SpaceX Raptor Engine Will Be Best on Cost and Nearly Best on ISP

Tim Dodd, Everyday Astronaut, has a detailed analysis of the SpaceX Raptor engine compared to the F-1 Saturn Five, Space Shuttle RS-25, the Russian RD-180 – used on US Atlas rockets and the Blue Origin BE-4.

SpaceX Merlin engine costs less than $1 million. Elon has said SpaceX produce the Raptor for cheaper than or close to the Merlin engine in later simplified Raptor versions. Tim estimates current Raptors at $2 million.

The Raptor will have a vacuum version with an ISP of 350 and a sea level version with an ISP of 330. ISP is rocket engine fuel efficiency.

The Raptor will have higher ISP than any rocket other than the Space Shuttle, but the Raptor will be 70 times cheaper than the Space Shuttle engine on a cost for constant power level basis.

SOURCES- Everyday Astronaut

32 thoughts on “SpaceX Raptor Engine Will Be Best on Cost and Nearly Best on ISP”

  1. Each fuel combination has a max theoretical ISP that cannot be exceeded. The shuttle engines do not achieve this limit but still achieves higher ISP than can theoretically be achieved with methalox. In other words even a ‘perfect’ best possible engine running on methalox cannot achieve the ISP that the shuttle engine actually achieves with hydrolox. The ultimate limit is in the fuel, not just in how well the engine is designed.

  2. ISP is specifically what it is an invalid comparison of overall engine design for because LH2 will always demonstrate better ISP than Methane, that’s not a function of a superior engine design, it’s a function of molecular physics.  Maximum exhaust velocity is inversely proportional to the square root of molecular weight. Across two identical designs, you will always get more Ve2 out of LH2 than methane. You’d have to royally screw the pooch so far off the map on an LH2 rocket engine for methane to beat you in ISP. Generally, once you choose your fuel, you’ve chosen your ISP range. There is no magical design guts that turns methane into a huge ISP performer.

  3. Look at their SIZE! Spacex chose to have multiple smaller motors and gets bonus…one or two can fail and still hit the target

  4. And mountains not requiring fuel to stay up there.

    The chief advantage of launching from a plane is actually that you can pick your launch site to insert into the right orbit when you want to. It adds a lot of complexity to the launch for not much reduction in required delta V.

    Not impressed at all with Stratolaunch. It’s not even super-sonic! That’s a lot of plane to not accomplish very much.

  5. Of course China and India have access to some seriously high mountains. And less concern for any “park” status.

  6. How does that particular design decision make the comparison of the overall performance invalid?

    It’s not like gravity is going to go easy on you because you chose to use a different fuel.

    I will concede that some measure of the required fuel tank mass would help the comparison.

  7. I was referring to the access the museum allowed, not the size.
    Because yes indeed you could live there, it was the size of many habitations.

  8. …which is why various people launch from an airplane. It’s even better than a mountain (it goes even higher, and you can be ON the equator). Well, except for the maximum size being a bit on the small side.

  9. And to add to Scarab and nbfdmd on the disadvantages of hydrogen:

    Hydrogen causes metal embrittlement so reliability would take a hit.

    Hydrogen has lousy thrust and is terrible for getting out of a gravity well– Hydrogen rockets like the Space Shuttle, Ariane 5, H3, etc. needs help from solid rocket boosters to get off the ground, or bundling multiple BIG hydrolox boosters together, like Delta IV Heavy. And even then DIVH can only lift half of what Falcon Heavy can to LEO).

  10. That, too, plays into their different targets. Bezos wants the Moon, and it isn’t 6-9 months away, not even 6-9 days. So Methane first stage, Hydrogen second, makes perfect sense for a lunar oriented mission.

  11. True, but given U.S. fears (paranoia?) about security, I ruled out non-U.S. based American rocket launches.

  12. “And given that latitude is more important than elevation, maybe Mt. Whitney in CA is a better choice than far north Denali, Alaska.”
    Or maybe people who want to launch rockets should arrange for a friendly relationship with Ecuador or Kenya.

  13. If the manufacture of methane is easy, the manufacture of hydrogen is too. Long term storage of methane is easier than hydrogen, which tends to seep through solid steel. I think that may be the bigger issue when drifting for 6-9 months and then trying to propulsively land.

  14. Both the BE-3 and BE-7 are hydrogen-LOX. The BE-4 is a methane-LOX engine, though. I expect Bezos will use the BE-4 for first stages, and the BE-3U and BE-7 for upper stages.

    Musk is giving hydrogen a pass, and focusing entirely on Methane for his future engines, except the draco.

    This is because Musk wants to go to Mars, where manufacture of Methane is easy, while Bezos wants to go to the Moon, which is carbon poor, but where Hydrogen/oxygen is easy to manufacture.

  15. Ah, so it does sound like there’re good advantages to launching at 20,000 feet or so (20kft), though not for the reasons I first thought. If the atmospheric drag is greatest at the launch, and increases as the rocket picks up speed, might not launching from 20kft also provide greater benefits because by the time greater speed is achieved, the rocket would be already well above 20kft and in even thinner atmosphere? I guess it depends on speed of acceleration, and that is a variable, with a tradeoff to fuel burned in taking time to accelerate.
    Blue Origin is launching spaceships from a custom plane specifically to save fuel, though I think that is over 30kft, which is impossible to do anywhere on Earth.
    I wonder what the sweet spot is where you can dispense with multistage rockets, and save that weight and the extra fuel to deliver it, and where you’re just dragging along deadweight empty fuel tanks and decreasing performance, and if that sweet spot is around 20kft?
    That’s a complex set of variables, and it also has to account for efficiency gains in today’s fuel to weight ratios.

    And given that latitude is more important than elevation, maybe Mt. Whitney in CA is a better choice than far north Denali, Alaska.

  16. One problem is that the peaks of really tall mountains tend to be parks. Getting authorization to build a launch facility on one, (Even underground with a sliding fake volcanic lake, which I suspect Bezos would be totally on board with doing.) would NOT be easy.

    Also, at least in the US the tallest mountains tend to be in Alaska, which is unfortunately far from the equator.

  17. Indeed, if I weren’t a space obsessed engineer, I’d have learned a lot from it. And even being one found it a nice summation of why Methane/Lox is actually a pretty sound choice, and the Raptor stands a good chance of being revolutionary.

    You can see why Musk chose Methane while Bezos chose Hydrogen, too: Different destinations in mind!

    One thing that really came out was that SpaceX is currently doing a lot of shipping back and forth. They really need to get test facilities co-located with their launch facilities, to reduce that.

  18. The benefit from elevation alone is trivial. Orbital velocity is about 7.8km/s; with other losses you can assume a launch from sea level needs about 10km/s total to reach orbit. Minimum stable orbit is about 250km, so total energy to reach orbit is gravitational potential + orbital kinetic = m*g*h + m * v^2 / 2 = 2.45MJ + 50MJ.So only about 5% of of the total energy needed to reach orbit is due to elevation; most of it is getting up enough sideways speed to fall around the Earth.

    Where high-elevation launch does help is in two ways:

    1) Sea-level engines can only have exit pressures of about 1 atm or else the air will rush into the nozzle turbulently and destroy it. Rockets designed to be launched at high altitude can be more efficient. Ambient pressure at 20kft is about 0.45atm, which translates to about a 22% increase in specific impulse.

    2) Lower maximum dynamic pressure. Most rockets launched from sea level need to aggressively throttle down around 35-50kft altitude to not get torn apart plowing through the atmosphere. Starting at say 20kft could eliminate that, which greatly reduces both boost stage complexity, and losses due to fighting gravity until you acheive orbit.

    Both of these could make or break a single stage to orbit vehicle – Space Shuttle could have been much more performant if launched from 20kft. For multiple stage vehicles there are diminishing returns for both, so it becomes a cost/benefit trade.

  19. If we constructed a spaceport on a mountaintop that was, say, 14,505 ft (Mt. Whitney, CA), or even 20,310 ft (Denali, Alaska), or slightly less after clearing a flat surface, would it significantly reduce the amount of the gravity well a rocket had to climb, and hence the amount of fuel needed to reach LEO? Would thinner air reduce drag and increase efficiency significantly as well?
    This would be tough to build and maintain, but would it be worth delivering spaceships to mountaintops by truck to reduce the need for fuel to escape Earth’s Gravity well?
    It seems to me the worst part of the gravity well is at the bottom, but I can’t find a calculation that shows you the exact amount by elevation.

  20. ⊕1 … I agree. An excellent video. The chap tends to talk too much, but it is still good stuff.

  21. True… but it did pretty good for 1969. Got that big roman candle off the pad and to LEO. And from there, to the Moon. And back. With rocks. Rockin!

  22. For the F1, 100% reliability over 17 flights is, statistically, perfectly consistent with it being the least reliable engine in that list.

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