SpaceX Falcon 9 / Dragon 2 : SpX-DM2 : May 30, 2020 : DISCUSSION

[Thorny]

Gulf of Mexico? I thought the primary splashdown site was the Atlantic off the eastern shore of the cape? Is this a weather related change or a misprint?

Still over ten days out. Very early for a weather call, I would think.

[Devlin98]

Current WB-57 and GV schedule has DM-2 Mission/Imaging Support on August 15-17.

[mlindner]

Will the Crew Dragon re-entry cause audible sonic booms like the Space Shuttle re-entry did? Or will it go from high in the atmosphere to subsonic too quickly for there to be audible sonic booms on the ground?

[gongora]

Via Twitter:
Eric Berger
@SciGuySpace
·
5m
NASA is currently targeting 2:35 p.m. ET (18:35 UTC) on Sunday, August 2, 2020 for the landing of the Dragonship Endeavour mission.

    *Behnken joined his fellow SpaceX Crew Dragon crewmate Doug Hurley and began packing for their return to Earth and splashdown in the Gulf of Mexico on Aug. 2.*
https://blogs.nasa.gov/spacestation/2020/07/22/crew-awaits-cargo-works-science-and-departure-following-spacewalk/

Gulf of Mexico? I thought the primary splashdown site was the Atlantic off the eastern shore of the cape? Is this a weather related change or a misprint?

They have recovery sites on both coasts of Florida and recovery ships in both places.  The Gulf side probably has more benign weather when there isn't a tropical storm going through.

[stcks]

Possibly this has already been talked about somewhere, but why is the trunk jettison before deorbit burn in the return timeline? Does this introduce a critical period where failure of propulsion results in loss of crew, stranded on orbit? Maybe failure of propulsion would always result in loc at this point? Any thoughts?

[The Vorlon]

The trunk is just that, no thrusters.  No trunk=less mass to slow down.

[Mandella]

I think what stcks is suggesting is that since the solar cells are on the trunk then once jettisoned the Dragon is now on battery, and thus has a relatively short time to get down. Any loss of propulsion would be LOC.

The only answer I have is, for all practical purposes, that would be true at any point after separation from the ISS. AFAIK there is nothing that could rescue them before life support expired, solar cells or no.

Simply put, critical systems must work at this part of the mission.

This is something I would certainly like to be wrong about.

[ShSch]

Possibly this has already been talked about somewhere, but why is the trunk jettison before deorbit burn in the return timeline? Does this introduce a critical period where failure of propulsion results in loss of crew, stranded on orbit? Maybe failure of propulsion would always result in loc at this point? Any thoughts?

Perhaps propulsion has more redundancies than the trunk jettison system and so is less likely to fail? After all, if the trunk fails to jettison after the deorbit burn, then it's most likely a LOC as well, while the crew has much less time to troubleshoot the problem.

[stcks]

Possibly this has already been talked about somewhere, but why is the trunk jettison before deorbit burn in the return timeline? Does this introduce a critical period where failure of propulsion results in loss of crew, stranded on orbit? Maybe failure of propulsion would always result in loc at this point? Any thoughts?

Perhaps propulsion has more redundancies than the trunk jettison system and so is less likely to fail? After all, if the trunk fails to jettison after the deorbit burn, then it's most likely a LOC as well, while the crew has much less time to troubleshoot the problem.

Yeah the more I think about it, the more I think any loss of prop like that is a LOC anyway, it really doesn't matter if it comes before of after trunk sep. And you're probably right regarding redundancies in prop vs trunk jettison.

[DistantTemple]

Possibly this has already been talked about somewhere, but why is the trunk jettison before deorbit burn in the return timeline? Does this introduce a critical period where failure of propulsion results in loss of crew, stranded on orbit? Maybe failure of propulsion would always result in loc at this point? Any thoughts?

Perhaps propulsion has more redundancies than the trunk jettison system and so is less likely to fail? After all, if the trunk fails to jettison after the deorbit burn, then it's most likely a LOC as well, while the crew has much less time to troubleshoot the problem.

Yeah the more I think about it, the more I think any loss of prop like that is a LOC anyway, it really doesn't matter if it comes before of after trunk sep. And you're probably right regarding redundancies in prop vs trunk jettison.

Sorry too lazy to look through previous threads, as this must have bee answered before! But I believe the D2 has loads of spare propellant, as this is unused from not aborting, and its various orbit adjustments only use a small amount. So if there is any problem prior to the de-orbit burn (like the trunk not jettisoning (but staying connected!)) they can just go back to the ISS, and they have several days (a week?) of supplies to do that in.... I think? So its not LOC. However once de-orbit has started all options are "reduced"!

[Mandella]

Possibly this has already been talked about somewhere, but why is the trunk jettison before deorbit burn in the return timeline? Does this introduce a critical period where failure of propulsion results in loss of crew, stranded on orbit? Maybe failure of propulsion would always result in loc at this point? Any thoughts?

Perhaps propulsion has more redundancies than the trunk jettison system and so is less likely to fail? After all, if the trunk fails to jettison after the deorbit burn, then it's most likely a LOC as well, while the crew has much less time to troubleshoot the problem.

Yeah the more I think about it, the more I think any loss of prop like that is a LOC anyway, it really doesn't matter if it comes before of after trunk sep. And you're probably right regarding redundancies in prop vs trunk jettison.

Sorry too lazy to look through previous threads, as this must have bee answered before! But I believe the D2 has loads of spare propellant, as this is unused from not aborting, and its various orbit adjustments only use a small amount. So if there is any problem prior to the de-orbit burn (like the trunk not jettisoning (but staying connected!)) they can just go back to the ISS, and they have several days (a week?) of supplies to do that in.... I think? So its not LOC. However once de-orbit has started all options are "reduced"!

I was just rereading and thinking I was being pretty dire for some reason. In reality the Dragon also has multiple thrusters so in case of a failure of one deorbit or possible return can be accomplished by the remaining thrusters.

[CorvusCorax]

Yeah worrying about loss of thrust is worrying about the wrong thing.

As far as I remember, all of Dragon's primary systems are triple redundant (as in any 2 of 3 can fail and it still works)

- battery/power
- avionic and computers
- propellant tanks and pressurization
- propellant feed lines
- thrusters

with additional independence between them (any power source could power any computer, any computer could control any thruster, etc...)

The way this system is designed any thinkable scenario that would cause a complete loss of thrust would be so severe - not to say catastrophic, that the resulting loss of thrust is the least of your worries.

On the other hand, a failure of getting rid of the trunk after the deorbit burn would be a very dangerous situation, as the capsule would reenter nose first instead of heatshield first.

Unlike Soyuz - which survived a failure of separation scenario multiple times when aerodynamic forces eventually broke the orbital module free of the landing module - Dragon2 with trunk attached would likely remain in nose forward orientation long enough for a complete burn-through and resulting LOC.

So - with respect to Soyuz TMA-10 and TMA-11 (Edit: And Soyuz 5 ) you definitely want to get rid of the trunk first, before initiating re-entry.

[erv]

On the other hand, a failure of getting rid of the trunk after the deorbit burn would be a very dangerous situation, as the capsule would reenter nose first instead of heatshield first.

Unlike Soyuz - which survived a failure of separation scenario multiple times when aerodynamic forces eventually broke the orbital module free of the landing module - Dragon2 with trunk attached would likely remain in nose forward orientation long enough for a complete burn-through and resulting LOC.

So - with respect to Soyuz TMA-10 and TMA-11 (Edit: And Soyuz 5 ) you definitely want to get rid of the trunk first, before initiating re-entry.

Why nose first? (assuming the RCS is working)
In the not getting rid if the trunk scenario I would be worried about disintegrating trunk damaging the heatshield or the capsule itself.

[CorvusCorax]

Why nose first? (assuming the RCS is working)
In the not getting rid if the trunk scenario I would be worried about disintegrating trunk damaging the heatshield or the capsule itself.

Same reason as with Soyuz. The aerodynamic forces during reentry are an order or two of magnitude stronger than anything the RCS is capable of. Even if it starts to re-enter tail-first - with the trunk attached the capsule would re-orient nose-first at atmospheric interface.

It's deliberately designed that way, since this is the launch-abort orientation. The trunk even has little fins for extra stability. It's basically like a shuttlecock and in the presence of any airstream will orient dragon in a stable, nose-first attitude.

Which, unfortunately, for re-entry at orbital speeds is absolutely deadly.

The Soyuz orbital module is attached to the landing module with struts - which are to be severed by explosive bolts prior to re-entry. The way they are presented to the airstream without insulation, the struts have shown to melt through before the forward facing crew hatch does.

In contrast, Dragon has a much smoother outer mold-line and capsule-trunk transition, since unlike Soyuz it doesn't have a launch shroud/fairing. Based on it's aerodynamic shape, and the likely formation of a shock front prior to the clamp that attaches the trunk, which is itself covered in thermal protection for ascent, I'd assume it as a given that the nosecone and forward hatch would melt through long before the clamp is affected - although to be certain you'd have to put the shape through some realistic re-entry sim.

Maybe the Superdraco's would have enough force, but certainly not enough propellant to keep Dragon tail-first against the airstream during all of reentry, RCS has absolutely no chance.

[meekGee]

Why nose first? (assuming the RCS is working)
In the not getting rid if the trunk scenario I would be worried about disintegrating trunk damaging the heatshield or the capsule itself.

Same reason as with Soyuz. The aerodynamic forces during reentry are an order or two of magnitude stronger than anything the RCS is capable of. Even if it starts to re-enter tail-first - with the trunk attached the capsule would re-orient nose-first at atmospheric interface.

It's deliberately designed that way, since this is the launch-abort orientation. The trunk even has little fins for extra stability. It's basically like a shuttlecock and in the presence of any airstream will orient dragon in a stable, nose-first attitude.

Which, unfortunately, for re-entry at orbital speeds is absolutely deadly.

The Soyuz orbital module is attached to the landing module with struts - which are to be severed by explosive bolts prior to re-entry. The way they are presented to the airstream without insulation, the struts have shown to melt through before the forward facing crew hatch does.

In contrast, Dragon has a much smoother outer mold-line and capsule-trunk transition, since unlike Soyuz it doesn't have a launch shroud/fairing. Based on it's aerodynamic shape, and the likely formation of a shock front prior to the clamp that attaches the trunk, which is itself covered in thermal protection for ascent, I'd assume it as a given that the nosecone and forward hatch would melt through long before the clamp is affected - although to be certain you'd have to put the shape through some realistic re-entry sim.

Maybe the Superdraco's would have enough force, but certainly not enough propellant to keep Dragon tail-first against the airstream during all of reentry, RCS has absolutely no chance.

On the flip side, both struts (Soyuz) and trunk wall (Dragon) support a similar capsule on ascent, the struts will be more massive than the distributed wall, so the wall has more surface area per cross section ("thinner") and potentially fails earlier.


Also, unlike struts, one hot spot and punch through and the entire wall peels off..


And, for a grand finale, isn't the trunk composite? I'm guessing the struts are metal?

As you say though - "simulation or it didn't happen"


 

[CorvusCorax]

On the flip side, both struts (Soyuz) and trunk wall (Dragon) support a similar capsule on ascent, the struts will be more massive than the distributed wall, so the wall has more surface area per cross section ("thinner") and potentially fails earlier.


Also, unlike struts, one hot spot and punch through and the entire wall peels off..


And, for a grand finale, isn't the trunk composite? I'm guessing the struts are metal?

As you say though - "simulation or it didn't happen"

1. I don't think those struts are solid, they'd be hollow pipes (more strength for weight, Spacecraft are typically weight optimized)

2. The struts have much more narrow curvature than a relatively flat trunk wall. Peak heating is defined by curvature - the more pointy and narrow something is, the hotter it gets. That's why reentry vehicles have a blunt front.

3. The struts - according to wikipedia - are said to have no thermal insulation, while the trunk airshell afaik has SPAM - That being said, there's some conflicting data. This diagram shows the struts between heatshield and service module completely exposed, while the picture on the same page shows thermal foil insulation around them. Of course the foil might not really last long with hollow space behind when exposed to airstream, so they might be exposed really quickly

4. The trunk is composite, but carbon composite typically lasts significantly longer than aluminum or other similar low melt point light metals, even if the resin starts to ablate and decompose. Now if those struts were titanium or stainless steel ...

Image source: https://en.wikipedia.org/wiki/Soyuz_(spacecraft)

[meekGee]

On the flip side, both struts (Soyuz) and trunk wall (Dragon) support a similar capsule on ascent, the struts will be more massive than the distributed wall, so the wall has more surface area per cross section ("thinner") and potentially fails earlier.


Also, unlike struts, one hot spot and punch through and the entire wall peels off..


And, for a grand finale, isn't the trunk composite? I'm guessing the struts are metal?

As you say though - "simulation or it didn't happen"

1. I don't think those struts are solid, they'd be hollow pipes (more strength for weight, Spacecraft are typically weight optimized)

2. The struts have much more narrow curvature than a relatively flat trunk wall. Peak heating is defined by curvature - the more pointy and narrow something is, the hotter it gets. That's why reentry vehicles have a blunt front.

3. The struts - according to wikipedia - are said to have no thermal insulation, while the trunk airshell afaik has SPAM - That being said, there's some conflicting data. This diagram shows the struts between heatshield and service module completely exposed, while the picture on the same page shows thermal foil insulation around them. Of course the foil might not really last long with hollow space behind when exposed to airstream, so they might be exposed really quickly

4. The trunk is composite, but carbon composite typically lasts significantly longer than aluminum or other similar low melt point light metals, even if the resin starts to ablate and decompose. Now if those struts were titanium or stainless steel ...

Image source: https://en.wikipedia.org/wiki/Soyuz_(spacecraft)

1 Still though
2 Curvature in general or the component in the direction of flow?
3 I have a sense that ascent requirements will make the two system perform similarly at the end - just enough insulation to hold up, but no more, for exactly the reentry scenario
4 A true race condition..

But to the original point, always better to separate before commit from a reentry standpoint, but also commit before separate from a TTL viewpoint in case the thruster don't fire..  Either situation should be exceedingly rare, but I'd trust the thrusters more since they can be tested just before separation..

[CorvusCorax]

2 Curvature in general or the component in the direction of flow?
3 I have a sense that ascent requirements will make the two system perform similarly at the end - just enough insulation to hold up, but no more, for exactly the reentry scenario

2. Curvature of the component facing the flow. A flat surface hitting the flow perpendicularly ( which at low speeds would be the aerodynamic worst case ) creates a detached shock wave in front of the object. This minimizes heating. (It makes a lot of hot plasma in front of and around the vehicle, but the surface itself stays much cooler) while maximizing drag.

A pointy tip is the other extreme, it would pierce through the air and form a closely attached shockfront that "hugs" the material. While this would be best at Mach 1.5 or Mach 2 since there's less drag, the surface heating is maximized.

This is why reentry-vehicles and heat-shields are blunt. You want to maximize drag and minimize heating.
This is why the SR71 is pointy. You want to minimize drag - and the titanium can still take it at Mach 3.3  (steady state but with active cooling using the fuel) (or for short times even Mach 6.7 - on the X 15 ) But not at Mach 20!

3. No, you got that completely wrong:

Dragon - and it's trunk - are designed to withstand supersonic flight and heating during ascent. It has a capsule integrated launch escape system and it's entire surface has a mechanically sturdy somewhat heat resistant coating.

The Soyuz spacecraft on the other hand is completely encapsulated in a launch shroud - which is integrated with the launch escape tower (complete with grid fins for stabilization) and jetisoned late during ascent. The spacecraft itself - safely protected within - is not designed - except for its landing module - to encounter or withstand any aerodynamic forces. The insulation it has is only designed to deal with in-orbit radiative heating -- metal-foil plus fluffy stuff, like you would have on a satellite.  You can see that nicely in that famous spacewalk where a Soyuz orbital module was gutted to look at a hole - spewing fluffy foam and foil insulation everywhere.

If you tried that with a dragon trunk, you'd ruin your knife/scissors and probably barely scratch it.

[Steven Pietrobon]

After trunk separation, if the main deorbit thrusters failed to work (the forward facing thrusters around the hatch), other thrusters could be used or possibly Dragon 2 makes an emergency return to ISS. If the attitude control thrusters also fail, a Starliner or another Dragon 2 present on ISS could try and mount a rescue mission, but it will be tricky without attitude control.

[Jorge]

After trunk separation, if the main deorbit thrusters failed to work (the forward facing thrusters around the hatch), other thrusters could be used or possibly Dragon 2 makes an emergency return to ISS. If the attitude control thrusters also fail, a Starliner or another Dragon 2 present on ISS could try and mount a rescue mission, but it will be tricky without attitude control.

Rescue is not possible. The SpaceX docking system and the NDS Block 1 used by Starliner are not fully androgynous; Dragon/Starliner can dock to ISS but not to each other.