HWM Singapore – August 2019

engine had to be adapted for hydrogen,

which is signi cantly less dense than RP-1

or even liquid oxygen, requiring a larger

pump to get the right amount to the

combustion chamber.

The diff ering densities and pump sizes

also meant that the engine now needed

two preburners and shafts, one each for

hydrogen and oxygen. The oxidizer still

goes directly to the combustion chamber,

where it meets the hydrogen fuel that

has passed through the preburner.

Either way, wastage is minimized as

the exhaust is recaptured and not simply

thrown out.

PUTTING THEM TOGETHER

But what if you could combine the best

elements of both designs? Put them

together, and you get the full- ow staged

combustion engine. You have the dual-

preburner design that the Americans

eventually put into the Space Shuttle’s

RS-25 engine, except that there’s now

independent oxidizer-rich and fuel-rich

preburners. The oxidizer-rich preburner

also means that a very strong metal alloy

is needed to withstand the heat, and

SpaceX developed something Musk calls

the “SX500 superalloy”.

On the fuel-rich side, you can’t use RP-1

because of all that soot, which is where

the liquid methane comes in.

You also have the freedom to pipe all

the fuel and oxidizer you need through

the preburners without worrying about

wastage as in the open-cycle design.

In eff ect, the fuel is burned twice, once

in the preburners and again at greater

effi ciency in the combustion chamber.

Wastage is completely minimized

and you end up making the most of

the available fuel and oxidizer. Both

of them also arrive in the combustion

chamber as hot gas, which further

improves combustion.

Finally, you have to worry less about

the seals between the turbines and the

pump, which was an area of concern on

the RS-25. The last thing you wanted was

for liquid oxygen to leak into hot fuel-rich

gas, but there is little risk of liquid fuel

leaking into already fuel-rich gas or liquid

oxygen coming into contact with oxidizer-

rich gas, as is the case with the respective

fuel-rich and oxidizer-rich preburners in

a full- ow staged combustion engine.

This improves the reliability of the Raptor

design, which is crucial given that SpaceX

expects to reuse the rocket.

OPENCYCLE

DIAGRAM

CLOSEDCYCLE

FUELRICH

CLOSEDCYCLE

OXIDIZERRICH

PUMPS

PREBURNER

TURBINE

SHAFT

FUEL OXIDIZER

COMBUSTIONCHAMBER

FUEL OXIDIZER

COMBUSTIONCHAMBER

PREBURNER PUMPS

TURBINE

SHAFT

FUEL OXIDIZER

COMBUSTIONCOMBUSTIONCHAMBERCHAMBER

FUEL OXIDIZER

COMBUSTIONCHAMBER

FULL FLOW

STAGED COMBUSTION

CYCLE ENGINE

The sooty exhaust is dumped

overboard in an open-cycle

engine, resulting in a lot of

propellant wastage.

This is the type of engine

used on the Space Shuttle

main engine, also known

as the RS-25.

The use of oxidizer-rich

preburners meant that a

new type of metal had to

be developed to help the

turbines withstand the heat.

This is the most effi cient

engine type, giving you full

control of propellant  ow

to the preburner.

AUGUST 2019 | HWM 41