Pure Fantasy: BFR as S1, SLS Core w/ J-2X as S2

[TomH]

As the title says, this is pure fantasy. I know it will never happen for an endless number of reasons. But, just for the fun of speculation, imagine a decision gets made to put disposable SLS core on top of reusable BFR (S1 of MCT). RS-25 gets swapped out for J-2X and the SLS core becomes air started S2. There are no solids. Maybe EUS is a third stage (perhaps a crasher stage on a massive lunar lander), but probably not. I know many are going to say Why? but this is nothing more than playful imagination. It is just an exercise in fun.

What mission architectures could you envision for this monster? What kind of costs might it entail? (Could it actually be cheaper than Block IIB since Dark Knights, RS-25E, and EUS never have to be developed?) Is it stackable in VAB? Other pros and cons? Remember-no need to argue against it; it's just for fun.

[llanitedave]

Legos?

[Arcas]

I'm pretty sure it wouldn't fit on top. It would snap in half if the wind didn't blow it over before launch. Maybe using it as side boosters or something Ariane 5 style.

[MATTBLAK]

Legos?

Yes, maybe but so what? Don't be a spoilsport and pejorative...

[TomH]

It would snap in half...

Yea, I forgot about the boosters lifting SLS core from the upper thrust beam. It would have to be strengthened.

Maybe using it as side boosters or something Ariane 5 style.

No, you do not want H for the booster fuel and CH₄ for the US fuel. It works the other way around: high ISP-Density at launch, high ISP on US. Besides, the BFR burns at a booster rate and SLS core at sustainer rate.

[deltaV]

The original plan for Ares I was to use a SSME as the upper stage but this turned out not to be feasible because it was too difficult to redesign SSME to be started in flight without the aid of ground equipment. An SLS Core uses SSMEs so it's not suitable as an upper stage for that reason.

[93143]

As I understood it, it wasn't the air start that was intractable (though it wasn't trivial).  It was the restart required by the Ares V EDS.  That brought in the J-2X, and everything kinda fell apart from there.

[Hauerg]

The original plan for Ares I was to use a SSME as the upper stage but this turned out not to be feasible because it was too difficult to redesign SSME to be started in flight without the aid of ground equipment. An SLS Core uses SSMEs so it's not suitable as an upper stage for that reason.

Yeah, that's why the opening post states: "RS-25 gets swapped out for J-2X"

[Zed_Noir]

If you don't have any solid motors. Then you don't need the MLP & the CT or the VAB. The whole stack can be assembled horizontally move to the pad on a TEL from a SX style HIF hangar.

However maybe the J-2X should be replaced with BE-3U since the Blue engine will be cheaper and in mass production. Also I am guessing the BE-3U will have better thrust to weight ratio & ISP then the J-2X. The J-2X was tailored for the Ares-1 upper stage after all.

[TomH]

...maybe the J-2X should be replaced with BE-3U since the Blue engine will be cheaper and in mass production. Also I am guessing the BE-3U will have better thrust to weight ratio & ISP then the J-2X. The J-2X was tailored for the Ares-1 upper stage after all.

BE-3U has just over 50% of thrust of J-2X. I cannot find ISP for BE-3U. I know this is hypothetical, but IDK if BO would sell engines for this LV even if NASA and SpaceX somehow agreed to combine these.

[Zed_Noir]

BE-3U has just over 50% of thrust of J-2X. I cannot find ISP for BE-3U. I know this is hypothetical, but IDK if BO would sell engines for this LV even if NASA and SpaceX somehow agreed to combine these.

If BO will supply BE-4 for the ULA Vulcan than I don't see why they would not supply the BE-3U to anyone with the cash. The more units from an engine line brings economy of scale to the manufacturing facility. Reducing the unit cost of each engine since there will be more units to cover the overhead cost.

[Patchouli]

I were to use the J-2X with BFR I'd go inline with a 3 or 4  engine J-2X second stage riding the BFR first stage for the basic vehicle.
Option two BFR as is and just use a J-2X EDS on top of it.
This is assuming BFR is to be roughly the size of a Saturn C5 and will have similar lift off thrust.

[EE Scott]

Legos?

What do you think SLS, Ares V, Ares I is/were?

[EE Scott]

As the title says, this is pure fantasy. I know it will never happen for an endless number of reasons. But, just for the fun of speculation, imagine a decision gets made to put disposable SLS core on top of reusable BFR (S1 of MCT). RS-25 gets swapped out for J-2X and the SLS core becomes air started S2. There are no solids. Maybe EUS is a third stage (perhaps a crasher stage on a massive lunar lander), but probably not. I know many are going to say Why? but this is nothing more than playful imagination. It is just an exercise in fun.

What mission architectures could you envision for this monster? What kind of costs might it entail? (Could it actually be cheaper than Block IIB since Dark Knights, RS-25E, and EUS never have to be developed?) Is it stackable in VAB? Other pros and cons? Remember-no need to argue against it; it's just for fun.

Sounds like an awesomely powerful LV! It pays to think big sometimes, and I would love to see the J-2X put to use. Sure, the SLS core would need to be reinforced, hopefully not radically re-designed. While J-2X would be expensive in multiples (maybe 5 engines per S2?), at least the J-2X costs and performance are known. I can imagine robust mission capabilities to the Lunar south pole and manned missions including Venus flyby and Deimos and Phobos landings in my lifetime. Of course this my imagination boosting NASA's budget significantly - who knows, maybe even to 0.8% of the total Federal budget?

[TomH]

Option two BFR as is and just use a J-2X EDS on top of it.

That's kind of what I'm envisioning, except the SLS core with J-2X is that US. I would think staging occurs at considerably < orbital V. The J-2X SLS-core based US then provides the remaining ΔV all the way to escape velocity.

As MCT comes closer to reality, if NASA really wants to keep its own rocket (or rather, if certain senators still want pork flowing to a certain state), this could provide a way to eliminate 5 segment solids, advanced solids, RS-25E, and perhaps also EUS by purchasing boost services from SpaceX on reusable BFR, swapping out the engine, strengthening the walls, and integrating flight controllers, among other things. And, all that money spent to develop J-2X.........is no longer wasted.

[Paul451]

imagine a decision gets made to put disposable SLS core on top of reusable BFR (S1 of MCT). RS-25 gets swapped out for J-2X and the SLS core becomes air started S2.

BFR is the core, so might as well swap the SLS core straight out and develop a dedicated second stage.

Particularly if you are going to swap the RS-25s for J-2Xs, and modify the modified SLS core so it can support its own weight, you already are developing a new upper-stage. So stop trying to turn the ET into yet another launcher and start with a clean-sheet built around your chosen engine.

Of course, the easiest second stage is to take your first stage engine and develop a vacuum-rated version, and your first stage prop tanks and shorten them slightly. Common manufacturing process, common diameter for ops. And at least you know the tanks can support their own weight.

Re: Lego¹ rockets.

Rockets can be Lego'ed. But only if they have sufficient design margin. The more specialised the design, the more it pushes the limits of engineering, the more expensive it will be to adapt. The Shuttle was... well, impossible. A 100+ tonne spaceplane? That they got it to fly at all clearly showed they were operating some kind of engineering voodoo. Expecting to come along 30 years later and use the components in a fundamentally different design is madness. It shouldn't have worked the first time.

^{¹ Not "legos" dammit!}

[Ronpur50]

I put this together rather quickly, so is this the general idea?  It is big!

[RonM]

From that graphic, it looks like the stack would be around 525 feet tall. The doors at the VAB are 456 feet, so where does this beast get assembled?

[EE Scott]

I put this together rather quickly, so is this the general idea?  It is big!

Cool, thanks for putting that together. Really provides perspective.

[spacenut]

I would think the upper stage would not be the length of the core of SLS but shorter.  Probably no more than 5 J2X's on the core upper stage for something like this to be practical.  Probably 1/3 longer than the Saturn V second stage since the J2X is about 300k lbs thrust vs 200k thrust each on Saturn V.  Then the rocket would much less higher.