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36 minutes ago, CatastrophicFailure said:

Always seemed odd to me that they ditch the trunk before deorbiting. :/

Seems like part way would be better. They need to make sure it’s well away from dragon, also can’t impact near recovery.

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49 minutes ago, tater said:

Seems like part way would be better. They need to make sure it’s well away from dragon, also can’t impact near recovery.

Given how much quicker it would decelerate, separation doesn't seem like it should be a concern if they ditch it normal/antinormal just after the deorbit burn, like Soyuz or any other capsule. I've heard they need the extra fuel margin, which in turn makes me wonder why they can't just use the SuperDrakes and their larger supply. 

I'm sure they actually have perfectly good reasons for doing things this way, I'm just curious what they are. :/

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Just now, CatastrophicFailure said:

I've heard they need the extra fuel margin, which in turn makes me wonder why they can't just use the SuperDrakes and their larger supply. 

IIRC Dracos and SuperDracos use the same supply. But I suppose I could be mistaken…

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10 hours ago, StrandedonEarth said:
10 hours ago, CatastrophicFailure said:

I've heard they need the extra fuel margin, which in turn makes me wonder why they can't just use the SuperDrakes and their larger supply. 

IIRC Dracos and SuperDracos use the same supply. But I suppose I could be mistaken…

You're correct, the Dracos and SuperDracos draw from the same tanks. That's the whole point of the system.

It's surprising to me that they don't have enough margin to deorbit the trunk, though.

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I think the trunk is *supposed* to burn up completely, which would explain why they haven't been concerned about uncontrolled deorbiting.

The fact it survived ought to be concerning.

It's probably not so much not having the margin, to why use margin on the bit that won't survive and save the margin for the bit containing crew?

Edited by RCgothic
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8 hours ago, sevenperforce said:

It's surprising to me that they don't have enough margin to deorbit the trunk, though.

I don't believe this is the case.  I believe it's more that they don't want to burn up margin that they don't have to.

Remember, these things have offset CoMs and don't have magic unrealistically powerful gyros for control on re-entry.  They use RCS for navigation when re-entering.  If something goes amiss during that phase, you want as much fuel as possible for corrections.

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3 hours ago, Geonovast said:

I don't believe this is the case.  I believe it's more that they don't want to burn up margin that they don't have to.

Remember, these things have offset CoMs and don't have magic unrealistically powerful gyros for control on re-entry.  They use RCS for navigation when re-entering.  If something goes amiss during that phase, you want as much fuel as possible for corrections.

Okay, yeah, that makes sense. If you had uncharacteristically asymmetric erosion on the heat shield then two continuously-firing Draco thrusters could mean the difference between a safe splashdown with lessons learned and a LOCV tumble.

2 hours ago, StrandedonEarth said:

I wonder how much of the Draco s fuel they use up with rendezvous and other maneuvering. As in, what percentage of their fuel do they usually have left before the deorbit burn?

Crew Dragon has something on the order of 400-500 m/s of dV. It uses 100 m/s to deorbit and so we can assume about 200 m/s for the phasing and orbit raising burns to match the ISS.

So probably half remains before the entry burn.

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Also, with a reusable spacecraft, I would expect that they don't want to spend more time than needed burning the retro thrusters, since that decreases their operational lifetime.

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On 7/30/2022 at 11:25 AM, RCgothic said:

I think the trunk is *supposed* to burn up completely, which would explain why they haven't been concerned about uncontrolled deorbiting.

The fact it survived ought to be concerning.

It's probably not so much not having the margin, to why use margin on the bit that won't survive and save the margin for the bit containing crew?

But they know how much fuel they have left and how much they will need to deorbit, add margins and you know how much you have left. 
Think its more that they would have to do to deorbit burns. One to deorbit the trunk then an second burn to avoid the trunk. 

A bit like in KSP you want to point north or south then  dropping service module after deorbit burn dropping it ahead of you is dangerous as its likely to have higher drag/ mass than your capsule so it will move towards you. 

Now I don't think this is much an issue with an dragon 2 capsule as I assume its designed to use gps.
Dragon is also a bit unique in that the capsule contains the engine part. The trunk is just for storage, power and cooling, (yes its also needed for an abort in the atmosphere)
All other capsules I know about need the service module to deorbit, Soyuz has some thrusters for control after separation and it also drop the orbital module, Soyuz also land on land so I assume the orbital module also comes down over land as Russia is hard to miss :) 
For the rest I assume the service module move out of the way after separating. 

 

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1 hour ago, magnemoe said:

For the rest I assume the service module move out of the way after separating. 

They probably have some sep motors (or similar).  With a suitable orientation at separation, even 2 m/s will get them a fair amount of spacing after 5 mins.  

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32 minutes ago, magnemoe said:

All other capsules I know about need the service module to deorbit, Soyuz has some thrusters for control after separation and it also drop the orbital module, Soyuz also land on land so I assume the orbital module also comes down over land as Russia is hard to miss

During the initial set of operational flights, Soyuz jettisoned the orbital module first, before the deorbit burn, and allowed its orbit to decay naturally because it would burn up entirely in the atmosphere. This was primarily to reduce propellant consumption in the service module during the deorbit burn, reserving more propellant for on-orbit activities. However, in 1988 there was a computer glitch which delayed the re-entry burn by 24 hours and so the cosmonauts were stuck without the orbital module facilities, which was very unpleasant. So since then they have jettisoned the orbital module post-burn.

It should be noted that the control thrusters on the Soyuz descend module are simple peroxide monoprop and are not connected to the service module. So the service module can always burn to depletion without impacting the descent propellant reserves, in contrast to Crew Dragon where the same tanks feed the thrusters used for re-entry RCS.

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Just now, Beccab said:

 

Notice the fence. How many know why its designed this way?

3 hours ago, sevenperforce said:

During the initial set of operational flights, Soyuz jettisoned the orbital module first, before the deorbit burn, and allowed its orbit to decay naturally because it would burn up entirely in the atmosphere. This was primarily to reduce propellant consumption in the service module during the deorbit burn, reserving more propellant for on-orbit activities. However, in 1988 there was a computer glitch which delayed the re-entry burn by 24 hours and so the cosmonauts were stuck without the orbital module facilities, which was very unpleasant. So since then they have jettisoned the orbital module post-burn.

It should be noted that the control thrusters on the Soyuz descend module are simple peroxide monoprop and are not connected to the service module. So the service module can always burn to depletion without impacting the descent propellant reserves, in contrast to Crew Dragon where the same tanks feed the thrusters used for re-entry RCS.

Yes getting stuck in the cramped return module for an day with no toilet does not sound fun. And you jettison it because you need the margin but once you done your deorbit burn you are committed. 

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On 7/30/2022 at 2:25 AM, RCgothic said:

I think the trunk is *supposed* to burn up completely, which would explain why they haven't been concerned about uncontrolled deorbiting.

LOTS of things fail to "burn completely". I can't think of a spacecraft that has ever come down from orbit where we know everything "burned up on re-entry".

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2 hours ago, mikegarrison said:

LOTS of things fail to "burn completely". I can't think of a spacecraft that has ever come down from orbit where we know everything "burned up on re-entry".

Soyuz? Hundreds of its descent and orbital stages have been burning in the atmosphere in  the last five decades

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2 hours ago, Beccab said:

Soyuz? Hundreds of its descent and orbital stages have been burning in the atmosphere in  the last five decades

Every one of them could have hit the ground and who would know? It's not like they are coming in over Paris or London or New York. That's some seriously empty land they re-enter into.

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5 hours ago, mikegarrison said:

Every one of them could have hit the ground and who would know? It's not like they are coming in over Paris or London or New York. That's some seriously empty land they re-enter into.

It does occasionally get visited by a group of very observant people riding trucks and helicopters.

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