Could upper stage engines be brought back to earth inside the Dragon?

[Joel]

If there is excess return cargo capacity for the Dragon, could this be used to bring upper stage engines back to earth? I figure that if you can detach the vacuum nozzle of Merlin Vac,  the diameter should be around 1.20 m (a third of the Falcon 9 diameter).

The common berthing mechanism allows 1.27 m diameter passage, so you might just be able to squeeze one through. Without nozzle and support structure, the full length of Merlin Vac should also fit inside the 2.9 meter pressurized space.

The weight of the engine is around 500 kg, maximum return payload is 3000 kg.

You might just be able to fit two or three Merlin Vac's inside a Dragon allowing you to recover the US engines from DragonRider and Dragons with return payload.

Would this be a viable alternative to trying to reenter and land the second stage?

[Silmfeanor]

Short answer : no.

the upper stage is not inserted near the ISS orbit, and you'd need a space station to somehow grab such a thing and bring it inside.

Also, if on-orbit capability is so great that they're able to detach a LOX/RP-1 from it's tanks and thrust structure safely, detach a nozzle that has fired ( and has to be sturdy enough to be able to withstand this firing ), safe this ( including it's igniters ) and bring it back....

Well, let's just say that we wouldn't be flying to the ISS.


And: Cargo is volume limited, not mass limited. So, if you'd be able to fit the engine in there somehow ( which you can't detach, safe or clean ) you'd eat up all the return capability.

[Joel]

Right... I was thinking long term. It's clear that it would require more in-orbit capability, and an engine redesign that makes it easily detachable from the support structure and nozzle.

And as you say, it doesn't make sense to bring up the US to ISS orbit, so you would have to do it at the altitude where the upper stage is inserted.

If you'd be able to fit more than one engine inside, you wouldn't eat up all the return capability. Only some Dragons would then return engines. Others would operate normally.

[Joel]

And is cleaning the engine really an issue? I thought that the vacuum of space would boil away any leftover liquids.

[apace]

Technically perhaps, but not economically. Merlin is a mass produced engine. They use on Falcon 9 10 of them, on Falcon Heavy 28. It will be always cheaper to produce new ones than recover the engine only. Of course, if you can reuse the first and upper stage completely, it's another question.

[Jim]

Right... I was thinking long term. It's clear that it would require more in-orbit capability, and an engine redesign that makes it easily detachable from the support structure and nozzle.

And as you say, it doesn't make sense to bring up the US to ISS orbit, so you would have to do it at the altitude where the upper stage is inserted.

If you'd be able to fit more than one engine inside, you wouldn't eat up all the return capability. Only some Dragons would then return engines. Others would operate normally.

no, there is no scenario where this is viable or even possible.  The upperstage is no where near the Dragon when either has completed its mission.

[Joel]

OK, I see. So it would make more sense to detach and bring the the 500 kg Merlin up to ISS orbit (as part of the unpressurized cargo) than bringing the 5000 kg Dragon down to the upper stage's orbit?

You could store upper stage engines at ISS (or in whatever future space station) and bring them down to earth in batches.

[Jason1701]

OK, I see. So it would make more sense to detach and bring the the 500 kg Merlin up to ISS orbit (as part of the unpressurized cargo) than bringing the 5000 kg Dragon down to the upper stage's orbit?

You could store upper stage engines at ISS (or in whatever future space station) and bring them down to earth in batches.

It would make more sense to do neither and forget about upper stage engine recovery.

[Jim]

OK, I see. So it would make more sense to detach and bring the the 500 kg Merlin up to ISS orbit (as part of the unpressurized cargo) than bringing the 5000 kg Dragon down to the upper stage's orbit?

No, either.  There is no sense to do either.  There is no room nor time nor benefit.

[Joel]

OK, I see. So it would make more sense to detach and bring the the 500 kg Merlin up to ISS orbit (as part of the unpressurized cargo) than bringing the 5000 kg Dragon down to the upper stage's orbit?

No, either.  There is no sense to do either.  There is no room nor time nor benefit.

I realize that the benefit is currently limited, but would you say that this concept is more (un)realistic than adding a heat shield to the second stage, reenter it and land vertically on the launch site?

[Jim]

I realize that the benefit is limited, would you say that this concept is more (un)realistic than adding a heat shield to the second stage, reenter it and land vertically on the launch site?

More un.  Dragon will only fly a couple of flights per year.  Even if Dragon flew more, there would be no room for the engine.  And idea killer is thinking that detaching the engine robotically would be simple.  That is more complex than reusable second stage.  Even EVA removal would be too.  Not to mention more expensive.

[Joel]

Just a clarification, what do you mean by "no room"? Inside the Dragon or in the ISS? I mean, you don't need to store the engines inside the station.

I guess a limitation would be to get the Merlin into the Harmony node, but that's just a design problem of the ISS. A future space station could be designed to allow this.

[ArbitraryConstant]

Just a clarification, what do you mean by "no room"? Inside the Dragon or in the ISS?

A MVac engine is huge. There's no room because it's nearly the same size as Dragon.

[Joel]

As I said above, the engine would have to be redesigned to allow the vacuum nozzle to be detached and discarded. Not being an expert, I recon that landing the upper stage on Earth vertically would require something similar (retracting the nozzle). Or would you be able to land the upper stage vertically with the vacuum nozzle attached/extended?

As for fitting it into the Dragon, see above. At least one, maybe more than one should fit from what I can tell. Without support structure and nozzle that is.

[Jim]

Just a clarification, what do you mean by "no room"? Inside the Dragon or in the ISS? I mean, you don't need to store the engines inside the station.

I guess a limitation would be to get the Merlin into the Harmony node, but that's just a design problem of the ISS. A future space station could be designed to allow this.

Inside the dragon, there is no room. It has up and down cargo.   And there is no design problem with the station, as there is no need for objects such as this to go inside.  So a future station would not designed for it.

[Jim]

As for fitting it into the Dragon, see above. At least one, maybe more than one should fit from what I can tell. Without support structure and nozzle that is.

And what is the benefit with all these changes?

[Joel]

The benefit would be that you're able to reuse the engine, which is the expensive part of the stage, without the dead weight associated with a heat shield, making the upper stage sturdy enough for reentry and extra propellant for reentry and landing.

Of course, some extra propellant would be needed to raise the altitude of the 500 kg engine, but that should be small in comparison.

Also, once you've designed the system, it should be cheap to operate, because you will probably want to bring a lot more cargo up into LEO than down from LEO. Especially if you are servicing a space station.

[apace]

The benefit would be that you're able to reuse the engine, which is the expensive part of the stage

Wrong.

[Jim]

The benefit would be that you're able to reuse the engine, which is the expensive part of the stage, without the dead weight associated with a heat shield, making the upper stage sturdy enough for reentry and extra propellant for reentry and landing.

Of course, some extra propellant would be needed to raise the altitude of the 500 kg engine, but that should be small in comparison.

Also, once you've designed the system, it should be cheap to operate, because you will probably want to bring a lot more cargo up into LEO than down from LEO. Especially if you are servicing a space station.

1.  It is not the most expensive part of the vehicle, the avionics is up there with it.
2. The modifications negate any savings

3.  No, it is not cheap to operate.  Again, Dragon only flies a few missions a year. The second stage is going to fly on 2 to 6 times more flights per year.

Again, how does the engine get inside.  ALso, the CBM is not an EVA hatch

This idea has no merit.

[go4mars]

It's an interesting idea, but I agree with others here: It may be technically possible to design things for such a scenario, but it might be more realistic and beneficial to design for re-entering the stage or part of the stage. 

If the high volume application of dragon is moving people up and down, then there is clearly an issue with the idea.

The only scenario I see where this might make some sense (not much) is if the engine and assembly also added some structural support within dragon, like as a middle column if there was a need for increased downmass of something very dense. Even then, I struggle to believe this would be the best solution.