Author Topic: Safe Intantaneous Impact Point Tracks for Starship Recovery  (Read 19695 times)

Offline Twark_Main

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #40 on: 12/04/2023 09:27 am »
You're talking about a more-or-less 90º turn here.

...

 I've put an example below, although it seems to require about 2600m/s of delta-v, which is horrid.

I agree this is not a good idea (again, the only good idea is "don't explode" :D ), but I have questions:


    1)  What equation did you use? I'm not able to reproduce you 2600 m/s number. I get something much bigger.

    2)  Why wait until so far off the coast to perform the burn? You could cut the required angle (and therefore the requires delta-v) if you start the burn right off the coast. If the idea is that you can't perform any aerodynamic maneuver over land then your map still doesn't work, because to start a turn right after reaching the coast means Starship must have been reentering over Mexico itself.

    3) Why 28.6°? If there's anything we know, it's that the inclination flying out of Boca Chica will not be at 28.6°, but more like 30-31°. I don't see a reason why they'd be required to support launching out of the Cape and then landing at BC.

« Last Edit: 12/04/2023 09:30 am by Twark_Main »
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Offline Twark_Main

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #41 on: 12/04/2023 09:37 am »
2) The ability to get some translation during the bellyflop, most likely by lowering the nose.  Anybody have a good guess for how many km this might be good for?

Presumably it's (belly flop starting height) × (subsonic L/D).
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Offline whitelancer64

We don't have to speculate too much about any of this, there is precedent in Shuttle landing ground tracks.

The most likely re-entry ground tracks for early Starship flights are over the Gulf and only crossing over Florida long after the danger of breaking up during re-entry is past, like the ground track for STS-125, STS-133, STS-134, and STS-135.

https://spaceflightnow.com/shuttle/sts125/090521tracks/
https://spaceflightnow.com/shuttle/sts133/groundtrack/
https://spaceflightnow.com/shuttle/sts134/groundtracks/index2.html
https://spaceflightnow.com/shuttle/sts135/groundtrack/

For landings at Boca Chica, they would likely get permission from Mexican authorities to fly over / through their airspace for a landing. The ground track would look similar to STS-125 and STS-134 above, but the landing would be at the southern tip of Texas.

At some point, probably fairly soon after the first few successful landings, Starship will be presumed to be safe enough for CONUS overflight during reentry, just like Dragon is now.

I think it's more than just permission from Mexico for an ascending-segment EDL.  The FAA still has to issue the EDL license, and the required Dead People Expected Value computation doesn't care what the nationality of the expected dead people is.

Still just trying to get the problem clear here:  For each point in the EDL trajectory, there will be an IIP.  (Well, it's actually an instantaneous impact ellipse, but we'll use the acronyms we have.)  Associated with that point will be two values: a probability of impact (i.e., how likely debris is to fall in that spot, which will vary by flight phase), and a probability that the impact will injure or kill somebody, which is basically a function of population density at that point.

Presumably, the reason that a D2 can get an EDL license that overflies Florida is because its demonstrated reliability is so high that the probability of impact along the Florida portion of the IIP track is so low that, even when convolved with the high population density, the DPEV is still low enough.

That obviously won't be the case with Starship for a while.  My problem is that it's equally unlikely to yield an IIP track through either Mexico or the Rio Grande Valley that will pass the DPEV requirement.

Maybe this means they have to bring down a whole bunch of Starships in the Kauai test range, until the reliability brings the DPEV down to an acceptable value?

I think in both cases, whether landing at Texas or Florida, chances of debris falling on people is low. Reentry happens mostly over the Pacific Ocean and by the time Starship reaches the part of the flight path over Mexico, the chances of it breaking up should be low. Risk is reduced as you say, by overflight of low population areas or a shorter chord of Mexico.

And to be clear: Dragon has overflown CONUS during reentry, not just Florida.
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Offline whitelancer64

We've talked a lot about needing negative lift to keep Starship in the atmosphere during interplanetary reentries.  Does it need positive lift to stay in the corridor for a merely orbital reentry?  I have a feeling that the answer to this is, "Yes, it needs positive lift," but it'd be good to confirm it.

Note that the curve in my cheesy picture is a curve because it's a flat-earth model.  In reality, it'd be closer to a chord through the atmosphere.

Another half-baked idea:  Will a multiple-skip reentry keep the IIP track far enough downrange?
Barring other concerns you would want to use most of your lift as positive lift to extend the trajectory as this lessens g-forces and is easier on the TPS (you keep some in reserve for steering). Going sideways to maximize cross range will lead to a harsher entry. Doing negative lift to maximize the overshoot of potential debris will most likely fry/flatten you Starship.

Makes sense.  There's a big difference between hitting the atmosphere with an almost-circular perigee speed of 7900m/s, vs. coming in on a hyperbolic with 12-13km/s.  The former looks like a rapidly-decaying orbit, which you make slightly less rapidly-decaying through the application of positive lift.  The latter will pretty much just drill a chord through the atmosphere and pop out, unless you apply negative lift to bend it into the atmosphere long enough to shed the energy.

I'm still not quite willing to give up on this.  Are you saying that Starship L/D is unlikely to be able to raise itself out of the corridor back to entry interface, even if it's doing the hypersonic equivalent of a stall?  Because that'd be sort of ideal for this particular harebrained scheme.

Remember that this doesn't necessarily have to work for a crewed mission, or even for a mission with down-mass.  Both of those requirements can come later in the program, when the debris probability along the IIP has fallen due to increased proven reliability.  So the only real limitation on drag is what the ship will take structurally.  Given that the load is distributed across the entire ventral surface of the vehicle, a brief (upward) pulse of 8-10G isn't necessarily fatal, if it pops the vehicle back into an attitude where it can do a boostback.¹

This would put a lot more hot-soak load on the TPS, since it'd be doing two separate hypersonic glides (although the second one will be much slower than the first).  But it does have the nice property of keeping debris way downrange, which is the problem we're trying to solve.

___________
¹After transitioning to bellyflop, would it be possible to yaw and pitch the vehicle around to start a boostback in the high atmosphere?  That certainly satisfies the requirement of having killed the downrange velocity component, even if it increases delta-v demands somewhat.  See attached.

Your drawing really needs a distance scale. Entry interface for the Shuttle was about 5,000 km from the landing point.
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Offline catdlr

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #44 on: 12/04/2023 05:54 pm »
We don't have to speculate too much about any of this, there is precedent in Shuttle landing ground tracks.

The most likely re-entry ground tracks for early Starship flights are over the Gulf and only crossing over Florida long after the danger of breaking up during re-entry is past, like the ground track for STS-125, STS-133, STS-134, and STS-135.

https://spaceflightnow.com/shuttle/sts125/090521tracks/
https://spaceflightnow.com/shuttle/sts133/groundtrack/
https://spaceflightnow.com/shuttle/sts134/groundtracks/index2.html
https://spaceflightnow.com/shuttle/sts135/groundtrack/

For landings at Boca Chica, they would likely get permission from Mexican authorities to fly over / through their airspace for a landing. The ground track would look similar to STS-125 and STS-134 above, but the landing would be at the southern tip of Texas.

At some point, probably fairly soon after the first few successful landings, Starship will be presumed to be safe enough for CONUS overflight during reentry, just like Dragon is now.

I think it's more than just permission from Mexico for an ascending-segment EDL.  The FAA still has to issue the EDL license, and the required Dead People Expected Value computation doesn't care what the nationality of the expected dead people is.

Still just trying to get the problem clear here:  For each point in the EDL trajectory, there will be an IIP.  (Well, it's actually an instantaneous impact ellipse, but we'll use the acronyms we have.)  Associated with that point will be two values: a probability of impact (i.e., how likely debris is to fall in that spot, which will vary by flight phase), and a probability that the impact will injure or kill somebody, which is basically a function of population density at that point.

Presumably, the reason that a D2 can get an EDL license that overflies Florida is because its demonstrated reliability is so high that the probability of impact along the Florida portion of the IIP track is so low that, even when convolved with the high population density, the DPEV is still low enough.

That obviously won't be the case with Starship for a while.  My problem is that it's equally unlikely to yield an IIP track through either Mexico or the Rio Grande Valley that will pass the DPEV requirement.

Maybe this means they have to bring down a whole bunch of Starships in the Kauai test range, until the reliability brings the DPEV down to an acceptable value?



And to be clear: Dragon has overflown CONUS during reentry, not just Florida.

https://forum.nasaspaceflight.com/index.php?topic=52608.msg2419472#msg2419472
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Offline whitelancer64

We don't have to speculate too much about any of this, there is precedent in Shuttle landing ground tracks.

The most likely re-entry ground tracks for early Starship flights are over the Gulf and only crossing over Florida long after the danger of breaking up during re-entry is past, like the ground track for STS-125, STS-133, STS-134, and STS-135.

https://spaceflightnow.com/shuttle/sts125/090521tracks/
https://spaceflightnow.com/shuttle/sts133/groundtrack/
https://spaceflightnow.com/shuttle/sts134/groundtracks/index2.html
https://spaceflightnow.com/shuttle/sts135/groundtrack/

For landings at Boca Chica, they would likely get permission from Mexican authorities to fly over / through their airspace for a landing. The ground track would look similar to STS-125 and STS-134 above, but the landing would be at the southern tip of Texas.

At some point, probably fairly soon after the first few successful landings, Starship will be presumed to be safe enough for CONUS overflight during reentry, just like Dragon is now.

I think it's more than just permission from Mexico for an ascending-segment EDL.  The FAA still has to issue the EDL license, and the required Dead People Expected Value computation doesn't care what the nationality of the expected dead people is.

Still just trying to get the problem clear here:  For each point in the EDL trajectory, there will be an IIP.  (Well, it's actually an instantaneous impact ellipse, but we'll use the acronyms we have.)  Associated with that point will be two values: a probability of impact (i.e., how likely debris is to fall in that spot, which will vary by flight phase), and a probability that the impact will injure or kill somebody, which is basically a function of population density at that point.

Presumably, the reason that a D2 can get an EDL license that overflies Florida is because its demonstrated reliability is so high that the probability of impact along the Florida portion of the IIP track is so low that, even when convolved with the high population density, the DPEV is still low enough.

That obviously won't be the case with Starship for a while.  My problem is that it's equally unlikely to yield an IIP track through either Mexico or the Rio Grande Valley that will pass the DPEV requirement.

Maybe this means they have to bring down a whole bunch of Starships in the Kauai test range, until the reliability brings the DPEV down to an acceptable value?



And to be clear: Dragon has overflown CONUS during reentry, not just Florida.

https://forum.nasaspaceflight.com/index.php?topic=52608.msg2419472#msg2419472

Exactly.

Near as I can tell, Dragon, from reentry interface to landing point, is about 2,000 km.

Note that this reentry path almost directly overflies St. Louis, Nashville, and Atlanta.

https://twitter.com/thesheetztweetz/status/1580969388072529922
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Offline catdlr

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #46 on: 12/04/2023 06:11 pm »
We don't have to speculate too much about any of this, there is precedent in Shuttle landing ground tracks.

The most likely re-entry ground tracks for early Starship flights are over the Gulf and only crossing over Florida long after the danger of breaking up during re-entry is past, like the ground track for STS-125, STS-133, STS-134, and STS-135.

https://spaceflightnow.com/shuttle/sts125/090521tracks/
https://spaceflightnow.com/shuttle/sts133/groundtrack/
https://spaceflightnow.com/shuttle/sts134/groundtracks/index2.html
https://spaceflightnow.com/shuttle/sts135/groundtrack/

For landings at Boca Chica, they would likely get permission from Mexican authorities to fly over / through their airspace for a landing. The ground track would look similar to STS-125 and STS-134 above, but the landing would be at the southern tip of Texas.

At some point, probably fairly soon after the first few successful landings, Starship will be presumed to be safe enough for CONUS overflight during reentry, just like Dragon is now.

I think it's more than just permission from Mexico for an ascending-segment EDL.  The FAA still has to issue the EDL license, and the required Dead People Expected Value computation doesn't care what the nationality of the expected dead people is.

Still just trying to get the problem clear here:  For each point in the EDL trajectory, there will be an IIP.  (Well, it's actually an instantaneous impact ellipse, but we'll use the acronyms we have.)  Associated with that point will be two values: a probability of impact (i.e., how likely debris is to fall in that spot, which will vary by flight phase), and a probability that the impact will injure or kill somebody, which is basically a function of population density at that point.

Presumably, the reason that a D2 can get an EDL license that overflies Florida is because its demonstrated reliability is so high that the probability of impact along the Florida portion of the IIP track is so low that, even when convolved with the high population density, the DPEV is still low enough.

That obviously won't be the case with Starship for a while.  My problem is that it's equally unlikely to yield an IIP track through either Mexico or the Rio Grande Valley that will pass the DPEV requirement.

Maybe this means they have to bring down a whole bunch of Starships in the Kauai test range, until the reliability brings the DPEV down to an acceptable value?



And to be clear: Dragon has overflown CONUS during reentry, not just Florida.

https://forum.nasaspaceflight.com/index.php?topic=52608.msg2419472#msg2419472

Exactly.

Near as I can tell, Dragon, from reentry interface to landing point, is about 2,000 km.

Note that this reentry path almost directly overflies St. Louis, Nashville, and Atlanta.

https://twitter.com/thesheetztweetz/status/1580969388072529922

and here is a map of population density

https://eoimages.gsfc.nasa.gov/images/imagerecords/7000/7052/us_population_2005_lrg.jpg

« Last Edit: 12/04/2023 06:12 pm by catdlr »
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Offline dwharder

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #47 on: 12/04/2023 06:25 pm »
Attached is an image intended to show two consecutive ground tracks for a satellite in a 200km orbit inclined at 32°. (It was obtained from the https://spacecalcs.com/calcs/orbit-visualizer website.)
The extent to which a returning vehicle would need to yaw to make a higher inclination crossing of Mexico across the neck of land separating the Pacific from the Gulf is ... unclear. The second zoomed in image is just for fun.

You're talking about a more-or-less 90º turn here.  Think about what that means:  You have to kill all of your downrange speed, while still preserving cross-range speed that you converted from that downrange speed, and it has to be enough to traverse hundreds if not thousands of km over the Gulf of Mexico.  That's a really high L/D coefficient.

I guess you could combine modest cross-range with some boostback, similar to my overshoot scheme above.  That might allow you to cross a thinner part of Mexico, although it's hard to find one less populated than the direct route to BC--other than you need to miss Matamoros.  I've put an example below, although it seems to require about 2600m/s of delta-v, which is horrid.

Keep in mind that 40% of the total population of Mexico is in the Mexico City-Puebla region.  If you're looking to avoid population centers, a track that brings you in over/near Puebla is probably not your best bet.

Online TheRadicalModerate

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #48 on: 12/04/2023 08:34 pm »
    1)  What equation did you use? I'm not able to reproduce you 2600 m/s number. I get something much bigger.

I'm just doing a simple ballistic trajectory at 45º.  If you were to do that at entry interface, the other point of entry interface would be R = Δv²/g in the direction of the impulsive maneuver.

Quote
    2)  Why wait until so far off the coast to perform the burn? You could cut the required angle (and therefore the requires delta-v) if you start the burn right off the coast. If the idea is that you can't perform any aerodynamic maneuver over land then your map still doesn't work, because to start a turn right after reaching the coast means Starship must have been reentering over Mexico itself.

It was mostly just an arm-wave.  I agree that you'd want to do the boost-over burn close to the coast.

The big question is where you'd have enough control authority to do a fairly extreme attitude change without tumbling.  That has to be at a point where you're almost ready to go pure belly-flop.  But that will depend on ensuring that the part of the IIP track going over land has a sufficiently low probability of debris.

Quote
    3) Why 28.6°? If there's anything we know, it's that the inclination flying out of Boca Chica will not be at 28.6°, but more like 30-31°. I don't see a reason why they'd be required to support launching out of the Cape and then landing at BC.

Based on this, I think a BC launch has a pretty good chance of getting a clear IIP for a descending azimuth to 28.6º inclination.  But you need a detailed IIP track (which we can't do).  The real question is whether the IIP track can thread the needle to miss the western tip of Cuba but still go between Jamaica and the western tip of Hispaniola.

If it can't quite do it, I suspect that there's a fairly cheap dogleg that can.  If not, then I agree that 30-31º lets you thread the needle between Yucatan and the Caymans.

Seems to me that BC is a great place from which to mount a Mars campaign.  For anything else, it's kinda dicey.


Online TheRadicalModerate

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #49 on: 12/04/2023 08:50 pm »
I think in both cases, whether landing at Texas or Florida, chances of debris falling on people is low. Reentry happens mostly over the Pacific Ocean and by the time Starship reaches the part of the flight path over Mexico, the chances of it breaking up should be low. Risk is reduced as you say, by overflight of low population areas or a shorter chord of Mexico.

And to be clear: Dragon has overflown CONUS during reentry, not just Florida.

For this to be correct (and it seems plausible if not 100% convincing to me), then:

1) For the last few hundred km of the IIP track, it needs to have a very low probability of anything falling.

2) If something does fall, it's one giant hunk of debris (i.e., the whole Starship). 

If Starship were anything like the D2, I wouldn't be worrying about this.  But it's not.  It has a trajectory and flight control regime that's poorly understood, and it will either generate a lot of debris or it will hit the ground with several tonnes of methalox, which will absolutely deflagrate.

I'm starting to like the "28.6º track, south of Matamoros, then find some way to do 30-50km of cross range" idea.  If the L/D will handle that during the bellyflop, then it's easy.  If you need some boost-over (-across?), that gets more complicated, since a Raptor restart will have the usual settling questions associated with it.

Offline whitelancer64

If either Dragon or Starship fails during reentry, there is like zero chance it will fall as one piece. They would both break apart and have a several hundreds of kilometers long debris track like Columbia did.
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Online TheRadicalModerate

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #51 on: 12/05/2023 04:40 am »
If either Dragon or Starship fails during reentry, there is like zero chance it will fall as one piece. They would both break apart and have a several hundreds of kilometers long debris track like Columbia did.

At reentry, yes, lots of tiny pieces.  Mid-EDL, probably the same.  Near the point where both vehicles go into vertical descent, the odds of big pieces or even intact vehicles gets pretty high, but the debris gets more localized.

But I still don't see how that allows you to decide what parts of the EDL are safe to have their corresponding IIP tracks over populated land.  There are a lot of different attributes to consider (see attached).

We have a ridiculous amount of data on blunt body reentry vehicles.  We have almost none on Starship.  We don't even have decent aircraft analogs to inform Starship's envelope.  I'm reasonably confident that it'll be safe to bellyflop just offshore (at least for the public), but I don't think you can say that it'll be safe in the mid-supersonic-to-transsonic regime as it approaches the bellyflop.  The real question is whether the risk of debris along that part of the IIP, combined with a sparse population density, stays below the Dead People Expected Value limit.  Having looked at the map, I think there's a decent chance of an adequately low DPEV if:

1) They use an ascending segment EDL across Mexico toward BC.
2) They can have the bellyflop point be about 50km south of Starbase.
3) They can get the bellyflop to cheat over close to the chopsticks across that distance.

I don't think there's a prayer of early flights coming back to KSC, with either ascending or descending segments.  After they get some actual flight data, the debris risk of modest-mach flight over Florida may be able to be revisited.

Update:  Fixed some errors and cleaned up some language in the table.
« Last Edit: 12/05/2023 07:56 pm by TheRadicalModerate »

Offline Slarty1080

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #52 on: 12/05/2023 09:25 am »
Perhaps some early test flights over the Pacific will provide some useful data for the likely debris spread pattern from Starship.

But pity the poor Dead People Expected Value limit calculators. They will have to deal with all of this depending on the population density at each point of the flight path, considering the altitude, velocity, the ballistics of the vehicle, potential break up modes and debris scatter ellipses etc. :o

On top of that very few people indeed will be interested in the details of their calculation, unless something goes badly wrong and someone is injured or killed. Then every armchair scientist, newshound and inspector of things will have an opinion on what they did wrong (even if they did it right).
« Last Edit: 12/05/2023 09:30 am by Slarty1080 »
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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #53 on: 12/05/2023 10:01 am »
<picture of table>

a) you wrote "hypersonic" instead of "subsonic" for vertical descent  (you really want to avoid a hypersonic terminal descent for something starship sized - although if you can manage, the military might be interested ;) )
b) can you put some approximate numbers to the risk? "quite modest" is so vague, i don't even know if thats low or high ;)
« Last Edit: 12/05/2023 10:02 am by CorvusCorax »

Offline whitelancer64

If either Dragon or Starship fails during reentry, there is like zero chance it will fall as one piece. They would both break apart and have a several hundreds of kilometers long debris track like Columbia did.

At reentry, yes, lots of tiny pieces.  Mid-EDL, probably the same.  Near the point where both vehicles go into vertical descent, the odds of big pieces or even intact vehicles gets pretty high, but the debris gets more localized.

But I still don't see how that allows you to decide what parts of the EDL are safe to have their corresponding IIP tracks over populated land.  There are a lot of different attributes to consider (see attached).

We have a ridiculous amount of data on blunt body reentry vehicles.  We have almost none on Starship.  We don't even have decent aircraft analogs to inform Starship's envelope.  I'm reasonably confident that it'll be safe to bellyflop just offshore (at least for the public), but I don't think you can say that it'll be safe in the mid-supersonic-to-transsonic regime as it approaches the bellyflop.  The real question is whether the risk of debris along that part of the IIP, combined with a sparse population density, stays below the Dead People Expected Value limit.  Having looked at the map, I think there's a decent chance of an adequately low DPEV if:

1) They use an ascending segment EDL across Mexico toward BC.
2) They can have the bellyflop point be about 50km south of Starbase.
3) They can get the bellyflop to cheat over close to the chopsticks across that distance.

I don't think there's a prayer of early flights coming back to KSC, with either ascending or descending segments.  After they get some actual flight data, the debris risk of modest-mach flight over Florida may be able to be revisited.

I would put the highest risk point - reentry interface - over the Pacific Ocean.

Overflight should be done over low populated areas or a small chord of Mexico. This is essentially what the Shuttle did which is why I posted a bunch of links to examples of ground tracks that went over Mexico. The levels of risk to people on the ground from falling debris, assuming breakup at some point during reentry, should be similar for both vehicles (Starship is ~85 metric tons dry, Shuttle 75 metric tons dry). Any work done on reentry risk to people on the ground from Shuttle should be broadly applicable to Starship.

I don't think introducing dramatic maneuvers, doglegs, or engine burns at any point during reentry is a great idea. These would put a lot more stress on Starship and increase the chances that something goes wrong. Generally, the simpler way to do things is better for safety.
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Online TheRadicalModerate

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #55 on: 12/05/2023 07:54 pm »
<picture of table>

a) you wrote "hypersonic" instead of "subsonic" for vertical descent  (you really want to avoid a hypersonic terminal descent for something starship sized - although if you can manage, the military might be interested ;) )

Oops.  Fixed.


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b) can you put some approximate numbers to the risk? "quite modest" is so vague, i don't even know if thats low or high ;)

I don't think I can do better than this arm-wave.  But I do think that the relative comparisons between D2 and Starship give us a small amount of information about whether Starship can hit the DPEV threshold or not--and, if not, where they need to think about fiddling with the conops.
« Last Edit: 12/05/2023 08:07 pm by TheRadicalModerate »

Online TheRadicalModerate

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #56 on: 12/05/2023 08:05 pm »
I would put the highest risk point - reentry interface - over the Pacific Ocean.

Agree.  But where's the highest acceptable risk point?

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Overflight should be done over low populated areas or a small chord of Mexico. This is essentially what the Shuttle did which is why I posted a bunch of links to examples of ground tracks that went over Mexico. The levels of risk to people on the ground from falling debris, assuming breakup at some point during reentry, should be similar for both vehicles (Starship is ~85 metric tons dry, Shuttle 75 metric tons dry). Any work done on reentry risk to people on the ground from Shuttle should be broadly applicable to Starship.

Do you really think the Shuttle could receive an FAA EDL license today?

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I don't think introducing dramatic maneuvers, doglegs, or engine burns at any point during reentry is a great idea. These would put a lot more stress on Starship and increase the chances that something goes wrong. Generally, the simpler way to do things is better for safety.

Agree it's not a great idea.  The question is whether it's the least bad idea.

Remember that we're going to have three tiers of Starship EDL service:

1) Returning empty.  Only the vehicle is at risk.
2) Returning with down-mass.
3) Returning with crews.

But all three have to conform with risk-to-public numbers.  And only the first one of them needs to work in the near term.  Longer-term, SpaceX will likely be able put more favorable numbers into the probability-of-failure inputs to the computation, and debris from a few failures will better characterize breakups.  Both of those are inputs into the actual computation, which may easily let recoveries cross the threshold needed so that type #2 and #3 recoveries can use more simplified, more direct approaches.

Offline whitelancer64

I would put the highest risk point - reentry interface - over the Pacific Ocean.

Agree.  But where's the highest acceptable risk point?

Quote
Overflight should be done over low populated areas or a small chord of Mexico. This is essentially what the Shuttle did which is why I posted a bunch of links to examples of ground tracks that went over Mexico. The levels of risk to people on the ground from falling debris, assuming breakup at some point during reentry, should be similar for both vehicles (Starship is ~85 metric tons dry, Shuttle 75 metric tons dry). Any work done on reentry risk to people on the ground from Shuttle should be broadly applicable to Starship.

Do you really think the Shuttle could receive an FAA EDL license today?

Quote
I don't think introducing dramatic maneuvers, doglegs, or engine burns at any point during reentry is a great idea. These would put a lot more stress on Starship and increase the chances that something goes wrong. Generally, the simpler way to do things is better for safety.

Agree it's not a great idea.  The question is whether it's the least bad idea.

Remember that we're going to have three tiers of Starship EDL service:

1) Returning empty.  Only the vehicle is at risk.
2) Returning with down-mass.
3) Returning with crews.

But all three have to conform with risk-to-public numbers.  And only the first one of them needs to work in the near term.  Longer-term, SpaceX will likely be able put more favorable numbers into the probability-of-failure inputs to the computation, and debris from a few failures will better characterize breakups.  Both of those are inputs into the actual computation, which may easily let recoveries cross the threshold needed so that type #2 and #3 recoveries can use more simplified, more direct approaches.

A. I have no idea. I assume NASA has worked that out for the Shuttle / commercial crew a long time ago.

B. Why wouldn't it?

C. By the time Starship is returning down-mass and crew, it will have a well demonstrated safety record. It'll be overflying CONUS during reentry on a regular basis long before then.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline whitelancer64

I also have a more fundamental question:
Has NASA or the FAA ever required the IIP during reentry to always be at sea?

IOW, are you running in circles here working on a problem that doesn't actually exist?

Doing some google searches, there are a bunch of hits for instantaneous impact point during launch, but NONE for reentry. All the NASA-related papers I can find are entirely about launch failures and launch abort scenarios, nothing for reentry. 
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Online TheRadicalModerate

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Re: Safe Intantaneous Impact Point Tracks for Starship Recovery
« Reply #59 on: 12/05/2023 09:53 pm »
I also have a more fundamental question:
Has NASA or the FAA ever required the IIP during reentry to always be at sea?

IOW, are you running in circles here working on a problem that doesn't actually exist?

Doing some google searches, there are a bunch of hits for instantaneous impact point during launch, but NONE for reentry. All the NASA-related papers I can find are entirely about launch failures and launch abort scenarios, nothing for reentry.

Your fundamental question is perfectly legitimate, and it's pretty much why I spun up this thread.

I don't even know whether there are differences in the licensing procedure the FAA uses for NASA-operated missions vs. commercial missions.  I do know that there was a guy in East Texas with an OMS COPV sitting next to his barn after the Columbia disaster, and several turbopumps lying around in fields.  If that didn't cause some people to update their priors, even if they did so well out of the public eye, I don't know what would.

Nonetheless, they kept flying Shuttles for another 8½ years.  So either the FAA decided that the Shuttle met its criteria, or it didn't have to license Shuttle reentries.

PS:  When you say "IIP always at sea," that's not really what we're talking about.  To get really pedantic, "IIP" is a point (it's in the acronym).  What we're interested is the locus of all IIPs.  Let's call that an "IIP track".

Each IIP in the IIP track will have a different probability of debris landing on it, which will be a function of the probability of something bad happening at the corresponding point in the trajectory and the size/density of debris that could result from the bad thing.  You have to integrate across all the probabilities along the whole track, factoring in the population density at each point, to get your Dead People Expected Value.

Some parts of the IIP track can be over land, as long as they don't contribute excessively to the DPEV.  So the question is whether the scary parts go over sufficiently dense populations to blow out your DPEV.
« Last Edit: 12/06/2023 04:20 am by TheRadicalModerate »

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