SpaceX - US Deorbit Vehicle (for ISS)

[FutureSpaceTourist]

This thread is for updates and discussion of the modified Dragon chosen by NASA for de-orbiting the ISS.

There is a separate thread for NASA’s procurement of the vehicle: https://forum.nasaspaceflight.com/index.php?topic=56996.0



https://twitter.com/sciguyspace/status/1806762777333076064

Bill Spetch, operations integration manager for NASA’s International Space Station Program, confirms that the US Deorbit Vehicle will be based on "Dragon heritage" hardware. It will involve modifications of the trunk.

[Nomadd]

 If they're going to need 7-8 tons of fuel, would it be a Heavy launch?

[Remes]

At 50:00:
Eric Berger: What is the architecture for usdv, will it be a modified dragon, a dragon xl, or something else?
Bill Spetch: that's based on dragon heritage design. They have do some modifications and some changes to the trunk.

• Bill Spetch, operations integration manager, NASA’s International Space Station Program

[Jim]

If they're going to need 7-8 tons of fuel, would it be a Heavy launch?

Closer to 15

[matthewkantar]

Could it be that it will take more than one launch? 

[vaporcobra]

If they're going to need 7-8 tons of fuel, would it be a Heavy launch?

Closer to 15

Welp, definitely a Falcon Heavy mission or maybe a fully expendable F9 if it needs ~15 tons of fuel and the usual ~9 tons of dry mass!

And if SpaceX plans on daisy-chaining existing Dragon 2 propellant COPVs to add that capacity, 15 tons would imply ~30 extra COPVs in the trunk in addition to the capsule's usual 8. Plenty doable with the roughly 4m x 3.5m available.

[FutureSpaceTourist]

It’s not very SpaceX-like to do a one-off project that doesn’t also support/contribute to other missions or tech development goals.

So was this modified Dragon also part of the Polaris program’s study for servicing Hubble? Maybe also with an eye on commercial space stations in development and/or propellant depots?

Or is SpaceX just doing the ISS de-orbit vehicle to support their biggest customer?

Edit to add:

SpaceX first approached NASA about possible Hubble mission in Spring/Summer 2022 and the ISS de-orbit RFI was issued in August 2022.

[Remes]

Polaris is a manned mission. I believe the usdv will be unmanned and maximize fuel/thruster/redundancy payload. More like Soyuz vs Progress.

In addition to one off and no future use: I believe the plan of SpaceX was to replace F9 et al with SS/SH. Albeit I believe having a smaller vehicle would make sense, because a coach with only 5 passengers is not lucrative. But then again having 2 launch systems... So SpaceX will have to deal with Dragon longer and then even in a special variant.

Other reasons to do it:
- It is good money.
- It is high profile PR.
- The project has to start now. There is only one provider with a proven and available vehicle which comes close to be doing it. So if you are the only one capable, if it has to be done, and you let down your nation's space program, then a whole bunch of people (at NASA and politics) will never forget that.

[Remes]

The ISS has 450t and the "SSP 51101 USDV SRD" requires 57m/s.

p = 57 m/s * 450000kg = 25.65M kg*m/s

Taking the Draco Isp with 2300m/s that is a required propellant mass

m = t*F/Isp = 11140kg

Super Draco has 73kN Thrust. Requirement is a minimum of 3236N (Soyuz/Progress have 2.95kN). Also I don't know, if a Super Draco is designed for such long burn times.

With one Super Draco burn time would be

t=p/F=351s

And with a speed of 7.8km/s that is somewhere around 2700km ground distance.

NTO is about 1.4t/m³ and MMH 0.875t/m3. So guessing 11t is about 11m³ and Dragon trunk having 37m³ it might fit into it. Now that is all without tanks, pressurization, plumbing, ... Redundancy will take some weight and extra fuel.

Dragon has 6t payload mass. If the existing dracos are removed, life support, ... a little more. So sounds to me like we are talking about a Dragon+F9 Heavy+strengthened trunk. Not sure if the Super Draco is an option, but I'm not aware of any alternatives available from SpaceX. Maybe they buy some existing smaller engines with proven track record? They anyway most likely want to have several thrusters for redundancy reasons. So a super draco placed in the middle of the trunk is most likely out. (I'm not up to date with SS/SH hot gas thrusters? Anythhing there with ~800N? 4 of them would fulfill the requiremens and provide redundancy).

[steveleach]

The ISS has 450t and the "SSP 51101 USDV SRD" requires 57m/s.

p = 57 m/s * 450000kg = 25.65M kg*m/s

Taking the Draco Isp with 2300m/s that is a required propellant mass

m = t*F/Isp = 11140kg

Super Draco has 73kN Thrust. Requirement is a minimum of 3236N (Soyuz/Progress have 2.95kN). Also I don't know, if a Super Draco is designed for such long burn times.

With one Super Draco burn time would be

t=p/F=351s

And with a speed of 7.8km/s that is somewhere around 2700km ground distance.

NTO is about 1.4t/m³ and MMH 0.875t/m3. So guessing 11t is about 11m³ and Dragon trunk having 37m³ it might fit into it. Now that is all without tanks, pressurization, plumbing, ... Redundancy will take some weight and extra fuel.

Dragon has 6t payload mass. If the existing dracos are removed, life support, ... a little more. So sounds to me like we are talking about a Dragon+F9 Heavy+strengthened trunk. Not sure if the Super Draco is an option, but I'm not aware of any alternatives available from SpaceX. Maybe they buy some existing smaller engines with proven track record? They anyway most likely want to have several thrusters for redundancy reasons. So a super draco placed in the middle of the trunk is most likely out. (I'm not up to date with SS/SH hot gas thrusters? Anythhing there with ~800N? 4 of them would fulfill the requiremens and provide redundancy).

Why wouldn't they just use 8 Dracos?

[vaporcobra]

It’s not very SpaceX-like to do a one-off project that doesn’t also support/contribute to other missions or tech development goals.

So was this modified Dragon also part of the Polaris program’s study for servicing Hubble? Maybe also with an eye on commercial space stations in development and/or propellant depots?

Or is SpaceX just doing the ISS de-orbit vehicle to support their biggest customer?

I was wondering the same thing. It seems unlikely that this modified Dragon concept popped out of nowhere just for the deorbit vehicle, especially given that (per an Eric Berger source) SpaceX may not have even bid on the first RFP. The contract value is also strange in that context: $843M is almost a quarter of the value (in 2024 $s) of SpaceX's initial 2014 contract to complete most Crew Dragon R&D, build half a dozen flight vehicles, and complete two major abort tests, two orbital test flights, and six operational missions!

That just seems at odds with the implication that it's more of a modified Dragon 2 than a bespoke spacecraft. So many possibilities, but my current best guess is that SpaceX will be building an all-new Dragon 2 (rather than modifying an old reused capsule) with substantial changes that borrow from concepts/designs meant for Dragon XL's propulsion section and whatever Crew Dragon tweaks it proposed for the Hubble reboost mission.

Maybe SpaceX is also thinking about ways to keep Dragon highly relevant and grow the markets it can access well into the 2030s? A Dragon w/ more dV could be useful for future destinations and maybe help increase SpaceX's tourism market by enabling service to more orbits/inclinations and providing more utility (orbit raising, mobility augmentation, etc) to entice private station customers. Maybe it could even outright supersede a bespoke Dragon XL and allow SpaceX to avoid that particular dead-end.

[ugordan]

Maybe SpaceX is also thinking about ways to keep Dragon highly relevant and grow the markets it can access well into the 2030s? A Dragon w/ more dV could be useful for future destinations and maybe help increase SpaceX's tourism market by enabling service to more orbits/inclinations and providing more utility (orbit raising, mobility augmentation, etc) to entice private station customers. Maybe it could even outright supersede a bespoke Dragon XL and allow SpaceX to avoid that particular dead-end.

Wouldn't that necessitate human-rating the FH as well? This deorbit vehicle will need many extra tonnes of prop (Jim says closer to 15) which is likely to be outside of reach for even an expendable single-stick F9.
Unless they make the whole thing more modular like fewer prop tanks for these future destinations that don't really require the oomph it takes to deorbit the behemoth that is the ISS?

[Remes]

Why wouldn't they just use 8 Dracos?

Are they qualified for a 30min burn? On youtube "Draco Thruster Vacuum Firing" shows a 10min burn. I wasn't aware of that. So might indeed be an option.

(And I'm sure SpaceX is happy not giving up their vertical integration and avoiding the contractual hazzle of outsourcing critical parts.)

Wouldn't that necessitate human-rating the FH as well?

I don't think usdv will be manned. Once it is seperated FH plays no role any more.
It's a little bit like Shuttle-Centaur. Astronauts might not be amused riding on so much fuel and so much plumbing.

[baldusi]

It’s not very SpaceX-like to do a one-off project that doesn’t also support/contribute to other missions or tech development goals.

So was this modified Dragon also part of the Polaris program’s study for servicing Hubble? Maybe also with an eye on commercial space stations in development and/or propellant depots?

Or is SpaceX just doing the ISS de-orbit vehicle to support their biggest customer?

I was wondering the same thing. It seems unlikely that this modified Dragon concept popped out of nowhere just for the deorbit vehicle, especially given that (per an Eric Berger source) SpaceX may not have even bid on the first RFP. The contract value is also strange in that context: $843M is almost a quarter of the value (in 2024 $s) of SpaceX's initial 2014 contract to complete most Crew Dragon R&D, build half a dozen flight vehicles, and complete two major abort tests, two orbital test flights, and six operational missions!

That just seems at odds with the implication that it's more of a modified Dragon 2 than a bespoke spacecraft. So many possibilities, but my current best guess is that SpaceX will be building an all-new Dragon 2 (rather than modifying an old reused capsule) with substantial changes that borrow from concepts/designs meant for Dragon XL's propulsion section and whatever Crew Dragon tweaks it proposed for the Hubble reboost mission.

Maybe SpaceX is also thinking about ways to keep Dragon highly relevant and grow the markets it can access well into the 2030s? A Dragon w/ more dV could be useful for future destinations and maybe help increase SpaceX's tourism market by enabling service to more orbits/inclinations and providing more utility (orbit raising, mobility augmentation, etc) to entice private station customers. Maybe it could even outright supersede a bespoke Dragon XL and allow SpaceX to avoid that particular dead-end.

One important contracting detail is that this will be a NASA vehicle and a NASA mission, not a service mission. Since NASA will have spaceship (and possibly design) ownership, costs have to count with NASA making the decisions and doing a full FAR 15 accounting. I assume this is very expensive but given the ridiculous amount of potential liabilities, it's the only way that any al the parties would be willing to accept the risk.

[geza]

It’s not very SpaceX-like to do a one-off project that doesn’t also support/contribute to other missions or tech development goals.

What if this development is coupled to a future SpaceX offer for Mars Sample Return? Imagine a modified Dragon with beefed up propulsion for a direct return to Earth. It should be delivered to the Martian surface by a Starship.

Cargo Starship to Mars is in the plans, anyway. If Starship-HLS for 2026 is serious, then cargo landing on Mars should be achievable in the '27, or in '29, at the latest, in time for MSR. Then a return spacecraft is needed. If it is a high-deltaV Dragon, then has a high commonality with the ISS-terminator. Having generous NASA money for both will help a lot. Both spacecraft are needed around the same time.

[DanClemmensen]

Would SpaceX consider a literal modified Dragon? They currently have four Crew Dragons and one being built. By the time USDV is needed, the demand for Crew Dragon flights will be going away, almost by definition, so SpaceX may be able to modify one of the existing Crew Dragon Capsules. It is especially true if Starliner is flying: NASA is committed to fly six Starliner missions and time is getting short, so Starliner could fly the last two crewed ISS missions.

More or less the same is true for Cargo Dragon if it is a better starting point: modify one of the three active Cargo Dragons and let Cygnus pick up the last resupply missions if needed.

[StraumliBlight]

Does the deorbit vehicle need to be expended?

Or to rephrase the question, could a standard Cargo Dragon be used and limit the modifications to an extended trunk with an engine? Once the ISS altitude has been lowered sufficiently it could undock and perform a normal re-entry and recovery.

[DanClemmensen]

Does the deorbit vehicle need to be expended?

Or to rephrase the question, could a standard Cargo Dragon be used and limit the modifications to an extended trunk with an engine? Once the ISS altitude has been lowered sufficiently it could undock and perform a normal re-entry and recovery.

Jim tersely said "no" much earlier in the thread. After reading the CONOPS paper recently, I think I understand why. The whole object of this excersize is to hit a very precise small area with extreme reliability, so the USDV must be able to make late corrections if necessary.

My opinion:
Separately, that Cargo (or Crew) Dragon will have very limited value if recovered, because its main customer, ISS, will be gone, the number of remaining customers, if any, will be small. The remaining Dragon fleet will likely be more than adequate. Thus, the cost of the extra design and production effort to retain recoverability is not likely to be (ahem) recovered.

[greybeardengineer]

Or is SpaceX just doing the ISS de-orbit vehicle to support their biggest customer?

SpaceX will be paid >$800m to do something 1) well within their competency, 2) likely costs them *way* less than $800m to do the NRE and build/launch the hardware, 3) does a vital task for NASA, a critical customer, that apparently had no other takers.

Seems like a no-brainer to me.

[DanClemmensen]

Or is SpaceX just doing the ISS de-orbit vehicle to support their biggest customer?

SpaceX will be paid >$800m to do something 1) well within their competency, 2) likely costs them *way* less than $800m to do the NRE and build/launch the hardware, 3) does a vital task for NASA, a critical customer, that apparently had no other takers.

Seems like a no-brainer to me.

The only issue would be the opportunity costs of diverting the engineers and other resources from more strategic tasks. I have zero insight into this, but presumably the $834 million will cover it.