

Under the Hood with the SpaceX Merlin Engine - pinehead
http://tuts.pinehead.tv/2013/01/14/under-the-hood-with-the-spacex-merlin-engine/

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stuff4ben
The livescribe link in the article is well worth watching. Fascinating, rocket
science is!

[http://www.livescribe.com/cgi-
bin/WebObjects/LDApp.woa/wa/ML...](http://www.livescribe.com/cgi-
bin/WebObjects/LDApp.woa/wa/MLSOverviewPage?sid=n7SMWV6PdG7r)

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dexter313
I think it's interesting that the engine is so small, are all engines of that
caliber that size or is this one special?

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ballooney
It's not really mentioned in the article but as a starting point they received
all the work done for Fastrac [1], a NASA program to build small, cheap,
expendable rockets - something like the Falcon 1. That got cancelled as NASA
projects are wont to do but it made sense to use that design as a baseline. A
clean sheet engine design is very complicated indeed - it's a very high
dimensional, highly coupled optimisation problem, so a known good starting
point is a very valuable thing to have.

Indeed the turbopump, which is one of the hardest bits of a rocket engine, is
made by Barber Nichols, who made the turbopump for Fastrac and who make the
turbopumps for the Merlin engines [2]. I've heard rocket engineers describe
rocket engines as turbopumps with some extra plumbing. Perhaps a slight
exaggeration but not far off, especially since their design is so tightly
coupled and dependent upon overall engine parameters. The degree of coupling
depends on the topology of the plumbing, or the 'cycle' of the engine, which
I'll try and explain:

There are three kinds of rocket engine cycle (well, there are maybe more but
these are the three that have been flown historically). The Expander Cycle,
the Staged Combustion Cycle, and the Gas Generator cycle. I'll mention the
last two.

Merlin, as the article mentions, is an example of a Gas Generator cycle [3].
In this cycle, you take off a little bit of fuel and oxidiser to burn outside
the main combustion chamber, to generate some hot energetic gases that you can
exhaust over a turbine. This spins the turbine up, which is connected to a
shaft with a compressor on the other end. The compressor increases the
pressure of the propellents so that they can be injected into the main
combustion chamber. This assembly (turbine, shaft, compressor) is called the
turbopump. It's necessary because the engines require very high flow rates to
get the thrust they need, and that has to be at a high pressure - higher than
the pressure of the combusting gases inside the combustion chamber, else you
wouldn't be able to inject it!

Back to the bleed-off to drive the turbine. You usually don't want a perfect
stoichiometric mix of fuel and oxidiser for this, or even close, because it
generates extraordinary hot gases that no turbine would last long in (The
turbines are spinning at many tens of thousands of RPM usually so would be
subject to much higher forces than the actively cooled walls of the main
combustion chamber). For this reason you usually have a large imbalance of one
propellent to the other to keep the temperature down. Usually you run with
excess fuel, or 'fuel-rich', as the opposite - oxidiser rich - means you have
hot oxidising gases which are harder on the metallurgy. I do know of some
russian exceptions to this, though, where fuel rich would have left sooty
deposits in the plumbing (The materials science employed in the turbines was
apparently so witchcraft that when the US got intelligence of oxidiser-rich
turbine precombustors, they thought is was deliberate counterintelligence from
the russians to get them to waste billions researching the impossible). The
gas generator cycle, as the article mentions, dumps this turbine exhaust
overboard separately. The problem with this is that there's a load of
uncombusted fuel in this exhaust, which you're just wasting, and this hits
your rocket performance - the Specific Impulse ( I_{sp} ), as you're not
getting as much bang out of a given mass of fuel as you could.

The answer to this is the Staged Combustion Cycle [4], where you also inject
the exhaust of the turbine into the combustion chamber to finish off
combustion. The performance of these engines is higher but the thermodynamic
balance to design a working system is a greater challenge, and some of the
engineering is a bit harder too. Staged Combustion engines are mostly russian,
although the Space Shuttle Main Engines are a US-design example of Staged
combustion.

SpaceX have been gradually and incrementally improving the Merlin's away from
their simpler beginnings, and it's been pleasing to watch as an interested
outsider. To bring it back to the OP question, "are all engines of that
caliber that size or is this one special?", Merlin didn't particularly stand
out in terms of power density in the early days, although it's been improving
and improving. Now there is the Merlin 1D [5], which claims to have the
highest thrust to weight of any rocket engine every made. One should take
these claims with a pinch of salt as what counts as 'engine' and what counts
as 'plumbing' and what counts as fuel tank is sort of open to debate and you
can do some creative accounting to make your numbers look better. However,
it's an impressive achievement regardless.

The metric that doesn't lie, from a performance point of view, is the mass
fraction of the rocket - that is the fraction of the all-up, fuelled-up mass
of the rocket on the pad that makes it into orbit. The higher the better -
i.e. you can take bigger payloads for a given size rocket. Note this is just
from a performance point of view, not an economics point of view.

However, increasing mass fraction will be important to SpaceXs staged aim of
re-usable rockets. That's because as well as the payload, each rocket stage as
to also carry the fuel it needs to land. I believe Falcon 9 can launch about
2% of its pad mass into orbit (i.e. the payload can be 2% of the total mass),
and Musk reckons if that could be increased to about 4 or 5%, there's be
enough margin to carry enough landing fuel and extra landing hardware like
legs.

So my bet, just for fun (I'm not connected with SpaceX, this is just sideline
speculation for the sake of interest) is that you might start to see
development of a Staged combustion engine instead of gas generator, and a
switch to methane fuel which, with LOX, has a slightly higher specific impulse
than Lox/Kerosene which they currently use. Maybe 20 seconds extra (seconds
being the unit of specific impulse), which is maybe 5-7% more than they might
be seeing now, which is worth having. Methane is nice because it has a similar
density to kerosene. Lox/LH2 has almost 50% better performance than
LOX/Kerosene in theory but LH2 is of a very much lower density, so the tanks
must be much bigger (the illustrates my earlier point about the slipperyness
of engine-only thrust to weight as a metric - how much bigger and heavier are
the tanks?).

[Edit: of course instead of building a higher performance engine you could
just built a much bigger rocket with the same mass fraction and have the cargo
be a smaller percentage of launch mass. probably cheaper than developing a
staged combustion engine. I suspect the rocket science equivalent of more
servers vs a rewrite in C]

This post has ended up being a bit longer than I thought it would. Hopefully
of some interest if you're new to the subject.

[1] <http://en.wikipedia.org/wiki/Fastrac_(engine)> [2] [http://www.barber-
nichols.com/products/rocket-engine-turbopu...](http://www.barber-
nichols.com/products/rocket-engine-turbopumps) [3]
<http://en.wikipedia.org/wiki/Gas-generator_cycle_(rocket)> [4]
[http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket...](http://en.wikipedia.org/wiki/Staged_combustion_cycle_\(rocket\))
[5]
[http://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#M...](http://en.wikipedia.org/wiki/Merlin_\(rocket_engine_family\)#Merlin_1D)

~~~
FrojoS
Thanks a lot. Very interesting!

> For this reason you usually have a large imbalance of one propellent to the
> other to keep the temperature down.

So how large is that imbalance usually, or in the case of the Merlin 1C?

> although the Space Shuttle Main Engines are a US-design example of Staged
> combustion.

Is this, why their exhaust is so clean? How much more efficient is this engine
compared to the Merlin?

PS: I'm excited, this engine will be used again in the new super heavy lift
[1] that NASA is developing, though obviously, I can't tell if its a good
idea.

[1] <http://en.wikipedia.org/wiki/Space_Launch_System#Core_stage>

~~~
schiffern
> _Is this, why their exhaust is so clean?_

The Space Shuttle Main Engines used LOX and LH2, which creates water when
burned.

> _How much more efficient is this engine compared to the Merlin?_

The specific impulse is the metric to look at, and ballooney already talked
about it.

