
Aerospike Engines: Why aren't we using them now? [video] - hinkley
https://www.youtube.com/watch?v=K4zFefh5T-8
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qdonnellan
We used an aerospike nozzle for our senior design project at Texas A&M. I
thoroughly enjoyed the research and theory behind the concept, but when it
came down to actually building the thing we ran into a lot of - obvious in
hindsight - complexity.

The nozzle must be made out of graphite, the manufacturing of which is much
more complicated than aluminum, so throw out the door any pre-conceived
notions you may have that making any shape is possible. Our nozzle was a
three-part component: the inner plug (think of one of those "nose suckers" you
have to get boogers out of a baby's nose) and the outer nest in which the plug
sat. Between the nest and the plug you have "struts" supporting the plug to
allow for a gap (through which your fuel will be exhausted). The plug is
secured to the nest via some high-heat epoxy.

So yeah, way more complicated than a single bell of graphite, which is
probably the #1 reason why these haven't seen industry use (the cost of
complexity must be weighed against the cost of inefficiency, both are
important factors in any rocket application).

I am very interested in the "linear aerospike"[1] engine though, which would
obviously have design implications on the rest of the spacecraft, but
manufacturing a linear aerospike would be much simpler than what we attempted.

[1]
[https://en.wikipedia.org/wiki/Aerospike_engine](https://en.wikipedia.org/wiki/Aerospike_engine)
[The main image]

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burfog
I can see that Texas A&M made you use graphite, but otherwise I don't see why
the nozzle must be made out of graphite.

For a turbine blade, you'd use a single-crystal superalloy. The machining is
just 3D printing of wax; you then use a lost-wax process to cast it. Cooling
channels are built into it.

That ought to work. You don't even need a particularly corrosion-resistant
superalloy if you cool it with liquid methane and then let that leak out to
form a moving film on the surface.

~~~
avmich
> Cooling channels are built into it.

How would you build cooling channels into something like this?

~~~
ThenAsNow
Conventional ways to do this include casting or, as is done on the RL-10 and
SSME, building the nozzle out of thin-walled tubes (very labor-intensive and
expensive). 3D printing is the future here, but there's still lots of
improvement required in terms of wall thickness and roughness control, not to
mention build volume.

~~~
avmich
The previous phrase was that the part is lost-wax cast. To me it's interesting
how one can build cooling channels in the part which is cast.

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hliyan
Summary:

1\. An aerospike engine tries to overcome a problem in the traditional bell-
shaped nozzles: traditional nozzle shape and size can only be optimized for
one specific pressure (e.g. either sea level or a higher altitude). At all
other altitudes, it is inefficient.

2\. An areospike is kind of an inverted bell -- there is a spike in the middle
and the rocket exhaust flows around it. This forms a sort of virtual bell
nozzle (the inner wall is the spike's surface, the outer wall is the air
itself). As the atmospheric pressure varies, the virtual nozzle shape adjusts
too. This makes the areospike efficient at all altitudes (but not as efficient
as a bell nozzle at it's most optimal).

3\. Ideal for single-stage-to-orbit

4\. Work stopped (author claims) because in the space industry, tried and
tested is favored. And apparently, fuel is not such a large component of the
cost of a rocket launch.

~~~
wolfram74
When sentiments like this [1] are floating around, it's an easy argument to
make that companies aren't interested in fundamental research, and at most
tweaking off the shelf parts.

\-- [1]
[https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2...](https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2340)

~~~
hinkley
Got another article without a paywall?

~~~
wolfram74
mea culpa, I wouldn't have linked it if I'd known an hour later it would be on
the wrong side of a paywall. That's a behavior I haven't encountered before.

~~~
leoc
Someone once said that the FT paywall resembled something from the Unseen
University.

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jpk
One of the premises of this video is wrong. The author claims that rockets are
staged primarily because the engine bells need to be a different shape at
different altitudes to maximize their efficiency.

While it is true that engines designed for atmospheric and vacuum flight
employ different bell shapes for the reasons the author describes, this is not
why rockets are staged. The primary driver behind staging is to shed a portion
of structural mass after it's no longer needed.

For instance, there's no reason most rockets couldn't be designed to lift
their payloads directly to orbit in one stage, but by the time the vehicle
reaches the end of its flight profile, it would be accelerating a great deal
of structural mass to orbital velocity before deploying the payload. The near
empty fuel tanks, large engines needed to lift all that fuel you've since
consumed, etc are all dead weight at that point. If instead the vehicle drops
that mass mid flight and continues on to orbit in a much lighter
configuration, the total possible deltaV of the system is greatly increased.
Using second stage engine with a bell designed for vacuum only makes sense
here, but that's not why rockets are staged.

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caio1982
Because two stage rockets are now cheaper and easier to develop given track
records of bazillions of launches until today. Reusable rockets have
substantially reduced interest in aerospike design because it simply does not
solve an actual problem anymore.

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phkahler
It looks like SpaceX are perfectly capable of doing simulation and testing of
an engine like this. If they have it would be great to see a presentation on
their results and conclusions.

~~~
valuearb
They’d tell you that staging reduces the efficiency if Aerospikes enough that
they are not worth the weight penalty.

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exabrial
Why did Firefly aerospace switch after their takeover

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jl2718
One thing I never understood about SpaceX is whether it’s really worth it to
build a multi-use rocket. It’s a really simple design, and the materials are
not that expensive. When I was studying this stuff, it seemed that we were
moving in the direction of things like cheap self-consuming solid fuel
engines. It’s just a controlled explosion; seems like trying to create a
refillable stick of dynamite. Not only that, but reuse increases weight, which
increases size, which increases weight.... We all knew why launches were
overpriced, and it had nothing to do with the cost of the fuselage.

It seemed like Elon didn’t understand a lot of fundamentals, and I observed
that he had a lot of trouble recruiting Astro engineering experts because of
this, so he ended up with mostly young MechE students out of Cornell’s SAE.
Retro-propulsive landing is an interesting show piece, but how does it change
the economics? Note that SpaceX is still being propped up by NASA COTS, so
it’s far too early to call it a commercial success.

Can anybody better-informed help me understand what we’ve learned since about
the advantages of reuse?

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pjc50
Reusable rockets make sense for the same reason that reusable aircraft make
sense.

~~~
bewo001
Reusable airplanes would make less sense if they had to fly back empty all the
time..

