HWM Singapore – August 2019

FEATURE

ILLUSTRATIONS

ASHRUDDIN SANI

PHOTO

SPACEX

 Starship’s ambitions also call for a
completely new engine design. The
Raptor engine is far more powerful
than the existing Merlin units used in
the Falcon 9 and Falcon Heavy. More
importantly, while Merlin uses a mix
of RP-1 propellant, a type of kerosene,
and liquid oxygen, Raptor runs off liquid
methane, which would allow it to be
refueled on Mars where methane is in
abundance. Similarly, Blue Origin’s new
BE-4 engine will also use methane.
The Raptor is built around a full- ow
staged combustion cycle, which has only
been used by two rocket engines so far,
including the Soviet RD-270 and Aerojet
and Rocketdyne’s integrated powerhead
demonstrator (IPD) project. However,
none of these have actually  own, so if all
goes according to plan, the Raptor would
be the  rst such engine to send a rocket
into space.
Instead of having just a single
preburner, the Raptor features one
oxidizer-rich and one fuel-rich preburner.
In modern rocket engines, small volumes
of fuel and oxidizer are piped to the
preburner, where the resulting reaction
powers a turbine that in turn drives the
pumps that send more fuel and oxidizer
into the combustion chamber.
The reaction in the combustion
chamber is what produces thrust, and
it’s what enables the rocket to lift off.
The full- ow staged combustion engine
is generally considered to be the pinnacle

of rocket design, where it allows for the

most effi cient use of its liquid propellants.

To understand why, you  rst have to look

at the history of rocket engine designs.

THE OPEN-CYCLE ENGINE

Some of the best known rockets, such

as the Soyuz, Saturn V, and Delta IV,

have used an open-cycle engine.

The technology has been immensely

successful, and it works. However, it had

one major  aw. While the preburner is

required to turn the turbine, you can’t

use the same ratio of fuel and oxidizer as

the engine, because the resulting exhaust

would be too hot for the turbine.

Instead, you have to use a fuel-rich

mixture, which brings its own set of

problems like incomplete combustion,

if you’re using a carbon-based fuel.

This sooty exhaust cannot be

recirculated through the engine, so it

ends up being dumped overboard. That’s

plenty of waste right there, and you may

have even noticed this in pictures as a

distinctive black plume next to the

 ery exhaust.

A BIT OF ROCKET SCIENCE

CLOSING THE CYCLE

That said, the last thing you want on

a rocket is wastage, and Soviet and

American scientists ended up solving

two halves of the same problem. To

get around the problem of incomplete

combustion, the Soviets decided they

would opt for an oxidizer-rich mixture

instead, where all the oxidizer, but

only some of the fuel, was shot at the

preburner. Unfortunately, this created

its own problem. The oxygen-rich gas

produced by the preburner was now so

hot that no metal turbine could survive it.

Fortunately, they managed to solve this

by creating a special alloy that could

actually stand up against the heat.

The cleaner exhaust produced by the

oxidizer-rich mixture also allowed them

to close the cycle and pipe the preburner

exhaust – comprising hot gaseous oxygen

• into the combustion chamber, where it
  reacts with the liquid fuel.
  On the other hand, the Americans
  stuck with a fuel-rich mixture, but they
  swapped out carbon-based kerosene
  for hydrogen instead. However, the

„ One of the  rst to set foot on

Starship will be Japanese billionaire

Yusaku Maezawa. Maezawa is also an

avid collector of art, and as part of a

project called Dear Moon, he will invite

and pay for a group of six to eight artists

to come with him to the Moon.

The artists will come from varying

backgrounds, including music,  lm,

painting, and fashion. He hopes that the

face of Earth’s companion will

inspire these artists to create

unique works. “These artists will be

asked to create something after

they return to Earth, and these

masterpieces will inspire the dreamer

within all of us,” Maezawa said. That’s a

lofty goal to be sure, and the billionaire

was reportedly inspired by imagining

what would have happened if his favorite

artists had had the chance to

visit the Moon.

The crew won’t be landing on

the Moon though, and will instead

 y around the satellite and return to

Earth. The lunar  ight is currently

scheduled for 2023, although it

remains to be seen whether SpaceX

will meet that launch date or sail

haplessly past it.

PAINTING THE MOON

Test- ring of the

Raptor engine.

40 HWM | AUGUST 2019