MiniRaptor / SC Merlin

[sevenperforce]

The higher density propellant is preferred to minimize tank weight which is proportional to volume.

Yes, but higher specific energy can more than make up the difference. If you shifted the bulkhead on the Falcon 9 first stage and filled it up with densified methalox, and slapped three Raptors underneath, it would have a 40% performance increase.

What are your assumptions behind this 40% number?

I have plotted in the attached image the bulk density required to achieve the Falcon 9 booster's delta-v with payload, assuming constant volume, stage dry mass, thrust, and outer mold line. Methalox Raptor is above the required Isp to get the same performance as Merlin, but not by nearly enough to get 40% improvement. More like 4% unless they also do a methane upper stage.

Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size, equal vehicle TWR, and altitude compensation.

Although when I run the numbers again, my math may have been too conservative.

For example, using the Falcon 9 first stage by itself, I get an estimated payload of 2.9 tonnes to LEO. If the same Falcon 9 first stage was filled with methalox, it would only need two Raptors to have the same vehicle TWR, due to the lower propellant mass, and it would be able to get 12.3 tonnes to LEO.

[Basto]

Seems like fantasy land to me. There are so many things in the vehicle that are specifically designed around the engines /fuel type.

The moment you start swapping these you are designing a new rocket. Makes no sense not to optimize this new vehicle around the new fuel / engines.

While it may seem logical that you are taking a shortcut you are actually making things harder.

[envy887]

Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size and equal vehicle TWR.

That's not a realistic comparison since Falcon 9 isn't a kerolox SSTO. The F9 booster's job is to push 125 tonnes from launch pad through staging, something kerolox is much better suited for than doing SSTO.

The I_sp efficiency for a vehicle is directly related to the delta-v it is required to operate through. The only reason a methalox Falcon could compete is because Raptor is FAR more advanced than Merlin. For a slow-staging booster like F9, a full-flow kerolox engine with Raptor-like pressures and would easily out-perform methalox Raptor engined booster.

[sevenperforce]

Seems like fantasy land to me. There are so many things in the vehicle that are specifically designed around the engines /fuel type.

The moment you start swapping these you are designing a new rocket. Makes no sense not to optimize this new vehicle around the new fuel / engines.

Oh, certainly. I'm not in any sense suggesting the Falcon 9 be retrofitted for methalox; I was just pointing out that the fluffiness of methane doesn't necessarily require a much larger tank, due to the higher specific energy and the lower thrust requirements.

[sevenperforce]

Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size and equal vehicle TWR.

That's not a realistic comparison since Falcon 9 isn't a kerolox SSTO. The F9 booster's job is to push 125 tonnes from launch pad through staging, something kerolox is much better suited for than doing SSTO.

The I_sp efficiency for a vehicle is directly related to the delta-v it is required to operate through. The only reason a methalox Falcon could compete is because Raptor is FAR more advanced than Merlin. For a slow-staging booster like F9, a full-flow kerolox engine with Raptor-like pressures and would easily out-perform methalox Raptor engined booster.

Full-flow kerolox may not be achievable, but ORSC kerolox has been done and redone by the Russians, so I'll compare that.

A pair of RD-180s can deliver the same thrust as all nine Merlin 1Ds with a vacuum specific impulse of 338 s, though dry mass will be quite a bit higher, at 29 tonnes. Using the same single-stage-vehicle comparison as before, it's a payload of 2.57 tonnes to LEO, lower than with Merlins.

But let's take the first-stage-booster-only example. My estimates put the Falcon 9 FT at 23.2 tonnes to LEO, max.

If you swap out the nine Merlins for two staged-combustion RD-180s, leaving the upper stage exactly the same, you'd get 25.1 tonnes to LEO.

If you swap out the Merlins for three Raptors and move the first-stage common bulkhead, then fill it up with methalox only on the first stage, you'd get 26.4 tonnes to LEO. So that's a 14% improvement, even still retaining the Merlin-based kerolox upper stage.

(this assumes a Raptor TWR of roughly 200:1)

[envy887]

Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size and equal vehicle TWR.

That's not a realistic comparison since Falcon 9 isn't a kerolox SSTO. The F9 booster's job is to push 125 tonnes from launch pad through staging, something kerolox is much better suited for than doing SSTO.

The I_sp efficiency for a vehicle is directly related to the delta-v it is required to operate through. The only reason a methalox Falcon could compete is because Raptor is FAR more advanced than Merlin. For a slow-staging booster like F9, a full-flow kerolox engine with Raptor-like pressures and would easily out-perform methalox Raptor engined booster.

Full-flow kerolox may not be achievable, but ORSC kerolox has been done and redone by the Russians, so I'll compare that.

A pair of RD-180s can deliver the same thrust as all nine Merlin 1Ds with a vacuum specific impulse of 338 s, though dry mass will be quite a bit higher, at 29 tonnes. Using the same single-stage-vehicle comparison as before, it's a payload of 2.57 tonnes to LEO, lower than with Merlins.

But let's take the first-stage-booster-only example. My estimates put the Falcon 9 FT at 23.2 tonnes to LEO, max.

If you swap out the nine Merlins for two staged-combustion RD-180s, leaving the upper stage exactly the same, you'd get 25.1 tonnes to LEO.

If you swap out the Merlins for three Raptors and move the first-stage common bulkhead, then fill it up with methalox only on the first stage, you'd get 26.4 tonnes to LEO. So that's a 14% improvement, even still retaining the Merlin-based kerolox upper stage.

(this assumes a Raptor TWR of roughly 200:1)

I get similar numbers, by the way. The 4% increase was for a dual-Raptor design; 3 Raptors would be hard to fit under Falcon.

But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

[TomH]

But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

Sometimes performance gains are from pumping and burning the prop faster. That means a shorter burn time if you don't enlarge tanks/densify, etc.. If you don't increase payload, you cut gravity losses, but wild comparisons raw thrust need to be balanced against other parts of the equation.

[OneSpeed]

... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.

[sevenperforce]

But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

I can't imagine that even a cutting-edge ORSC would be able to significantly outperform the RD-180 in SL isp; it has 275% the chamber pressure of a Merlin 1D. But otherwise, yes, it's a somewhat unfair comparison. Of course, TWR really only affects dry mass.

... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.

Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

[hamerad]

But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

I can't imagine that even a cutting-edge ORSC would be able to significantly outperform the RD-180 in SL isp; it has 275% the chamber pressure of a Merlin 1D. But otherwise, yes, it's a somewhat unfair comparison. Of course, TWR really only affects dry mass.

... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.

Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

Could they perhaps use some of the thrusters they need for the BFS to land? Or are they too weak?

[hkultala]

But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

AFAIK nothing official has been said about Raptor TWR.

People have just been speculating that it will be in the same range as Merlin, though RD-180 might be more reasonabale basis for TWR speculation.

FFSC needs three relatively high-pressure chambers and much bigger turbines than GG. These all have weight. And bigger pressure allowing relatively smaller chamber does help much when the chamber needs much stronger walls due much higher pressure.

[sevenperforce]

Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

Could they perhaps use some of the thrusters they need for the BFS to land? Or are they too weak?

Right, that's precisely what I was thinking. We don't have any specifics on the actual size of the thrusters, but pressure-fed hot-gas thrusters typically have a very good TWR. Mass penalty is very low, since they are pressurized from the existing autogenously-pressurized intertanks. SL Isp wouldn't be greater, but that's hardly a problem.

A single Raptor will be able to downthrottle to about 62 tonnes SL thrust. In contrast, the Merlin 1D can downthrottle to about 35 tonnes thrust, which is a TWR of 1.55 for the suicide burn/hoverslam. Using the same metric, a single Raptor could be used for an (unmanned) suicide burn down to a stage dry mass of 40 tonnes; below that, you'd need auxiliary thrusters.

I wonder if SpaceX would do altitude-compensating nozzles on the hot-gas thrusters.

AFAIK nothing official has been said about Raptor TWR.

People have just been speculating that it will be in the same range as Merlin, though RD-180 might be more reasonabale basis for TWR speculation.

FFSC needs three relatively high-pressure chambers and much bigger turbines than GG. These all have weight. And bigger pressure allowing relatively smaller chamber does help much when the chamber needs much stronger walls due much higher pressure.

Elon has stated categorically that the Raptor is expected to have a TWR better than the Merlin 1D.

While Elon's projections have been overoptimistic on plenty of occasions, I don't think his optimism is misplaced here. The RD-270, the only other full-flow staged-combustion engine to reach a test stand, had a TWR of 190:1, higher than the Merlin 1D. Of course, this is partly because the RD-270 was pushing hypergolics, but even with the higher isp of a methalox engine, this would translate to an expected TWR of about 171:1 on the Raptor. They'd only need about a 7% increase to beat the Merlin 1D, which is hardly aggressive considering it's been fifty-five years since the RD-270 was designed.

[envy887]

... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.

You're right. I was thinking they were 1.7m diameter for some reason. Those must be the ship landing Raptors.

[sevenperforce]

You're right. I was thinking they were 1.7m diameter for some reason. Those must be the ship launch clamp landing Raptors.

See, fixed that for you!

[OneSpeed]

Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

From the Reddit AMA, the ITS vernier thrusters are 10mT thrust each, so 6 of them would be sufficient to land a Falcon with some fuel. Then again, rather than rely on two separate thrust systems for a successful landing, 7 half scale Raptors (the current development engine) would have comparable thrust to three full scale, and be able to land on the centre engine. Not saying they'll actually do that either ...

[sevenperforce]

Partly inspired by this thread, here's a dedicated thread on second-stage reuse configurations in the future: https://forum.nasaspaceflight.com/index.php?topic=42783.0