
The Tech Behind SpaceX’s New Engine - _JamesA_
https://hackaday.com/2019/02/13/the-impossible-tech-behind-spacexs-new-engine/
======
JumpCrisscross
“Benefits of the full-flow staged combustion cycle include turbines that run
cooler and at lower pressure, due to increased mass flow, leading to a longer
engine life and higher reliability.”

TL; DR Full flow lowers turbine temperatures at the expense of parts
complexity. Given turbopumps are the devil’s ass part of rocketry, this has
been a sought-after technology. The pay-off isn’t so much efficiency as much
as longevity. (This also explains why full flow hasn’t been a priority for
anyone until SpaceX.)

[https://en.m.wikipedia.org/wiki/Staged_combustion_cycle#Full...](https://en.m.wikipedia.org/wiki/Staged_combustion_cycle#Full-
flow_staged_combustion_cycle)

~~~
Tuna-Fish
> TL; DR Full flow lowers turbine temperatures at the expense of parts
> complexity.

Ehhh. I'd argue it also lowers part complexity, because it eliminates
interpropellant seals.

> The pay-off isn’t so much efficiency

It improves achievable chamber pressure (assuming similar maximum pressures
and temperatures at the turbines), which improves both thrust and Isp.

> (This also explains why full flow hasn’t been a priority for anyone until
> SpaceX.)

The reason it hasn't been a priority is more that no new development has been
a priority since the 70's.

~~~
avmich
> ...it also lowers part complexity, because it eliminates interpropellant
> seals.

But it adds the whole second gas generator and turbine. No, it doesn't lower
part complexity as a whole :) .

> It improves achievable chamber pressure

Yes, I'd even argue that's the penultimate goal of full-flow scheme.

1) For a rocket engine you usually (almost always) want as high Isp (which is
approximately the speed of gasses flowing from the rocket engine) as possible.

2) To get that Isp, you need as high pressure in the chamber as possible.

3) To get that high pressure, you want to supply your pumps with as much power
(in Watts) as possible from the given overall fuel flow (in kilograms per
second) - while not melting turbines blades (so fuel:oxidizer ratio is
limited).

4) To get most power to the turbines blades at fixed (maximum) gas temperature
and fixed overall fuel flow, you want to use all of that flow (classical
staged combustion uses only one component, which is essentially not using the
whole fuel flow) and adjust ratios fuel:oxidizer at both gas generators so
that total power would be maximum. You want maximum flow both because that
increases turbine efficiency and because that actually provides more power to
the turbine.

~~~
Tuna-Fish
> But it adds the whole second gas generator and turbine. No, it doesn't lower
> part complexity as a whole :) .

Because the fuel side and oxidizer side typically have a very different flow
rates, many engines have separate pumps and turbines for them on separate
shafts even if both sides use same fluids in the preburner. For example, see
the SSME.

With FFSC, each of the turbines have no sealing requirements. This means,
among other things, that bearings become easier and safer, as you can afford
larger margins in your design. In many ways, FFSC allows the individual parts
of the engine to be simpler.

The main cost of doing it over gas generators (other than oxidation-resistant
superalloys...) is that all parts of the engine sort of circularly depend on
each other -- when the Merlin engine was in very early development, they drove
into the desert and ran the gas generator and fuel pump on it's own with
basically no support infrastructure. Testing Raptor in a similar way is just
not possible. It would have never gotten built without the NASA Stennis
facility that can provide all the intermediate fluids at the pressures
required.

I fully agree with the rest.

~~~
avmich
> Because the fuel side and oxidizer side typically have a very different flow
> rates, many engines have separate pumps and turbines for them on separate
> shafts even if both sides use same fluids in the preburner. For example, see
> the SSME.

By "many" you perhaps mean "many American". In Russian engines it's mostly a
single shaft.

> The main cost of doing it over gas generators (other than oxidation-
> resistant superalloys...) is that all parts of the engine sort of circularly
> depend on each other...

Yes, rocket engine is usually a complex dynamic system, with deep feedback
loops. You can still get gas generator and turbines with pumps tested
separately, but you need to measure dynamic properties - like resonance
frequencies - in order to have better chances of the good work when the system
is integrated.

~~~
Agathos
> By "many" you perhaps mean "many American".

And by "many American," do you perhaps mean "pretty much just the Space
Shuttle Main Engine?" Since that one, has America launched any new staged
combustion engine off the ground?

~~~
avmich
I believe GP meant that not for closed cycle engines, but for all turbo-pump
driven engines, which in USA mostly means open loop (gas generator) engines.
In USA the tradition was to add complexity in form of gears and extra axes to
get pumps running with more optimal speed, while in Russia (USSR) it was to
reduce efficiency and gain in simplicity, robustness and mass.

~~~
Agathos
So even U.S. gas-generator engines like the RS-68 have this extra gearing?

------
paulsutter
To model Raptor's hypersonic turbulent combustion SpaceX used an internally
developed simulator, which uses wavelet compression to vary resolution across
many orders of magnitude in both time and physical dimensions:

[https://www.nextplatform.com/2015/03/27/rockets-shake-and-
ra...](https://www.nextplatform.com/2015/03/27/rockets-shake-and-rattle-so-
spacex-rolls-homegrown-cfd/)

Here is a fantastic talk from the NVIDIA conference:

[https://www.youtube.com/watch?v=txk-
VO1hzBY](https://www.youtube.com/watch?v=txk-VO1hzBY)

------
prando
[https://www.netflix.com/title/80119093](https://www.netflix.com/title/80119093)
My rocket-science knowledge is abysmal, but I thoroughly enjoyed this article
and the Netflix documentary that I have linked to was incredible. Anyone even
remotely interested in rockets should check it out :).

~~~
jbkkd
Unavailable for me in the UK, what's the name of the documentary?

~~~
jessriedel
"Cosmodrome".

------
russellbeattie
If you live in SoCal (or even if you're just visiting), definitely take a
quick trip by the SpaceX facilities in Hawthorne [1] where they have a Falcon
rocket sitting outside. From a distance it looks like and industrial chimney,
but as you pull up, you can see it's an actual rocket. Standing next to it
gives a great sense of scale the next time you're watching a SpaceX video.

1\.
[https://www.google.com/maps/place/SpaceX,+Rocket+Rd,+Hawthor...](https://www.google.com/maps/place/SpaceX,+Rocket+Rd,+Hawthorne,+CA+90250/@33.9212982,-118.3277877,17z/data=!4m2!3m1!1s0x80c2b5dee46db32d:0x5589bf4232c10232)

~~~
dmix
The street view:
[https://www.google.com/maps/place/SpaceX/@33.9206589,-118.32...](https://www.google.com/maps/place/SpaceX/@33.9206589,-118.3282779,3a,75y/data=!3m8!1e2!3m6!1sAF1QipP_m588OmCKOeDeZhEx_DECOZTqkZYNvMKWly_i!2e10!3e12!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipP_m588OmCKOeDeZhEx_DECOZTqkZYNvMKWly_i%3Dw203-h135-k-no!7i4608!8i3072!4m12!1m6!3m5!1s0x80c2b5dee46db32d:0x5589bf4232c10232!2sSpaceX!8m2!3d33.9212982!4d-118.3277877!3m4!1s0x80c2b5dee46db32d:0x5589bf4232c10232!8m2!3d33.9212982!4d-118.3277877)

------
mabbo
> While the Space Shuttle has long since retired, a variation of the engine
> itself will go on to power the Space Launch System. It will be the most
> powerful rocket NASA has ever built and is slated to begin missions in 2020.

2020? I honestly doubt we'll see the SLS launch before 2024 at it's current
rate.

~~~
Rebelgecko
I would bet money that SLS+Orion launches before Falcon Super Heavy+Starship.
It looks like SLS is already doing integration testing for the various cores
and starting to assemble the main components. I'd guess 2021 at the latest, as
long as there's no multi-month government shutdowns in the meantime.

~~~
btilly
Well, sanity check.

In 2017 the SLS launch was about 2 years out, but likely to slip:
[https://www.nasaspaceflight.com/2017/11/sls-managers-
troops-...](https://www.nasaspaceflight.com/2017/11/sls-managers-troops-
slip-2020/)

In 2019 the SLS launch is a bit under 2 years out, but likely to slip:
[https://arstechnica.com/science/2019/02/nasa-still-
working-t...](https://arstechnica.com/science/2019/02/nasa-still-working-
toward-2020-launch-of-massive-sls-rocket/)

This strongly reminds me of the fact that late software projects are promised
to be on time until about 6 weeks before launch, and then launch keeps getting
delayed. And this is true no matter how late it eventually turns out to be.
The reason why 6 weeks is the magic figure is that for a software project,
that's the point where you can no longer paper over the inevitability of
failure with wishful thinking. Rockets have slower schedule, but it is
strongly looking like 2 years is a similar magic figure in that industry, for
similar reasons.

In that light, the money quote from the second article is this:

 _However, the agency and its prime contractor for the core stage, Boeing, are
on a tight timeline that has little margin for technical problems that might
occur during the structural tests of the tank or the green run tests.
Historically, during this integration and test process with other large rocket
programs, major problems have often occurred._

I am generally a believer that what happened historically shouldn't be
ignored. There is therefore no way that the SLS will launch in 2020. Or 2021.
In fact nobody really knows when it will launch. Furthermore the upper stage,
aka Orion, is apparently in even worse shape. Right now they are going back to
the drawing board to try to find a design that gets costs down.

Admittedly SpaceX itself is promising the BFR in about 2 years, and also had a
history of overruns. I don't think that they will launch on time. But they
have a better history of getting launch vehicles up.

I will therefore happily take your bet for $100, but I would like to formalize
it a bit. I win if Falcon Super Heavy+Starship or whatever it gets renamed to
gets successfully launched to orbit first OR if SLS+Orion gets canceled first.
Vice versa you win if SLS+Orion gets successfully launched first OR if Falcon
Super Heavy+Starship or whatever gets canceled first. Note that "SpaceX goes
out of business" counts as canceled, even if someone else (eg Bezos) buys the
remnants and then launches something based on the work.

If my version is acceptable, you can contact me by email per my profile.

~~~
Hypx
That's not quite correct. SLS is still slated to launch June of next year. It
may get delayed to 2021 if problems are found. Also, Orion already flown in
2014, and will undergo an abort test in April.

~~~
btilly
You are right. After poking around for a bit, I found that SLS+Orion can
launch without the upper stage. The upper stage allows more to be carried on
the launch.

That said, I'm still on the side of betting with history. In most
organizations, the deadline is the first date that nobody can (yet) disprove.
When a deadline depends on problems not happening on this project that
historically have been common, I think it is safe to bet that history will
repeat itself.

This goes doubly for the SLS. Which is more ambitious than past launch
systems, and is being built so long after the last new launch system was
designed by the companies involved that there is little institutional
knowledge left about how to do it. (Furthermore building with competing
companies contracting for pieces that need to integrate just sounds like a
recipe for expensive overruns to me.)

As opposed to the BFR. Which is being designed by a company with more recent
experience of how to build new launch systems than the rest of the planet put
together.

~~~
Hypx
Most of the structural and integration tests are already complete though.
We're far into the "beta testing" phase as it were. Also, the BFR isn't
anywhere near the state that the SLS is in right now.

------
Gravityloss
The article is a bit incomplete. The space shuttle main engine already had two
main turbopumps. IIRC this is because the optimal pump speed is different for
pumping hydrogen and oxygen. They have very different densities. You want to
avoid gearing as much as possible.

And the main problem problem in the staged combustion cycle - in feeding the
gas generator exhaust to the main chamber is not that it's fuel rich or
oxidizer rich - it's that it's usually much lower pressure than in the main
combustion chamber, because it had to go through the turbine, which causes the
pressure to lower.

That's why engines like NK-33 have a separate boost pump for the gas
generator. RD-180 has more pump stages for the fuel entering the gas generator
(all oxidizer passes through the gas generator).

Full flow staged combustion is another way to solve this - put all the
propellants through the gas generators and turbines.

I realize it's hard to write popular technical articles about medium
complexity subjects and sometimes you have to take some shortcuts.

[https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#/med...](https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#/media/File:Ssme_schematic_\(updated\).svg)

------
mannykannot
The F1 engines of the Saturn V 1st. stage used a rich-mixture gas generator to
drive the turbopump, but its exhaust was fed into the engine's nozzle about
halfway down, through an annular manifold [1]. Up to that point, the
combustion chamber and nozzle were cooled by circulating fuel through them,
but beyond that point, the cooler turbopump exhaust layer protected the nozzle
extension.

In pictures of launches [2], you can make out the brown smoky annulus of the
turbopump exhaust, for a distance about equal to the length of the nozzle
extension, until it either mixes with the hot exhaust, or with ambient air and
then burns, at which point the smoke particles become incandescent.

[1]
[https://history.msfc.nasa.gov/saturn_apollo/documents/F-1_En...](https://history.msfc.nasa.gov/saturn_apollo/documents/F-1_Engine.pdf)

[2] [https://images.nasa.gov/details-
ksc-69pc-442.html](https://images.nasa.gov/details-ksc-69pc-442.html)

------
kumarvvr
This is an excellent article. Just curious, how much more efficient is the
full-flow engine?

And, what exactly is the deal with the seals the article is talking about?

Anyone have more info?

~~~
Symmetry
A staged combustion engine is intrinsically a lot more efficient than an
engine where you dump some of your fuel to power the pumps, like in a tapoff
or gas-generator engine.

Compared to a oxygen-rich full flow you can reach a higher pressure while
having better safety margins which will allow you to re-use the engine. The
higher your chamber pressure the closer the rocket's thrust at sea level is to
its maximum thrust in vacuum. Also higher pressure tends to correlate with
higher thrust to weight ratios.

Also, having both inputs already be gasses mean they mix better. This will
have a tiny benefit in them burning more completely. But more importantly
they'll burn better across a wider range of thrusts, which could be important
for throttling down the engine for landing.

~~~
SECProto
> Also, having both inputs already be gasses mean they mix better. This will
> have a tiny benefit in them burning more completely. But more importantly
> they'll burn better across a wider range of thrusts, which could be
> important for throttling down the engine for landing.

Also, and importantly for the design of the engine, it's much much easier* to
computationally model the mixing of two gases, to ensure complete mixing and
no combustion instability (especially when the reaction components are limited
to ch4, o2, and partial- and complete-reaction products like c02, c0, h2o,
oh-, ...).

* by much easier, i mean less impossible

~~~
Symmetry
Have you seen SpaceX's talk at some NVidia conference about their homegrown
Computational Fluid Dynamics simulator? It was really interesting.

[https://www.nextplatform.com/2015/03/27/rockets-shake-and-
ra...](https://www.nextplatform.com/2015/03/27/rockets-shake-and-rattle-so-
spacex-rolls-homegrown-cfd/)

~~~
SECProto
I hadnt seen that exact article, but i recall reading (or listening to a
lecture) that mentions how one advantage of methane is the possibility to use
CFD in design, while that was still outside the realm of possibility with RP-1
due to complexity of reactants (differing phases and many compounds)

------
tehsauce
Rocket lab's Rutherford engine uses a closed cycle with battery powered fuel
pumps. They were actually to fly an engine like this in their "electron"
rocket, as early as 2017.
[https://en.m.wikipedia.org/wiki/Rutherford_(rocket_engine)](https://en.m.wikipedia.org/wiki/Rutherford_\(rocket_engine\))

~~~
wolf550e
This is only viable for small engines, like expander cycle is only viable for
small engines. The specific energy of electric batteries cannot compare to
that of kerolox or methalox.

Closed cycle is not a new idea:
[https://en.wikipedia.org/wiki/Staged_combustion_cycle#Histor...](https://en.wikipedia.org/wiki/Staged_combustion_cycle#History)

~~~
twic
I still have a soft spot for expander cycles - they seem so elegant compared
to staged combustion cycles, and apparently i am not the only person who
thinks that:

[https://blogs.nasa.gov/J2X/2014/03/24/inside-the-leo-
doghous...](https://blogs.nasa.gov/J2X/2014/03/24/inside-the-leo-doghouse-the-
art-of-expander-cycle-engines/)

~~~
CarVac
Interestingly the expander bleed cycle mentioned at the end is going to be
used in the Blue Origin BE-3U, a change from the combustion tap-off of the
older BE-3.

------
debt
"For example, the turbine of the V-2 rocket was spun with steam ..."

Steam technology and rocket technology have a shared history.

That kind of bottleneck shape that a is quintessential shape of a rocket
engine, is actually a Steam-Engine-era technology called a de Laval nozzle.

I didn't know they used a tiny steam engine inside a V-2.

I think it's cool that the design of such an old technology, the steam engine,
lives on inside the design of such a new technology, the rocket.

~~~
trhway
>I didn't know they used a tiny steam engine inside a V-2.

it wasnt steam engine per.se., it was catalytic peroxide decomposition.

~~~
InclinedPlane
The Soyuz first stage uses the same system even today.

------
taf2
Love this article and if you read it with Curious Droids' voice it's even
better:
[https://www.youtube.com/channel/UC726J5A0LLFRxQ0SZqr2mYQ](https://www.youtube.com/channel/UC726J5A0LLFRxQ0SZqr2mYQ)

------
twic
_Either approach, whether it recaptures the oxidizer or fuel rich preburner
exhaust, is clearly an improvement over dumping everything overboard. But
neither is an ideal solution as there’s still potentially combustible products
being wasted._

Are there? Here's a diagram of the RD-180, which uses an oxygen-rich
preburner:

[https://en.wikipedia.org/wiki/RD-180#/media/File:Rd180schema...](https://en.wikipedia.org/wiki/RD-180#/media/File:Rd180schematic.png)

Where is anything escaping other than through the combustion chamber?

~~~
CarVac
There's no wasted propellant with normal staged combustion.

But full flow staged combustion lets you fully vaporize both propellants
before they mix, leading to more optimal burning.

~~~
avmich
More importantly, supply more power to the pumps thus allowing higher pressure
in the chamber, and so higher Isp.

------
w8vY7ER
As only a casual fan, this was revealing on what makes a full-flow staged
combustion engine so promising. Thanks for sharing

------
burfog
Looking at the list of cycles on Wikipedia, I'm surprised that nobody seems to
have used preburners to pressurize the tanks. Use a fuel-rich preburner to
pressurize the fuel tank, and an oxygen-rich preburner to pressurize the
oxygen tank.

Mixing should be limited even if nothing special is done, due to the
temperature and phase of matter and short timeframe. One could of course pay
the weight penalty of a piston (need not have a perfect seal) or collapsing
bag.

Doing a heat exchanger (to boil and thus pressurize) is another option, but
then you're back to needing a place for the exhaust. It would let you do a
sort of full-flow engine without turbopumps however, which is great. All those
issues with cavitation and lubrication and stress cracking just go away.

~~~
JumpCrisscross
> _I 'm surprised that nobody seems to have used preburners to pressurize the
> tanks_

This combines the worst of both worlds. You get the weight and complexity of
turbopumps. And you get the weight, explosion risk and leakiness of pressure
vessels.

~~~
burfog
Skip the turbopump. That is the whole point of pressurizing.

Pressurizing has weight advantages. You can use a balloon tank. The tank no
longer has to have the rigidity to support itself.

Rockets with typical cycles have used balloon tanks. Even modern ones like the
Falcon 9 are partially that way, with just enough rigidity to be erected empty
on the pad. The Falcon 9 uses helium to pressurize; that could be changed to
preburner exhaust even if you did keep the moderate pressure and the
turbopumps.

~~~
rbanffy
This is one of the reasons SpaceX uses supercooled fuel and oxydizer (and one
of the reasons for going methalox in the first place) - the tanks self-
pressurize, so they can skip the Helium system.

Pumps are still good because you really don't want the tanks to pressurize at
the same pressure as the combustion chamber. If you do, you'll see a rapid
disassembly, but your claim on it being unscheduled will be questioned.

~~~
ballooney
This is completely, completely wrong. Really honestly confused nonesense.
Supercooling lowers the ullage pressure, if anything increasing the need to
pressurise (self or otherwise), because the self pressurisiation (the vapour
pressure) is lower. So you have to do additional work to feed the pumps. The
only reason to supercool is to increase the density. Your comment is quite
incorrect.

~~~
jessriedel
I really appreciate your expertise and that you're taking the time to correct
the sometimes overeager amateurs in this thread. But can I suggest you tone
down the unnecessary invectives? It's true that there ought to be some social
penalty paid when people give confident wrong answers -- it's a pet peeve of
mine -- but I think it's sufficient to just say something like "Your comment
is incorrect. Here's why, in detail. Please don't project so much false
confidence next time." That still stings for the other person to hear, as it
should, but it leaves them much more likely to want to learn more. And it's
much less likely to escalate into a fight that distracts from the main
subject.

~~~
ballooney
All fair, valid and noted. It’s the eve of Opportunity’s demise, and i worked
on mars EDL, and the infinite tide of javascripters making statesmanly-yet-
quite-mistaken proclamations on physics can feel quite disrespectful to the
actual engineers who work on this stuff now, worked on the stuff on both sides
in the cold war, and everyone back to goddard, tchiolkovsky and moore. But you
are right. More whisky and no more HN.

------
le-mark
Nice article, but this part is a bit misleading:

> American engineers went in the opposite direction. They believed that a
> fuel-rich mixture in the preburner was possible and could be done with
> existing metal alloys, so long as hydrogen was used as the fuel instead of
> kerosene. This ultimately lead to the development of the Space Shuttle Main
> Engine, which to date remains the most efficient liquid fuel rocket engine
> ever flown.

SSME performance was due to H2 vs kerosene, it was not a full flow engine.

Edit; also no mention of Blue Origins BE-4 which is also a full flow engine.

~~~
Palptine
Not true. Be-4 is a staged combustion metholox engine.

~~~
itp
Be careful with terminology. BE-4 and Raptor are _both_ staged combustion
methalox engines. Raptor is full-flow staged combustion, while BE-4 is
oxidizer-rich staged combustion.

------
nisten
The reason this is important is not the really the few % savings in fuel
weight.

Any small improvement in exhaust velocity of the engine makes a huge
difference(sort of exponential) in the amount of payload it can take to orbit.
In this case the 2 pre-burners also make relighting the engine in a vacuum a
lot more reliable.

For more on the math:
[https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation](https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation)

~~~
avmich
Right, another reason to use full-flow scheme is that both components get into
chamber in the gaseous form - the heated gaseous form. And fuel usually
doesn't have a problem igniting in the hot oxygen flow, especially if that
fuel is in gas form itself, within milliseconds.

------
rbanffy
The RD-270 got me scared. That much hydrazine flowing through anything can't
be good.

------
buboard
its not impossible, considering that both the US and the russians built them
(but never flown). These articles usually go overboard with the hero-worship
and fail to mention that those are incremental improvements on the immense
rocketry feats of the 60s.

------
kiba
I don't like the usage of 'impossible'. Impossible by what standard?

Laws of physics forbidding it is understandable. We may likely never see the
development of FTL because our understanding as we know it would see it
impossible to do due to its strange implication about cause and effect.

~~~
maneesh
someone didn't read the article, or notice the quotes around the word
"Impossible"

~~~
kiba
There wasn't quotes around 'impossible' in the article.

~~~
mynameisvlad
Literally in the headline.

And they used other words next to the unquoted impossible in the article
itself like " _deemed by scientists to be next to_ impossible". Context
matters.

~~~
jandrese
The US scientists didn't think there was a metallurgy that could survive the
oxygen rich environment, but the Russians did the work to find it.

The part I'm confused about is how it talks about all of these 60s rockets
that close the cycle but aren't as efficient because they mess up the mixture
in the combustion chamber, but shouldn't the injectors be tuned for the excess
fuel/oxidizer from the get go? It doesn't seem like you should be exhausting
unburnt fuel/oxidizer if you have it tuned correctly.

