# What's so special about SpaceX's Raptor rocket engine with 300 Bar chamber pressure?

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?

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.

• 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.

• Oxygen rich staged combustion is hardly "a technological development unique to Raptor", the Soviets have had oxygen rich staged combustion engines for decades. – Organic Marble Oct 25 '16 at 23:11
• This answer has more helpful information on this topic, as well as the video in that question: youtu.be/4QXZ2RzN_Oo – uhoh Oct 25 '16 at 23:22
• @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. – John Howell Oct 26 '16 at 4:29
• V@T.J. 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? – John Howell Oct 26 '16 at 4:41
• "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! – SF. Jan 24 '17 at 12:31

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.

• 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". – Nathan Tuggy Jan 25 '17 at 0:11
• @NathanTuggy it used Hydrolox not Methane and LOX – Raze Jan 25 '17 at 0:16
• For clarity, when I say "may want to qualify", I mean put it in the answer. Comments are transient. – Nathan Tuggy Jan 25 '17 at 0:17
• "so you may want to qualify in what ways" — I did. – Nathan Tuggy Jan 25 '17 at 1:05
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.