Hobby-sized Aerospike Rocket Engine?

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deandome

Well-Known Member
Sometimes Wiki is a blast....

I looked up the Saturn V cuzza this weekends big launch....there was a link to aerospike propulsion https://en.wikipedia.org/wiki/Aerospike_engine , and there were these photos:

https://en.wikipedia.org/wiki/File:Non-truncated_toroidal_aerospike_nozzle.jpg
https://en.wikipedia.org/wiki/File:Aerospike_close-up.jpg

I get that the idea that there are 5-8" research rockets (can we get 'em?), but that rail looks, well, amateur...like we use. And that's pretty skinny for a liquid fuel rocket testing a fundamentally new motor...isn't it? Is the photo real?

Mike Di Venti

Well-Known Member
The first picture is. CTI did the testing or was at least involved with it.
They did that testing several years ago. Try digging around the archives on Rocketry Planet for more info on it.
I believe I may have seen an article in Extreme Rocketry magazine back then also.

Handeman

Well-Known Member
TRF Supporter
Sometimes Wiki is a blast....

I looked up the Saturn V cuzza this weekends big launch....there was a link to aerospike propulsion https://en.wikipedia.org/wiki/Aerospike_engine , and there were these photos:

https://en.wikipedia.org/wiki/File:Non-truncated_toroidal_aerospike_nozzle.jpg
https://en.wikipedia.org/wiki/File:Aerospike_close-up.jpg

I get that the idea that there are 5-8" research rockets (can we get 'em?), but that rail looks, well, amateur...like we use. And that's pretty skinny for a liquid fuel rocket testing a fundamentally new motor...isn't it? Is the photo real?
Very interesting pics. Looking at the ....nozzle.jpg I would estimate it at 14" dia based on a 1" high lettering on the fin identifications. As for the rail, that looks like a 3 or 4 inch by 2 inch machined steel rail mounted to a large aluminum support. Of course if the lettering is 1/2 inch tall, the sizes would be smaller. But the text did mention that the engine was 1000 lbs. so I suspect the larger size.

TWRackers

Well-Known Member
Very interesting pics. Looking at the ....nozzle.jpg I would estimate it at 14" dia based on a 1" high lettering on the fin identifications. As for the rail, that looks like a 3 or 4 inch by 2 inch machined steel rail mounted to a large aluminum support. Of course if the lettering is 1/2 inch tall, the sizes would be smaller. But the text did mention that the engine was 1000 lbs. so I suspect the larger size.
Here's a picture of the rocket, which was 10 feet long. It looks a little thinner than my 10 foot long, 6 inch diameter L3 project, so I'd say the aerospike rocket's diameter had to be no larger than 6 inches. I think the lettering is on 3/4" self-adhesive tape from a label printer similar to ones we have at work, so the lettering might be 1/2" tall.

https://www.dfrc.nasa.gov/Gallery/Photo/Aerospike_Rocket/HTML/EC04-0113-171.html

Handeman

Well-Known Member
TRF Supporter
Here's a picture of the rocket, which was 10 feet long. It looks a little thinner than my 10 foot long, 6 inch diameter L3 project, so I'd say the aerospike rocket's diameter had to be no larger than 6 inches. I think the lettering is on 3/4" self-adhesive tape from a label printer similar to ones we have at work, so the lettering might be 1/2" tall.

https://www.dfrc.nasa.gov/Gallery/Photo/Aerospike_Rocket/HTML/EC04-0113-171.html
If that's ten feet long, it certainly looks about 6" diameter. Would have to be 1/2" letters then.

daveyfire

Piled Higher and Deeper
The first is a rocket built by Scott Bartel (formerly of blacksky) and CTI in tandem with NASA Dryden for a Joint Propulsion Conference paper ("Flight Research of an Aerospike Nozzle Using High Power Solid Rockets" -- it's public domain if you want to read it). The motor is a CTI O5100 that was modified with the spike nozzle.

The pictured image is not really an aerospike -- it's technically a full plug nozzle, since the point of the spike is present. An aerospike is truncated at the base, letting the flow properties generate the "spike" instead (hence the name). Anthony has said that the plan was to progressively truncate the spike in subsequent tests, but the project was halted due to NASA budgetary reasons.

The second image is the base of a biprop aerospike built by CSULB/Garvey. Liquid rockets can be any size you want them -- I have one sitting on my desk right now that's 1.5" diameter and about 40" long -- a 6" liquid chamber is pretty typical for hobby and development-scale work. More info on the flight at https://www.garvspace.com/Kimbo7-A Folder/K7A.htm -- or here's a pic of the engine being static fired:

tagerton

Well-Known Member
I am quite interested in building an aerospike engine. However, I can't seem to find much in the way of designs for smallish aerospike enignes. If I understand it correctly, for a solid fuel rocket, the only real change would be the nozzle design.

In terms of the nozzle, I'm having difficultly visualizing how the 'spike' attaches to the rest of the motor / body. I can think of at least two different ways, but both of them would be difficult and present some problems with regards to flow of the fuel.

Any thoughts?
-Ted

RandyHood

Member
I have plans to incorporate an aerospike, or actually a full plug, into a 3/4" sugar rocket motor. My plan is to machine an aluminum rod into a "tear drop" shape so that it fits into the bottom of the motor with the appropriate gap for exhaust flow. It would probably be mounted with 3 screws through the motor casing, tapped into the aluminum plug to keep it in place. Thoughts?

Does anyone know of a reason this is not more commonly tried with hobby rockets? Difficult to implement? No increase in performance?
Even if those reasons are true, I still want to experiment with it, because it feels like I'm pushing the envelope somehow.

boatgeek

Well-Known Member
I have plans to incorporate an aerospike, or actually a full plug, into a 3/4" sugar rocket motor. My plan is to machine an aluminum rod into a "tear drop" shape so that it fits into the bottom of the motor with the appropriate gap for exhaust flow. It would probably be mounted with 3 screws through the motor casing, tapped into the aluminum plug to keep it in place. Thoughts?

Does anyone know of a reason this is not more commonly tried with hobby rockets? Difficult to implement? No increase in performance?
Even if those reasons are true, I still want to experiment with it, because it feels like I'm pushing the envelope somehow.
I think you'll need to go to a different spike material. Aluminum will probably melt at the exhaust temperatures you're looking at, unless it's a very short burn and the rest of the spike can be a heat sink for the surface.

RandyHood

Member
Thanks, boatgeek. I was wondering about that. Full burn will last 5 seconds max, then 5 second delay. The exhaust temp will probably be well in excess of the aluminum melting point. Any suggestions for materials that would work? Or perhaps adjust the propellant mix to operate at lower temps?

djs

Well-Known Member
Does anyone know of a reason this is not more commonly tried with hobby rockets? Difficult to implement? No increase in performance?.
Scott from Binder Design has said that when he tried an aerospike, there wasn't any measurable increase in performance.

RandyHood

Member
Scott from Binder Design has said that when he tried an aerospike, there wasn't any measurable increase in performance.
I kinda wish you hadn't told me that :eyeroll:.
Buuuut...I still wanna try it .

rharshberger

Well-Known Member
I kinda wish you hadn't told me that :eyeroll:.
Buuuut...I still wanna try it .
Wasn't it Mike Fisher current owner that did it. Scott Binder was the original owner of Binder Design, (Scott is the owner of Fusion Rockets). Mike Fisher is pretty heavy into EX motor building.

RocketFeller

Well-Known Member
Wasn't it Mike Fisher current owner that did it. Scott Binder was the original owner of Binder Design, (Scott is the owner of Fusion Rockets). Mike Fisher is pretty heavy into EX motor building.
That is correct, Mike is the one who has built Aerospike nozzles. I believe he didn't pursue it due to the small performance gains versus the amount of labor machining the nozzles.

dhbarr

Amateur Professional
In order for a spike to be significantly useful, you'd still need to be producing thrust post-30kft. Below that it's mostly a very heavy boattail, I should think?

djs

Well-Known Member
Wasn't it Mike Fisher current owner that did it. Scott Binder was the original owner of Binder Design, (Scott is the owner of Fusion Rockets). Mike Fisher is pretty heavy into EX motor building.
so yeah, i knew that, but somehow my brain didn't

RGClark

Mathematician
Scott from Binder Design has said that when he tried an aerospike, there wasn't any measurable increase in performance.

The increase in performance is under vacuum conditions. That's why it's difficult to test for amateurs. But some amateurs have sent rockets to 100k ft. At that altitude, the air pressure is about 1/100th that at sea level. Under those conditions there would be significant improvement in Isp.

Bob Clark

LithosphereRocketry

Pining for the Fjords
The increase in performance is under vacuum conditions. That's why it's difficult to test for amateurs. But some amateurs have sent rockets to 100k ft. At that altitude, the air pressure is about 1/100th that at sea level. Under those conditions there would be significant improvement in Isp.

Bob Clark
Not really. The increase in performance for an aerospike is over a range of altitudes- it doesn't get over- or underexpanded like a de Laval nozzle would, so it is closer to optimal at more different altitudes. De Laval designs are great for when you are dealing with one elevation. This is why aerospikes have been considered for spaceplanes more than normal staged launchers.

The fact that HPR projects have gone to 100K is somewhat irrelevant to this as well. Sure, they reach that altitude, but long after motor burnout. The motor burn is all in basically 100% atmosphere.

To the OP- I absolutely think you should try it! Even if it doesn't gain anything, it would be pretty darn cool just to make it at all!

boatgeek

Well-Known Member
Thanks, boatgeek. I was wondering about that. Full burn will last 5 seconds max, then 5 second delay. The exhaust temp will probably be well in excess of the aluminum melting point. Any suggestions for materials that would work? Or perhaps adjust the propellant mix to operate at lower temps?
I don't actually know. You would need to know your exhaust temperature. I just know it's hot. Titanium (!) is traditional for high temp applications, but it's possible some steel alloys would work too. There's some more info here: https://e-reports-ext.llnl.gov/pdf/233980.pdf

daveyfire

Piled Higher and Deeper
Ha, the green arrow from my previous contribution on this thread is nine years old! Let's keep it going...

Mods, censor this if it's too detailed, but it's nothing you can't learn from NASA docs or a rockets class.

You can get a decent approximation of exhaust temperature using an equilibrium thermochemical code.

https://cearun.grc.nasa.gov/
https://lekstutis.com/Artie/PEP/Index.html
https://rimworld.com/loggerusb/propep3/ProPep 3 Manual.pdf

Metal will probably work for a short burn time. The Bartz relation is good for hot-side convective heat transfer, equation 30 here:

https://ocw.mit.edu/courses/aeronau...pulsion-fall-2005/lecture-notes/lecture_7.pdf

or equation 4-13 here:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710019929.pdf

Don't forget about radiation, though if you're using sugar that'll be less of a problem. A thermal boundary coating may be helpful to mitigate this.

For the thermal profile in the metal, you'll need to think about the conduction equations in your particular spike configuration. I recommend Carslaw & Jaeger for some nice approximations that are fairly plug-and-chug.

Good luck.

RGClark

Mathematician
Not really. The increase in performance for an aerospike is over a range of altitudes- it doesn't get over- or underexpanded like a de Laval nozzle would, so it is closer to optimal at more different altitudes. De Laval designs are great for when you are dealing with one elevation. This is why aerospikes have been considered for spaceplanes more than normal staged launchers.
The fact that HPR projects have gone to 100K is somewhat irrelevant to this as well. Sure, they reach that altitude, but long after motor burnout. The motor burn is all in basically 100% atmosphere.
To the OP- I absolutely think you should try it! Even if it doesn't gain anything, it would be pretty darn cool just to make it at all!
You're right. I always thought it was mainly at high altitude that it had an effect. But this graphic shows it has importance at low altitude as well:

If you had an aerospike nozzle and were testing it at sea level, you can confirm it gave you nearly the thrust of a small diameter bell nozzle. You could test both types concurrently for example or depend on engine estimation programs for the small diameter bell nozzle case.

But we know such a small diameter bell nozzle loses drastically thrust at high altitude, near vacuum conditions. But by the graphic the high altitude performance for the aerospike will approximate instead the greater thrust of the large diameter bell nozzle.

Another comparison you can make is doing the aerospike sea level tests compared to a large diameter bell nozzle. As the graphic shows, the large diameter bell nozzle loses drastically thrust at sea level. In this case though if you were doing actual experiments with the large diameter bell nozzle also, you need to be aware that instabilities result when they are operated at sea level that can damage or even destroy the engine. Adequate safety protocols need to be followed in that case. Again from the graphic, you can conclude the aerospike will give comparable performance to the large diameter bell nozzle at high altitude, near vacuum conditions.

Note that in both these cases, you're still not really testing the aerospike at the high altitude, near vacuum conditions though. As Litho notes, most solids burn out well before they reach their max altitude. Then to test this scenario, perhaps long burning solid motors can be used that would still be burning when the rocket reached high altitude. Or perhaps the aerospike could be used on the second stage, which would fire only at high altitude.

Bob Clark

RGClark

Mathematician
The first is a rocket built by Scott Bartel (formerly of blacksky) and CTI in tandem with NASA Dryden for a Joint Propulsion Conference paper ("Flight Research of an Aerospike Nozzle Using High Power Solid Rockets" -- it's public domain if you want to read it). The motor is a CTI O5100 that was modified with the spike nozzle.

The pictured image is not really an aerospike -- it's technically a full plug nozzle, since the point of the spike is present. An aerospike is truncated at the base, letting the flow properties generate the "spike" instead (hence the name). Anthony has said that the plan was to progressively truncate the spike in subsequent tests, but the project was halted due to NASA budgetary reasons.
I think the terminology is the reverse of this. The aerospike is the one that comes to a sharp point, and the aeroplug or simply plug nozzle is the one that is cut off before coming to a point. The advantage of the aeroplug is that it is lighter in weight and there are not as great heating problems when you don't have the sharp point. It turns out also that the loss in efficiency is relatively small.

BTW, it's not just the aerospike that can do altitude compensation. There are lots of ways it can be done. For some other possible ways, on my blog Exoscientist.blogspot.com, type in the search box the phrase: altitude compensation.

Or use your own imagination to come up with some other ways.

Bob Clark

LithosphereRocketry

Pining for the Fjords
I think the terminology is the reverse of this. The aerospike is the one that comes to a sharp point, and the aeroplug or simply plug nozzle is the one that is cut off before coming to a point. The advantage of the aeroplug is that it is lighter in weight and there are not as great heating problems when you don't have the sharp point. It turns out also that the loss in efficiency is relatively small.

BTW, it's not just the aerospike that can do altitude compensation. There are lots of ways it can be done. For some other possible ways, on my blog Exoscientist.blogspot.com, type in the search box the phrase: altitude compensation.

Or use your own imagination to come up with some other ways.

Bob Clark
No, I think that's the right terminology.
https://en.wikipedia.org/wiki/Aerospike_engine

James Duffy

Well-Known Member
Here's a picture of the rocket, which was 10 feet long. It looks a little thinner than my 10 foot long, 6 inch diameter L3 project, so I'd say the aerospike rocket's diameter had to be no larger than 6 inches. I think the lettering is on 3/4" self-adhesive tape from a label printer similar to ones we have at work, so the lettering might be 1/2" tall.

https://www.dfrc.nasa.gov/Gallery/Photo/Aerospike_Rocket/HTML/EC04-0113-171.html
I whipped up a scale model of one of the Dryden Aerospace Rocket Test vehicles last summer. Airframe is a custom 33mm OD glass tube, nose and tail cones are 3d-printed parts from Shapeways. The mission logo is a bit of a hoot!

The workshop drawing of the model is attached as a PDF below.

Attachments

• 837.5 KB Views: 76

RGClark

Mathematician
No, I think that's the right terminology.
https://en.wikipedia.org/wiki/Aerospike_engine
Hmm. After a web search I've seen the two terms used interchangeably, so the terminology may not be standardized.

By the way here's another altitude compensation method that could be tested by amateurs:

Altitude compensation attachments for standard rocket engines, and applications, Page 2: impulse pressurization methods.
https://exoscientist.blogspot.com/2016/01/altitude-compensation-attachments-for.html

Take a look at this image showing the effect of the size of the nozzle dependent on ambient pressure:

https://en.wikipedia.org/wiki/Rocket_engine_nozzle#Aerostatic_back-pressure_and_optimal_expansion

The bottom one shows what happens when you use a long, vacuum optimized nozzle at sea level. This results in a significant loss of thrust at sea level and also results in dangerous flow instabilities that can even destroy an engine.

It's the impingement of the air pressure on the exhaust flow that causes the appearance of this overexpanded flow. Then the idea behind this other altitude compensation method is use a shelf or vanes inside the nozzle to counteract the force of the air pressure pressing on the exhaust flow.

Bob Clark

RandyHood

Member
Great discussion and resources. Thanks for everyone's contributions.
Back on the topic of material for my small, 3/4" motor: I had the consideration of forming a ceramic nozzle. To make it durable enough to withstand the exhaust velocity, I would try combining clay with high strength fibers of some kind. Perhaps steel wool, or even a ceramic fiber insulation I have on hand.

FredA

Well-Known Member
I've seen and talked with Mike Fisher about his Aerospikes.
Agree that for OUR usage, they are a ton of work for minimal gains.

They also tend to be single use as the spike doesn't do well as the first point of impact on landing.

Net result is too much work for one flight.....

manixFan

Not a rocket scientist
I'm pretty sure Pat Gordzelik was involved with that launch, in fact I remember seeing photos taken with a NASA supplied tracking camera. It was pretty cool but obviously never really went anywhere.

Tony

RGClark

Mathematician
Not really. The increase in performance for an aerospike is over a range of altitudes- it doesn't get over- or underexpanded like a de Laval nozzle would, so it is closer to optimal at more different altitudes. De Laval designs are great for when you are dealing with one elevation. This is why aerospikes have been considered for spaceplanes more than normal staged launchers.

The fact that HPR projects have gone to 100K is somewhat irrelevant to this as well. Sure, they reach that altitude, but long after motor burnout. The motor burn is all in basically 100% atmosphere.

To the OP- I absolutely think you should try it! Even if it doesn't gain anything, it would be pretty darn cool just to make it at all!
Surprisingly, eventhough the aerospike has been known about since the 60’s, there still has not been a test of the aerospike on an actual rocket at near vacuum conditions.
The best that has been done on a rocket so far is to about 30,000 feet, but this just reduces the ambient pressure to about 1/3rd that of sea level. But getting to 100,000 feet reduces the air pressure to about 1/100th sea level, where the ISP of a rocket is close to that of the vacuum value.
But flights to 100k are within the capabilities of amateur and student teams, at least for solid rockets. This has been by coasting to that altitude though, i.e., motor not firing then. Then to test the aerospike ISP at the high altitude would require attaching an additional stage to a rocket that is 100K capable, and igniting this stage only at the 100K altitude. This would require removing some recovery and/or instrumentation mass on the lower stage rocket so the original 100K altitude could still be achieved.
(Note Bene: given the dangerous premature ignitions at the Fireballs 27 meet of upper stage solid motors used by student teams it is strongly advised that this only be undertaken under supervision of professional rocket engineers experienced with upper stage rocket motors since to get reliable ignition at high altitude requires different and more dangerous methods than those commonly used on solids.)

To illustrate, how important it is to test the vacuum aerospike, the Falcon 9 first stage has a vacuum ISP of 312s. But the upper stage has a vacuum ISP of 342s. But a rule of thumb among propulsion engineers is that “every 10% increase in ISP gives a 100% increase in payload.”

Then amateur or student teams could make a key experimental advance that neither NASA nor the billion-dollar launch companies have made.

Bob Clark