automated landmarks recognition (this is a new tech, by already successfully used by Perseverance, for example, and also expected to be used for upcoming Moon landings). 3 string unlike vs unlike vs unlike redundancy. Should be far from worries.

My guess is that primary causes of missing predesignated landing spot would be emergencies during hypersonic part of the reentry and maybe sometimes emergency re-entries (but it's hard to fathom what would really require immediate reentry rather than say venting affected part of the cabin to vacuum (in the case of toxic fumes, fire, and likes) or say waiting 15 minutes more to get in range of a safe and medically equipped landing spot.

Survivable re-entry emergencies would be stuff like when one of the fins seized and controls are now very limited and surviving hypersonic portion takes priority and let's worry about landing spot once the vehicle is transsonic.

Offline sebk

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #554 on: 12/12/2022 06:44 pm »


Wrong. It could emergency land on any flat patch of land. Contrary to space planes which are runway or bust. But the whole reasoning is fallacious in the first place anyway.

Without legs that could be a problem as the current version of starship is using a catch system

Belly (or rather skirt) landing is a valid form of emergency landing.

Sn-10 inadvertently demonstrated landing without legs (legs have failed, multiple of them even didn't deploy). It didn't topple. It also demonstrated that hard landing at 8-10m/s is problematic (as it exploded few minutes later as a result of the damage), but it also clearly shown that skirt landing is possible.

Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #555 on: 12/12/2022 06:56 pm »
Inflatable slide like on airplanes would’ve allowed people to get out before it exploded, just like airplanes have those things to get out before an airplane sinks or the cabin burns.
« Last Edit: 12/12/2022 06:57 pm by Robotbeat »
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Offline Robotbeat

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #556 on: 12/12/2022 07:01 pm »
SpaceX WILL have variants of Starship that land (after reentry) on legs. Point to point cargo for the military, Mars Starship. Lunar HLS, though it won’t have heatshield tiles, it will definitely have legs.

SpaceX has, of course, tested various Starship prototypes with legs on multiple occasions successfully.

“SpaceX may sometimes try to catch Starship with the catching arms” is not the same thing as “it’s not feasible for SpaceX’s Starship to use legs if required for some reason.” And I’m not sure why people have difficulty with this logic.
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

Online Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #557 on: 12/12/2022 07:25 pm »
And "reasonably flat land" isn't as common as you think.

and runways are all over the place.

Reasonably flat land is at least as common as runways

Runways are reasonably flat, if you can land a spaceplane you can land vertically. 

That assumes you can steer to reasonably flat land.  SS has very little cross range.  A space plane can steer to a runway because it has enormous cross range - in the 1000km range.

SS has about half of cross range of Shuttle, i.e. right with the other space planes (Shuttle had an extreme cross range even for a spaceplane because AOA for polar orbits requirement; its cross range was about 2500km).

I've no idea where did't you come with the notion that SS entry is ballistic (or anywhere close to that).

Elon said so, and published a video demonstrating that.  It comes in with an AOA of about 90 degrees, which means no lift.

Online Lee Jay

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #558 on: 12/12/2022 07:27 pm »
SpaceX WILL have variants of Starship that land (after reentry) on legs. Point to point cargo for the military, Mars Starship. Lunar HLS, though it won’t have heatshield tiles, it will definitely have legs.

SpaceX has, of course, tested various Starship prototypes with legs on multiple occasions successfully.

“SpaceX may sometimes try to catch Starship with the catching arms” is not the same thing as “it’s not feasible for SpaceX’s Starship to use legs if required for some reason.” And I’m not sure why people have difficulty with this logic.

The legs they've tested have basically all failed, and they've made a big deal about getting rid of them for the chopsticks.

I haven't seen any drawings or sketches of legs that are wider than the base of the vehicle, which isn't wide enough to land on anything but pretty flat ground.  I'm not sure legs like on F9 are viable on a vehicle this size.

Offline Barley

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Re: Abort options for Starship and Starship/SuperHeavy
« Reply #559 on: 12/12/2022 07:32 pm »

Survivable re-entry emergencies would be stuff like when one of the fins seized and controls are now very limited and surviving hypersonic portion takes priority and let's worry about landing spot once the vehicle is transsonic.
And at this point you have no idea what the cross range capacity is and very little time to find out.  There will not be a lot of choices, the automation needs to pick one of the less awful ones PDQ.

Tags: LAS black zones 
 

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