Space Debris (from Dragon trunk) lands near Dalgety, NSW July 2022

[gongora]

When discussing the dangers of space debris, it's a bit offtopic to start talking about the risks of completely separate activities such as traffic and refrigerators.  Please try to stay on topic.  Small trim made.

[pierre]

The Dragon trunk is hardly a 'low-density 30kg debris' (a figure you've just pulled from nowhere in a transparent attempt to create a strawman).

It's completely fine to ask for sources if you don't believe what I write, but please don't accuse me of commenting in bad faith.

It was widely reported in local Australian news that indeed the biggest chunk of the Dragon trunk that reached the ground was: "about three metres long, about 30kg".

For something bigger than a person to be so lightweight, it's clearly low-density. Definitely not a solid block of metal.

This might also explain why it survived reentry: it may have slowed down more than most pieces of debris in the upper atmosphere, and passed through the denser atmosphere at slower speeds, with lower heating.

Source for the size and mass, one of several that reported this: https://www.illawarramercury.com.au/story/7840773/the-biggest-piece-of-space-junk-to-hit-australia-in-40-years-might-not-be-the-last/

[edzieba]

The Dragon trunk is hardly a 'low-density 30kg debris' (a figure you've just pulled from nowhere in a transparent attempt to create a strawman).

It's completely fine to ask for sources if you don't believe what I write, but please don't accuse me of commenting in bad faith.

It was widely reported in local Australian news that indeed the biggest chunk of the Dragon trunk that reached the ground was: "about three metres long, about 30kg".

For something bigger than a person to be so lightweight, it's clearly low-density. Definitely not a solid block of metal.

This might also explain why it survived reentry: it may have slowed down more than most pieces of debris in the upper atmosphere, and passed through the denser atmosphere at slower speeds, with lower heating.

Source for the size and mass, one of several that reported this: https://www.illawarramercury.com.au/story/7840773/the-biggest-piece-of-space-junk-to-hit-australia-in-40-years-might-not-be-the-last/

The Dragon trunk was not disassembled prior to disposal, it entered as a single ~3000kg (plus any disposal cargo) object. And we have just seen a direct demonstration that it is not fully demisable. Nor were the resultant fragments harmless - any object that can embed itself into soil for 20-30cm of its length when falling from height will embed a significant distances into flesh too - and any 30kg object dropped from even non-orbital heights will happily injure or kill.

[sdsds]

Since the trunk is expended with each mission, can SpX redesign this particular component to make it more fragile, i.e. more likely to disintegrate into smaller chunks during reentry?

[randomly]

It's more likely that Congress will award Boeing a cost plus contract to redesign the Dragon trunk.

[sdsds]

It's more likely that Congress will award Boeing a cost plus contract to redesign the Dragon trunk.

Ha ha.

Put more pointedly, is it now incumbent on the FAA Office of Commercial Space Transportation to require that SpX implement some mitigation?

[edzieba]

Since the trunk is expended with each mission, can SpX redesign this particular component to make it more fragile, i.e. more likely to disintegrate into smaller chunks during reentry?

There are plenty of options:
1) Improve demisability (works for the trunk, disposed cargo inside the trunk may still survive depending on its demisability)
2) Improve targeting of the trunk prior to separation from Dragon.
3) Add propulsion to the trunk to target deorbit after separation (e.g. transplant the fairing RCS)
4) Delay trunk separation until after the deorbit burn, or split the deorbit burn into two halves with separation occurring during a midway coast phase.

4) is likely not viable, as it adds too much direct risk to the crew inside Dragon (entry with the trunk attached is not survivable).
3) is expensive and adds both additional systems and additional testing, as well as additional crew risk (inadvertent activation whilst attached).
1) requires modification to the trunk design. May be viable as part of a block upgrade (and new trunks continue to roll off the line, unlike capsules), and solves the problem - apart from disposed cargo - even if any deorbit targeting fails.
2) is the lowest cost and lowest risk (to Dragon crew) option which also solves for any disposal cargo inside the trunk. It's also what they're currently doing, and what clearly did not work in this recent case, so first they'd ned to figure out why their entry targeting was off before they can solve the problem. May require a few flights with active tracking and monitoring of the trunk to directly measure behaviour through decay and entry. May result in a "entry does not behave like we thought it did" finding as was uncovered around overoptimistic parachute modelling, unexpected Titanium NTO ignition, unobserved solid oxygen behaviour, etc.

[Bananas_on_Mars]

AFAIK only crewed flights separate the trunk prior to de-orbit burn.

So the total numbers are small, crewed flights don‘t carry cargo in the trunk for disposal.

It‘s quite possible they won‘t do anything about it for the remaining 2 handful of crewed flights?

[su27k]

2) is the lowest cost and lowest risk (to Dragon crew) option which also solves for any disposal cargo inside the trunk. It's also what they're currently doing, and what clearly did not work in this recent case, so first they'd ned to figure out why their entry targeting was off before they can solve the problem.

Not sure what you mean by this. There is no targeted reentry for trunks left in orbit. To do a targeted reentry you need to make a deorbit burn, trunk has no propulsion, and capsule only does the burn after separation from the trunk. QED, trunk is left in a low orbit and reentry location will be random, it's no different from what they do for F9 S2 stages used in GTO missions BTW.

[DanClemmensen]

2) is the lowest cost and lowest risk (to Dragon crew) option which also solves for any disposal cargo inside the trunk. It's also what they're currently doing, and what clearly did not work in this recent case, so first they'd ned to figure out why their entry targeting was off before they can solve the problem.

Not sure what you mean by this. There is no targeted reentry for trunks left in orbit. To do a targeted reentry you need to make a deorbit burn, trunk has no propulsion, and capsule only does the burn after separation from the trunk. QED, trunk is left in a low orbit and reentry location will be random, it's no different from what they do for F9 S2 stages used in GTO missions BTW.

This is clearly not precisely correct. The capsule+trunk separate from the ISS, so they end up together in some specific orbit. The spacecraft may or may not perform additional maneuvers prior to separation, and the this will leave the trunk is some specific orbit after separation. That orbit will result in the trunk deorbiting. The question is therefore what maneuvers are performed prior to separation, and are they calculated for a particular planned trunk re-entry?

[Zed_Noir]

Since the trunk is expended with each mission, can SpX redesign this particular component to make it more fragile, i.e. more likely to disintegrate into smaller chunks during reentry?

There are plenty of options:
<snip>
3) Add propulsion to the trunk to target deorbit after separation (e.g. transplant the fairing RCS)
<snip>

<snip>
3) is expensive and adds both additional systems and additional testing, as well as additional crew risk (inadvertent activation whilst attached).
<snip>

Adding very low impulse propulsion to the Dragon trunk isn't really that complicated. Just transplant the ion thrusters from the Starlink satcoms to take advantage of the solar arrays on the trunk. Two or three ion thrusters should be enough to de-orbit the trunk relatively quickly.

[DanClemmensen]

Since the trunk is expended with each mission, can SpX redesign this particular component to make it more fragile, i.e. more likely to disintegrate into smaller chunks during reentry?

There are plenty of options:
<snip>
3) Add propulsion to the trunk to target deorbit after separation (e.g. transplant the fairing RCS)
<snip>

<snip>
3) is expensive and adds both additional systems and additional testing, as well as additional crew risk (inadvertent activation whilst attached).
<snip>

Adding very low impulse propulsion to the Dragon trunk isn't really that complicated. Just transplant the ion thrusters from the Starlink satcoms to take advantage of the solar arrays on the trunk. Two or three ion thrusters should be enough to de-orbit the trunk relatively quickly.

It's not about the elapsed time. It's about the accuracy of hitting the proper dump zone. I would think that a higher-power shorter-duration maneuver is preferred, either by having the spacecraft place the trunk into the proper trajectory before separation or by putting a larger short-duration thruster in the trunk.

[TheRadicalModerate]

Since the trunk is expended with each mission, can SpX redesign this particular component to make it more fragile, i.e. more likely to disintegrate into smaller chunks during reentry?

There are plenty of options:
1) Improve demisability (works for the trunk, disposed cargo inside the trunk may still survive depending on its demisability)
2) Improve targeting of the trunk prior to separation from Dragon.
3) Add propulsion to the trunk to target deorbit after separation (e.g. transplant the fairing RCS)
4) Delay trunk separation until after the deorbit burn, or split the deorbit burn into two halves with separation occurring during a midway coast phase.

4) is likely not viable, as it adds too much direct risk to the crew inside Dragon (entry with the trunk attached is not survivable).
3) is expensive and adds both additional systems and additional testing, as well as additional crew risk (inadvertent activation whilst attached).
1) requires modification to the trunk design. May be viable as part of a block upgrade (and new trunks continue to roll off the line, unlike capsules), and solves the problem - apart from disposed cargo - even if any deorbit targeting fails.
2) is the lowest cost and lowest risk (to Dragon crew) option which also solves for any disposal cargo inside the trunk. It's also what they're currently doing, and what clearly did not work in this recent case, so first they'd ned to figure out why their entry targeting was off before they can solve the problem. May require a few flights with active tracking and monitoring of the trunk to directly measure behaviour through decay and entry. May result in a "entry does not behave like we thought it did" finding as was uncovered around overoptimistic parachute modelling, unexpected Titanium NTO ignition, unobserved solid oxygen behaviour, etc.

I think #4 may be viable, as long as there's sufficient delta-v in the D2.  Here's a nominal conops, with an off-nominal recovery branch:

a) D2 does deorbit burn for trunk.
b) Trunk is jettisoned into an engineered demise.
c) If (trunk jettison succeeds)
cS1) Make deorbit shallower, so D2 enters farther downrange.
cS2) Finish EDL.
else (trunk jettison fails)
cF1) Boost back into a clean orbit.
cF2) Debug trunk jettison until it works.
cF3) Deorbit again.
cF4) Finish EDL.

This requires enough extra prop to boost back to LEO, and then deorbit a second time.  I get the impression that D2 has way, way more delta-v than it needs, because it has a bunch of prop on board in case they need to do a SuperDraco-based escape.

One key issue here is if you need to dump the trunk in the Eastern Pacific but still need to put the D2 down in the Eastern Gulf of Mexico.  That will require reshaping the deorbit by a fair amount.

Note that if you follow the jettison failure branch, you wind up with a non-engineered entry for the trunk, which is what we're trying to avoid.  However, that will only be true if there's a problem with the trunk jettison, which should be very rare.  Bottom line, you have an engineered trunk demise about 99.9% of the time, and current harm to public probability the other 0.1% of the time.

[sdsds]

There are plenty of options:
[... cleverness ...]

I get the impression that D2 has way, way more delta-v than it needs, because it has a bunch of prop on board in case they need to do a SuperDraco-based escape.

If this were true there might/should be visible evidence of it. Propellant dumps by other spacecraft get observed from the ground. Have there been observations like that on Dragon missions?

[DaveS]

There are plenty of options:
[... cleverness ...]

I get the impression that D2 has way, way more delta-v than it needs, because it has a bunch of prop on board in case they need to do a SuperDraco-based escape.

If this were true there might/should be visible evidence of it. Propellant dumps by other spacecraft get observed from the ground. Have there been observations like that on Dragon missions?

No for the simple reason that that Dragon doesn't do prop dumps. It lands with whatever remains at EOM, just like Gemini and Shuttle. The Apollo CM did dump its RCS propellant after main chute deploy (something that caused one of the three chutes to fail on Apollo 15 and caused the crew of ASTP to briefly lose consciousness when an air valve was mistakenly left open and NTO fumes leaked into the cabin). The Soyuz DM does the same, dump its H2O2 overboard after main chute deploy.

And as far as SM/trunk debris are concerned, Soyuz does its "tri-module" separation after the de-orbit burn and there have been no debris found ever of the Soyuz SM or the Orbital/Habitation Module even though both follow the same overland trajectory as the DM.

[edzieba]

And as far as SM/trunk debris are concerned, Soyuz does its "tri-module" separation after the de-orbit burn and there have been no debris found ever of the Soyuz SM or the Orbital/Habitation Module even though both follow the same overland trajectory as the DM.

Soyuz also has the feature (with some debate as to whether by design or by happenstance) that in the event of a separation failure, the Soyuz capsule will survive for sufficient time in a nose-forward orientation for entry heating to break the truss between the descent module and instrument module - and on at least two occasions this has occurred in practice.
This is not the case for Dragon: the forward TPS will fail before the trunk attachment mechanism. Adding additional TPS to Dragon's backshell and/or modifying the trunk attachment mechanism to be more vulnerable to entry heating are fairly major design changes to a craft that is for the most part 'complete' and has ended production.

[TheRadicalModerate]

I was just going over an old Crew mission, and they close the nose cone after the deorbit burn--which makes sense, since they lose the nose thrusters once it's closed.  But that implies that there must be enough prop left to stabilize the orbit and try again if there's a problem with the nose cone, since leaving it open during EDL is probably an LOC event.

That said, F9 inserts Dragon into 190x210x51.6, and the ISS is at 413x422x51.6, so the boost burn has to be 126m/s.  Not sure of the sequence of orbit-lowering and phasing maneuvers prior to EDL, but it has to be at least 100m/s.  SWAG in phasing and co-elliptic burns for another... 100m/s?  Then maybe 20m/s in the keep-out zone.  Total:  about 350m/s.

With a 12t liftoff mass with 300s Isp for Dracos, 350m/s requires 1350kg of MMH/NTO.  That's very close to the 1390kg that was in the EIS for the SuperDraco testing.  I assume that the Dracos and SuperDracos share propellant COPVs, so they're not as prop-rich as I thought they'd be.

If this back-of-napkin is close, then it probably rules out dropping the trunk into an engineered disposal orbit.  Stabilizing in case of a trunk jettison contingency might leave you high (but not high enough) and dry.

One better-than-nothing possibility would be to assess prop prior to trunk disposal.  If things have gone well and there's enough prop to engineer the disposal safely, then they do it.  If not, the trunk does the current uncontrolled demise and Australian sheep sheds are on their own.

[Nomadd]

 Put heat shields and chutes on the trunks so people can make fun and useful stuff out of them when they land.

[EspenU]

I was just going over an old Crew mission, and they close the nose cone after the deorbit burn--which makes sense, since they lose the nose thrusters once it's closed.  But that implies that there must be enough prop left to stabilize the orbit and try again if there's a problem with the nose cone, since leaving it open during EDL is probably an LOC event.

If I recall correctly, the crew can completely jettison the nose cone if it doesn't close correctly. This would probably damage thrusters and the docking mechanism, but the crew would be fine.

[TheRadicalModerate]

I was just going over an old Crew mission, and they close the nose cone after the deorbit burn--which makes sense, since they lose the nose thrusters once it's closed.  But that implies that there must be enough prop left to stabilize the orbit and try again if there's a problem with the nose cone, since leaving it open during EDL is probably an LOC event.

If I recall correctly, the crew can completely jettison the nose cone if it doesn't close correctly. This would probably damage thrusters and the docking mechanism, but the crew would be fine.

Is the backup nose cone jettison pyro-assisted?  If so, I wonder if they could go to backup pyros for the trunk jettison.  That'd probably be a pretty simple modification to enable a safe-disposal conops.