What allows the Raptor engine to have a high chamber pressure of 300 Bar? Compared to other rocket engines which have been or that are now operational, it has the highest chamber pressure. What does it have different from other engines, and what kinds of new developments were made in this engine?


3 Answers 3


The Raptor is a full-flow staged combustion methalox engine. There is a lot of new technology in that sentence.

  • Staged Combustion
    Fuel and/or oxidizer is ignited to run a turbine that spins a turbopump, increasing the feed pressure into the combustion chamber. The gases in the turbine are then fed into the combustion chamber instead of being discarded. Usually the fuel-rich side goes through this process but there are also oxidizer rich staged combustion engines.

    More on Staged Combustion on Wikipedia.

  • Full-Flow Staged Combustion
    All of the mass of propellants in the engines passes through the turbopumps and/or turbines before entering the combustion chamber. That means there is a fuel-rich turbopump and a oxidizer rich turbopump.

  • Methalox
    The engine uses liquid methane and liquid oxygen as propellant. No methane-powered engines have been used for orbital rockets.

Now the big surprise from the IAC talk was the high chamber pressure of the rocket. This allows for many advantages.

  • Size: The Raptor engine is roughly the same size as the Merlin engine but has 3x the chamber pressure and 3x the thrust.
  • Pressure: The de Leval nozzles that rocket engines use take highly pressurized, low velocity gas and transforms it to lower pressure, high velocity gas. The better the ratio between chamber pressure and exit pressure, the more efficient the engine.

Raptor has a very high chamber pressure compared to other high-performance engines. The SSME has a chamber pressure of ~20MPa for instance.

As for technological developments unique to Raptor, check out this quote by Elon Musk in his AMA

It used to be developing a new metal alloy that is extremely resistant to oxidation for the hot oxygen-rich turbopump, which is operating at insane pressure to feed a 300 bar main chamber. Anything that can burn, will burn. We seem to have that under control, as the Raptor turbopump didn't show erosion in the test firings, but there is still room for optimization.

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    $\begingroup$ Oxygen rich staged combustion is hardly "a technological development unique to Raptor", the Soviets have had oxygen rich staged combustion engines for decades. $\endgroup$ Commented Oct 25, 2016 at 23:11
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    $\begingroup$ This answer has more helpful information on this topic, as well as the video in that question: youtu.be/4QXZ2RzN_Oo $\endgroup$
    – uhoh
    Commented Oct 25, 2016 at 23:22
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    $\begingroup$ @OrganicMarble yeah first were used at RD-270, but it has a chamber pressure 26.1 MPa compared to Raptor 30 MPa. Maybe that new metal alloy mentioned by Elon Musk makes the difference. $\endgroup$ Commented Oct 26, 2016 at 4:29
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    $\begingroup$ [email protected]. Tarazevits this new resistant metal alloy in which parts of the engine it is used, in the turbopump, in the combustion main chamber, in both of them, or also in other parts? $\endgroup$ Commented Oct 26, 2016 at 4:41
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    $\begingroup$ "Anything that can burn, will burn." A layman will not get the pain,tears and sweat of that sentence. Getting all of what should burn to burn is half the work, the other half being getting whatever shouldn't burn not to burn! $\endgroup$
    – SF.
    Commented Jan 24, 2017 at 12:31

The big deal with SpaceX's engines is not the $I_{SP}$. It's the Thrust/Mass ratio SpaceX is getting that is truly significant.
T/M is arguably more important for getting payloads to LEO than $I_{SP}$ is, and SpaceX is setting new records for T/M in their designs.

Merlin 1D has a T/M $\approx200$. Raptor is likely to have a T/M $\approx400$. Nothing else has ever come close to attaining T/M values like these.

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    $\begingroup$ You should really explain why TWR aka T/M is so important for LEO; it's commonly accepted that it's useful for boosters (at least to a certain point) but less so for second/third stages. $\endgroup$ Commented May 6, 2017 at 23:23
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    $\begingroup$ I haven't seen any recent specifications for the thrust or the mass of the ITS Raptor; do you have a citation for this ≈ 400:1 claim? $\endgroup$ Commented May 7, 2017 at 2:31
  • $\begingroup$ T/M is not important for LEO. It's important for =getting= to LEO. Once you are in orbit, ISP becomes more important. "Orbit is 1/2 way to anywhere". The energy requirements for getting from the surface to LEO are enormous. The longer it takes you to get from the surface to LEO, the worse it gets. Thus you want to get to LEO as quickly as your physical structures can survive. Hence T/M being so important. $\endgroup$
    – Ron
    Commented Sep 5, 2017 at 16:47
  • $\begingroup$ Merlin 1D is documented as having a T/M of ~200. Raptor is said to provide 3x the thrust in the same physical form factor as Merlin. That would be a T/M of ~600. But Raptor is going to require some strengthening, hence extra mass, over Merlin. Hence Raptor is likely to come in at ~400 and not ~600 for T/M. $\endgroup$
    – Ron
    Commented Sep 5, 2017 at 16:51
  • $\begingroup$ I'm not sure the long term goal as I haven't read anything on it but their short term goal (for the ITS / BFR) is TWR of 200 or higher. I don't think they are going to reach 400 anytime soon, I think that 250 for medium term is reasonable. (Remember that you would have an engine that needs to sustain a 3 times higher pressure, turbo-pumps that can send about ~ 2,5 times the weight and ~3,5 times the volume at a pressure differential of 3 times the previous one. With the same design and no optimizations it should be about 10,5 times the weight for turbo-pump. $\endgroup$ Commented Oct 3, 2017 at 4:37

One very simple thing, exceptionally high specific impulse due to high chamber pressure.

Merlin 1D has a chamber pressure of 97 ATM and atmospheric specific impulse of 282 seconds. Raptor has an atmospheric specific impulse of overwhelmingly 334 seconds (which is dreamlike). Suppose an imaginary rocket with the Merlin-1D has a $\Delta V$ of 10000m/s. If the Raptor was mounted on the rocket the $\Delta V$ would be increased by 1843 m/s!

That is life-changing for SpaceX and I highly doubt anything in two years would beat Raptor's exceptional high specific impulse.

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    $\begingroup$ Note that the SSME, with a lower chamber pressure of 20.64 MPa, achieved a sea-level ISP of 366 s, so you may want to qualify in what ways 334 s is "dreamlike". $\endgroup$ Commented Jan 25, 2017 at 0:11
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    $\begingroup$ @NathanTuggy it used Hydrolox not Methane and LOX $\endgroup$
    – Raze
    Commented Jan 25, 2017 at 0:16
  • $\begingroup$ For clarity, when I say "may want to qualify", I mean put it in the answer. Comments are transient. $\endgroup$ Commented Jan 25, 2017 at 0:17
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    $\begingroup$ "so you may want to qualify in what ways" — I did. $\endgroup$ Commented Jan 25, 2017 at 1:05
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    $\begingroup$ @Raze please include the Hydrolox info in your answer. I had the same question after reading the phrase. $\endgroup$
    – nitsujri
    Commented Mar 12, 2017 at 14:14

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