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Subject: Re: Aerospike (repost article)
From: (Ben Muniz (586-3578) ; /home/auspex_d0/usr2/yqq7121) 

In article <>,
Kevin W. Ryan <> wrote:
>   In regards to the design of and difference between plug and aerospike
>engines: it's been a while, but I think I can shed _some_ light on this.
>Feel free to correct me if I'm totally bogus on this.

You are correct.  It seems that I forgot to include when I 
posted the following a while back on  For those of 
you who've read it already, please excude my duplicate post.
--------------- cut here ---------------

A while back (pre split) I posted in regard to the following:

In article <1992Dec3.143759.2535@ke4zv.uucp> gary@ke4zv.UUCP (Gary 
Coffman) writes:
>tested. Later it intends to use aerospike engine designs that have
>*never* been tested, even on the ground. It will be difficult for 

On Date: Fri,  1 Jan 93 17:02:56 PST, Brian Stuart Thorn <BrianT@cup.> writes:
>   Much has been said recently about aerospike engines, specifically
>   in regard to potential use on the DC-1. I've never heard of them
>   before, can someone give me (and anyone else in the dark) a brief
>   description of what an Aerospike Engine is?
I've consolidated parts of my replys and expanded others.  Maybe someone
would like to edit this for the FAQ?:

The following information comes from an article "Nozzle Design" by R. A. 
O'Leary and James E. Bech in the Spring 1992 (No. 8) issue of Rocketdyne's 
*Threshold* magazine (call (818) 586-2380/2771 or write to Rockwell Aerospace/
Rocketdyne Division, 6633 Canoga Avenue, Mail Code AB57, Canoga Park, CA, USA, 
91304-7922 to get a copy).

What is an aerospike?

Briefly, an spike (or "plug") engine uses an exhaust nozzle that can be thought
of as a conventional bell shaped nozzle turned inside-out.  The aerospike 
nozzle is a truncated version of an ideal spike. Here is a cross-sectional 

       Bell           Ideal Spike           Aerospike

      xxxxx         x------------x       x------------x
     |     |         \          /         \          /
      \   /           \        /           \        /
     /    \            \      /             \______/
   /        \           \    /
 /            \          \  /

                               (x indicates the combustion location).

For a better diagram, see <Rocket Propulsion Elements> by George P. Sutton,
6th edition, J. Wiley & Sons, 1992, p. 70.

In a bell nozzle combustion gases flow through a constriction (throat) and 
then the expansion away from the centerline is contained by the diverging 
walls of the nozzle up to the exit plane. Bells nozzles are a point design 
with optimum performance at one specific ambient pressure (i.e., altitude).  
Careful design is needed to achieve desired high altitude performance while 
avoiding flow separation at the walls of the nozzle near the exit when 
operating at low altitudes (launch), which can lead to loss of performance and 
possible structural failure of the nozzle due to dynamic loads [flow 
separation is responsible for the large nozzle motion on the SSMEs during 
startup transient - watch closely during next launch].  Therefore a compromise 
altitude must be used for the design point of a bell nozzle.

In an spike nozzle the opposite takes place - the gas flow is directed 
radially inward from an annulus at some diameter away from the centerline.  
This flow is directly exposed to ambient pressure and its expansion is thus 
directly coupled to the external environment (continuous altitude 
compensation with no moving parts). Thus, a very high area ratio nozzle (high
vacuum performance) can also operate efficiently and safely at sea-level.

Truncating the ideal spike to save weight results in a wake at the base which 
has some performance loss.  This can be offset by pumping secondary flow (about
1% of primary flow) into the base region to elongate the wake which then forms 
an aerodynamic countour similar to the truncated structure (hence the name 

Have aerospike nozzles been been tested?

The article "Nozzle Design" states "During the 1960's, Rocketdyne tested 
numerous aerospike engines, ranging in size from subscale, cold-flow models 
to this 250,000-pound-thrust oxygen/hydrogen shown at a test stand in Nevada 
(picture of engine firing).  The low altitude performance advantage of 
the aerospike over conventional bell nozzle is clearly seen".  The curve 
presented looked like this:

             1.0 .----.----.----.----.----.----.----.----.----.----. 
                 |            + T  +T T   +   +/   +/   +/   +/ T +/
                 |    T +                   /
                 :  +                    /                  
                 |                    /           
             0.9 |                  / 
                 :                 /      Area Ratio = 75:1
  Nozzle         |                /       nozzle length = 25% equivalent 
  Efficiency     |              /                         conical
                 :              /
             0.8 |            /            /  Bell Nozzle
                 |           /             +  Aerospike (predicted)
                 :          /              T  Aerospike (test data)
                 |         /
             0.7 :----.--/-.----.----.----.----.----.----.----.----.
                 |      |     |     |     |       |     |       |
                 10    100   200   400   800    2000  4000    10000
                             Pressure Ratio:  Pc/Pa

Various propellants and both conical (1-D) and axial (2-D) models were 
been tested.  I have heard from several sources that Rocketdyne's 
*original* proposal for the Space Shuttle Main Engines used an aerospike 
design based on these tests.  At the California Space Development Council's
"Making Spaceflight Affordable" conference held in San Diego in February 1992, 
Vern Larson from Rocketdyne gave a presentation on the aerospike test program.
Also, I've heard reports that the Germans experimented with them during WWII, 
but I have not seen documentation to confirm this.

Why aren't they used?

At that same conference, I asked Max Hunter ("father" of the Delta rocket 
and a major player in the SSTO field) why it seemed that an aerospike was 
not baselined for the DC-X or the proposed DC-Y and DC-1.  He replied 
that there was concern regarding the lack of *flight-test data* (he 
acknowledged that there was plenty of ground test data), in particular 
for the transonic regime.  However, the Rocketdyne article states ". . . 
from Mach 1 to about Mach 3, *wind tunnel tests* (emphasis mine) indicate 
a drop in nozzle efficiency due to the slipstream turning into the nozzle 
region . . . Nevertheless, the interval of time that is spent in this 
adverse flight regime is short for typical flight trajectories, and 
overall performance of the aerospike nozzle remains well above that of a 
conventional bell-type nozzle".  Note that wind tunnel test results combined
with CFD simulations are usually sufficient for preliminary design of 
experimental aircraft flight vehicles.

Where can I get more information?

There are many references to aerospikes in the open technical literature:
one that I've seen noted but haven't had a chance to get is: Ballard, R. O., 
"The Aerospike/Aeroplug Engine: A Technology Development Summary", Sverdrup 
Technology Inc., MSFC Group, Contract NAS8-37814, 1991.

Ben Muniz (ISU '94)  w(818)586-3578
International Space Station Alpha: Structural Loads and Dynamics
Views/Commitments expressed do not represent Rockwell Aerospace/Rocketdyne

From: Henry Spencer <>
Subject: Re: What is the aerospike?
Date: Thu, 2 May 1996 21:15:47 GMT

In article <4m5ods$> writes:
>What's the concept behind the aerospike?

Actually, there are two different concepts for which that name is used.

The less frequent is a trick used in missiles which have very tight volume
constraints.  Long pointy noses are aerodynamically superior to short
blunt ones, but a short blunt nose with a spike sticking forward at the
center is much better than the short blunt nose alone.  You can improve it
further by continuously injecting a bit of pressurized gas at the base of
the spike.  This has been referred to as an aerospike. 

The more usual concept, in the space context, is an innovative nozzle
concept developed in the 1960s which has never quite made it to flight
(although at one point it was the leading candidate for the shuttle
propulsion system).  If your engine exhausts hot gas rearward and inward
around a tapered afterbody, it functions somewhat like an inside-out bell
nozzle:  the gas expands against the afterbody and supplies thrust.  The
nice part is that in a dense atmosphere, the flow hugs the surface of the
afterbody, while in a thin one, it expands widely... giving you much the
effect of a short bell nozzle at low altitude and a long one at high
altitude, compensating for the change in pressure.  One problem is that
the afterbody is heavy and difficult to cool... but you can chop it off
short and just exhaust a bit of gas (e.g., spent turbopump drive gas)
into the base, giving much the same effect.  That's an aerospike nozzle.
It's the best-known of the "altitude-compensating" nozzles.

(So why hasn't it flown?  A combination of politics, historical accident,
the stagnation of rocket propulsion in the US since the 1960s, and some
lingering doubts about aerodynamics at low supersonic speeds.  The latter
may be resolved soon -- LockMart, which wants to use an aerospike on its
X-33 design, is about to fly a test model on the back of a NASA SR-71.)
Americans proved to be more bureaucratic           |       Henry Spencer
than I ever thought.  --Valery Ryumin, RKK Energia |

From: Henry Spencer <>
Subject: Re: Technical Aerospike Question
Date: Sun, 21 Jul 1996 21:36:56 GMT

In article <4sensv$> (Todd Macdermid) writes:
>I'm still left with one question. I've got the impression that the
>gas around the aerospike is already supersonic as it travels over the

Correct.  It goes supersonic in the throat, and depending on design details
it may undergo some further acceleration before exiting the chamber(s) and
commencing to expand against the spike.

>However, it appears that the pressure just upon leaving would
>be higher than the atmospheric pressure...

Correct, necessarily so.

>So, anyways, why doesn't this gas instantly go through an expansion
>wave, instead of propogating down the spike in an orderly manner?
>Or does the expansion wave interation have to do with said propogation?
>(I highly suspect the latter, because they've worked in wind tunnels).

At the outer lip, the gas expands to ambient pressure immediately.  This
results in a series of expansion waves propagating inward, at an angle,
through the gas stream.  Where the last one hits the spike, gas pressure
is approximately ambient.  But the spike is still there, and is still
turning the gas outward, so a series of compression waves propagates
outward, again at an angle.  They reflect off the jet boundary as
expansion waves, and the whole process starts again.  The detailed flow
behavior is complex, and in fact the pressure on the spike oscillates as
you move downward, but the bottom line, seen from a distance :-), is
fairly orderly propagation down the spike and an *average* spike pressure
greater than ambient.

Detailed design of these things is not as simple as it looks.
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: Henry Spencer <>
Subject: Re: Technical Aerospike Question
Date: Tue, 23 Jul 1996 22:24:12 GMT

In article <4t02uq$> al jackson <> writes:
>>At the outer lip, the gas expands to ambient pressure immediately...
>When you say "outer lip",  is that the outer lip of the combustion

Correct -- the rim of the combustion chamber, farthest away from the spike.

>Which makes wonder do Aerospike combusion chambers suffer from the
>same kind of possible instability modes as do conventional rocket
>motor chambers?

Yes and no.  One fairly obvious way to build an aerospike chamber is as a
ring of more-or-less conventional small rocket engines with the nozzles
truncated just after the throat.  In that case, the instabilities are
those of the small engines, i.e. typically not much.  (Small engines have
much less trouble with instability than big ones.)

If you try to be cute and clever and build a continuous ring-shaped
chamber, or a ring broken up into a small number of segments, then yes,
instability is a worry.  Some of Rocketdyne's aerospike experiments with a
ring chamber had a fairly strong oscillation running around the ring.
(The linear configuration a la X-33, interestingly enough, had no trouble.)

It helps a lot to inject one propellant as a warm gas; expander and
staged-combustion cycles have an advantage here.  Neither the RL10 nor the
SSME had stability problems, but both the J-2 and J-2S had some, and the
less said about the LOX/kerosene and N2O4/hydrazine engines, the better. 
(This is one area, by the way, where hypergolics are probably worse rather
than better.)
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: "Jeff Greason" <>
Subject: Re: Combustion Chamber Design
Date: Thu, 7 Sep 2000 08:40:40 -0700

Michael J Wise <> wrote in message
> What is interesting me is, the RL10B and the AeroSpike seem to have thrown
> the whole concept of a "traditional" combustion chamber to the winds, or
> is it just me missing something? The RL10B's combustion chamber must be
> very small, if it can be missed in the picture with all the other plumbing
> around it.

Neither aerospike engines nor the RL10B do anything particularly unusual
with the combustion chamber.  The link you post to the RL10B is simply
showing an extendable high-expansion nozzle (which is what is covering
the whole assembly).  High expansion nozzles do tend to be large compared
to everything else.  The combustion chamber is pretty much the same as
all the other RL10's.

Aerospike engines (at least ones which appear to work), generally have
a number of combustion chambers of more or less conventional design;
it's the nozzle which is unusual, not the chambers.  There have been
designs for unusual chambers (toroids, for example), but they have a number
of practical problems with stability, cooling, or both.

> Could one use the aerospike configuration with other
> fuels, like H2O2/Kerosene? What kinds of modifications would need to be
> made to the design?

A nozzle doesn't care what fuel it's running on, as long as the shape is
appropriate to the task at hand.  Certainly, to the extent that aerospikes
work (a not fully resolved question), they should work equally well,
with the propellant combination of your choice.

> Or should I just enroll first in an engineering program, and ask my
> questions afterwards?

If you're actually thinking of building something, a background in
engineering is certainly something you want to have.
Engineering programs which really teach rocket design are pretty
scarce, though.  A combination of a good engineering education
and a large dollop of practical experience with high-pressure
plumbing and mechanical design seems to be a good foundation
to start from.

Before going much further, I'd recommend a thorough reading and
*understanding* of:
  _Rocket Propulsion Elements, 6th. ed._, G. Sutton

  _Modern Engineering for Design of Liquid-Propellant
     Rocket Engines_, Huzel & Huang

Not everything in them is gospel (Huzel & Huang, in particular,
tend to equate "how Rocketdyne did it" with "the only way
to do it", even when other companies had better results with
other methods).  However, they're pretty much the only good
books out there, and are definitely the place to start.

"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <>                <>

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