An Engine beyond Raptor?

[meekGee]

Maybe this belongs in its own thread:

November 16th 2021 Updates: NEW ENGINE

Elon Musk @elonmusk
True, although it will look clean with close out panels installed.

Raptor 2 has significant improvements in every way, but a complete design overhaul is necessary for the engine that can actually make life multiplanetary. It won’t be called Raptor

Ok I give.

Obvious questions:
What will this engine be attached to?
Are we still talking methalox?
Are we still talking about high thrust engines or is this an in-space high-ISP engine?

Shades of tHHGttG: "Behold Raptor, the second most advanced rocket engine designed by man"

[DigitalMan]

I’d like to suggest talking about what are the primary issues with the mission “making life multi-planetary” that would be drastically improved by something beyond Raptor

[KilroySmith]

I’m going to go with speed and ISP.  Speed to get you to Mars in less than 7 months.  ISP to minimize the amount of propellant you need to carry, which helps with the speed thing.
How is the question-lots of answers in hard Science Fiction novels, but it’s not clear where reality intersects with that sphere.

[DigitalMan]

I think the amount of mass to Mars is likely also a major consideration.

If engine count is a major issue, how does one value reducing engine count on the booster versus great improvements in Starship efficiency and/or performance?

[Robotbeat]

I kind of think it's still methalox, but you never know. Methalox trades well for a first stage. Hydrolox does well for an upper stage. Carbon monoxide/oxygen trades well on Mars (pearl-clutching aside, it's not any more toxic than, say, ammonia and is better than hydrazine).

It's methalox. (Still not comfortable saying that... methane/LOx isn't much longer...)

[meekGee]

I kind of think it's still methalox, but you never know. Methalox trades well for a first stage. Hydrolox does well for an upper stage. Carbon monoxide/oxygen trades well on Mars (pearl-clutching aside, it's not any more toxic than, say, ammonia and is better than hydrazine).

It's methalox. (Still not comfortable saying that... methane/LOx isn't much longer...)

I went on a flight of fancy there for a minute, but he did say "design overhaul", not "paradigm shift".

So yeah, I think you're right.

[Morgun]

They were always ok with brute forcing it with numbers as long as they could build enough Raptors cheaply. Switching to a new engine design would add years of development.

[Robotbeat]

They were always ok with brute forcing it with numbers as long as they could build enough Raptors cheaply. Switching to a new engine design would add years of development.

Plus the dry mass disadvantage of hydrogen means it buys you less than you think it does.

[meekGee]

I’d like to suggest talking about what are the primary issues with the mission “making life multi-planetary” that would be drastically improved by something beyond Raptor

Good point.

IMO, the biggest problem is propellant fraction. Even without the (IMO misguided) desire to get the ships back to Earth on a single synod, it's still 6 SH launches per one outbound trip, and then a full ship's propellant ISRU'd in order to get it back.

A high ISP engine means a combination of faster transits, less fuel, less ISRU fuel, more payload.

But:  you don't get high ISP from a "design overhaul".  You get high ISP by changing propellant or going non-chemical.

And there are so many problems with either of these...

EDIT:

https://twitter.com/elonmusk/status/1460816308463878148

Limiting factor for first launch is regulatory approval. Thereafter, fundamental issue is solving engine production.

Prototypes are easy, production is hard.

Sounds like it's just optimization for mass production, not anything crazy.

Musk's opinion is that the hardest problem is engine production.

Well, the man's earned the right to have an opinion, right? 

So the remaining question is why the name change...

[Morgun]

IMO, the biggest problem is propellant fraction. Even without the (IMO misguided) desire to get the ships back to Earth on a single synod, it's still 6 SH launches per one outbound trip, and then a full ship's propellant ISRU'd in order to get it back.

Unless there's a problem we don't know about with Raptor. Reliability? Something about the design that makes it work around Earth but unable to survive for 2 years on Mars?

[bstrong]

IMO, the biggest problem is propellant fraction. Even without the (IMO misguided) desire to get the ships back to Earth on a single synod, it's still 6 SH launches per one outbound trip, and then a full ship's propellant ISRU'd in order to get it back.

Unless there's a problem we don't know about with Raptor. Reliability? Something about the design that makes it work around Earth but unable to survive for 2 years on Mars?

IMO, it's all about the reliability and durability of the booster engines, since they will see much more use. Maybe they don't see a path to getting 1000 flights out of Raptor and have decided to go with a methalox Merlin for the booster.

If they took everything they learned from raptor and applied it to maximizing the T/W of a GG engine, how big a hit to performance would it even be?

[Tomness]

Nuclear Powered Engine

[DanClemmensen]

The engine needs a new way to start. Helium is hard to get on Mars, and all of that helium plumbing is complicated.

[butters]

I’d like to suggest talking about what are the primary issues with the mission “making life multi-planetary” that would be drastically improved by something beyond Raptor

Keep in mind that when Elon says something needs a redesign in order to scale to his ambitions, it's often more about cost, complexity, and manufacturability than it is about performance. Weight is more likely to come down with iteration, but Raptor as we know it is not very far from theoretical limits of specific impulse for a methalox engine, and whatever replaces it might not be much better in this respect. However, they do need to be able to produce many thousands of them per year for the big Mars fleets that he's envisioning.

The goal for a major redesign should be to enable production of 10 times the engines for the same cost. It that also comes with a drastically lower part count, lower weight and higher reliability, that would be great, too.

[Dancing Dog]

Maybe they'll build a viable rotating detonation engine... if anyone's got the CFD horsepower, they do.

[dnavas]

My bet is on cost, not capability.  The six that make the transit aren't going to have a hundred flights to amortize cost over.

[M.E.T.]

The main barrier to making life multi-planetary is cost.

Elon doesn’t need Raptor to be more powerful. He needs it to be cheap and reliable. And to do that, he not only needs it to be reusable, but FULLY and RAPIDLY reusable.

Those are the design changes that are required. So reduced complexity, increased durability and ultimately, reduced cost.

So in short - simplification, not performance enhancement. It might even result in REDUCED performance, if the trade off is significant simplification.

[MATTBLAK]

Has anyone done number-crunching for how well Raptor might scale up to double or even triple the thrust? And how physically large would that be?

[docmordrid]

Maybe they'll build a viable rotating detonation engine... if anyone's got the CFD horsepower, they do.

That or a Pulse Detonation Engine. JAXA launched both on a sounding rocket earlier this year.

https://forum.nasaspaceflight.com/index.php?topic=54595.0

[492]

Could this redesign involve a different kind of turbine?
Maybe a Tesla Turbine?
Or maybe some kind of internal ramjet or scramjet to counteract the back pressure?