Aerospike engine to be tested at high altitude (Finally!)

The new start-up Arca Space Corporation will test this year for the very
first time, more than 50 years after the aerospike was developed, the high
altitude performance of an aerospike on their liquid-fueled demonstrator for
their planned SSTO:

Flight of the Aerospike: Episode 1.
https://www.youtube.com/watch?v=L1hnImvI2gw

ARCA to perform historic space flight of aerospike engine.
Published by Klaus Schmidt on Sat Jun 17, 2017 9:30 am

For 60 years the aerospace industry has been waiting for this moment, a
moment ARCA Space Corporation will offer to the scientific community in
August this year by launching the Demonstrator 3 space vehicle: the first
space flight of an aerospike rocket engine. Soon we are going to know if the
hopes and dreams of generations of aerospace engineers, in their pursuit to
create what is supposed to be the most efficient rocket engine in the world,
will materialize. We hope to confirm that aerospike rocket engines, which
are significantly more fuel efficient than the current engines, are
achievable and that they can lead the way to the creation of Single Stage to
Orbit rockets, which are more cost efficient and responsive.

https://spacefellowship.com/news/art...ke-engine.html

Rocket engine nozzles now are of a fixed bell shape. However, it is known a
bell nozzle is most efficient at a certain altitude and less efficient at
other altitudes. As an example, the SpaceX Merin engines for sea level use
have an Isp of 312 s. But the Merlin Vacuum optimized for vacuum use given a
much longer nozzle has a vacuum Isp of 348 s.

Since all orbital rockets operate in stages, this lack of efficiency at
varying altitudes was considered acceptable. But the SSTO concept
(single-stage-to-orbit) would use a single engine all the way to orbit. So
there were investigated methods such as the aerospike that could maintain
high efficiency at all altitudes.

However, it came to be believed that SSTO's were not feasible, either
technically or economically. So little research went into the aerospike.
This is extremely unfortunate. In fact, the aerospike can increase payload
for all rockets including staged ones. If the aerospike or other altitude
compensation had been used for existing rockets, it would have been observed
that at the increased Isp, several *existing* first stages could be SSTO's.
For instance, the famous Saturn 1C first stage of the of the Saturn V could
be SSTO with altitude compensation to give it a max vacuum Isp of 360 s
instead of its 304 s.

As I said this increase in payload using the aerospike can be significant. I
estimated it could be 25% for the Falcon 9. How much is the increase is
dependent on the individual rocket. But for a solid rocket launch system I
was investigating I found the aerospike doubled the payload to orbit!

This is because solid rocket motors typically get about 285 s vacuum ISP.
But using a rocket simulation program, I found the vacuum Isp could get as
as high as 325 s and above. The required bell size though would be quite
large, and would extend far outside the width of the rocket body, making it
impractical. But the aerospike could do this without extending outside the
rocket body's width.

In short the reason why the aerospike hasn't been used for any rocket liquid
or solid is because of the idea it is only useful for SSTO's and because of
the idea that SSTO's aren't useful.

Both of these ideas are incorrect.

Bob Clark

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