Aerospike Nozzles

[Ryan S.]

I was searching the net the other day and looking at pictures. then I came across the video of a firing of on Aerospike nozzle. I have looked but ccould not find how these things work. Does anyone know? Maybe a cut away drawing?



Im not planning on making one or something I just want to know about them....you know furthher my rocketry knowlegde

heres the URL

https://www.ddeville.com/aerospike.htm

 

[Juerg]

The common "bell shaped nozzle" is a compromise.
For ideal performance the exhaust gas should expand within the bell just enough tho have the pressure going down to ambient pressure.
During the flight the ambient pressure changes, so bell shaped nozzles are just perfect at one specific altitude.
For hobby rocketry this isn't an issue (maximum ISP isn't a development goal that would make big sense), for space flight however it is.
The Aerospike nozzle basically is an "inverted bell". One wall is the central cone, the other "wall" is the ambient pressure.
It is evident that the "ambient wall" adjusts to the ambient pressure, this giving near to optimal performance at any altitude.

Search the web for more info, I think Boeing has quite something online.

Juerg

 

[Zippy]

Thanks Ryan,

Seeing that video of the Aerospike makes me wonder...
Would an AeroTech composite motor benefit from having a scale like shroud aft of the nozzle? Not a spike as in the video but a bell shaped shroud as you would see on the engines of a Saturn V for example. I would like to make shrouds mainly for scale realism but is it possible they might help performance as well? Doesn't a shroud (on an actuall rocket) do the same thing basically as the Aerospike nozzle does? The string of pearls effect. I don't know what it's really called. Just throwing it out there.

Zippy

 

[Juerg]

Would an AeroTech composite motor benefit from having a scale like shroud aft of the nozzle?

Yes, they would gain some ISP. But the weight would go up and it is much more expensive to build such a nozzle. After all, not worth the effort. An optimized motor would make sense only in a (computer)optimized rocket. As long as we build rockets way to heavy and with plenty of unused space inside, it does not make sense to try to improve the ISP by expensive means. (Some additional propellant is much cheaper than the larger nozzle)

I would like to make shrouds mainly for scale realism but is it possible they might help performance as well?

You may do it like this:
https://www.argoshpr.ch/Images/Ariane020714_LaunchMikeWrobelPrep/41_Noisy_End.jpg
The motors are hidden (large boosters, barely visible) in the nozzle but they are less than half a nozzle diameter recessed into the nozzle. Otherwise you are risking what has happened here:
https://www.argoshpr.ch/Images/ALRS3_20021027_Dany/PA270098.JPG
The nozzle was used as motor retainer, but it influenced the exhaust and caused the gas stream to follow one side of the nozzle bell, thus creating a thrust vector not in line with the rocket
https://www.argoshpr.ch/Images/ALRS3_20021027_Dany/PA270081.JPG
Result: Disaster. I guess RocketMan had similar problems with the RocketBoy rocket.

Juerg

 

[Zippy]

Juerg,

Great pictures. I like that machine work on the last particularly. Too bad it doesn't work well. If I ever add shrouds to a model I guess they'll have to be of the inefective but pretty variety.

Zippy

 

[powderburner]

Originally posted by ZippyOgiveHead
Would an AeroTech composite motor benefit from having a scale like shroud aft of the nozzle?

Motor nozzles are already designed with throats and (very) short lengths of divergent nozzle. Attempts to add more external nozzle expansion chamber length to a hobby-quality motor would not likely be successful, and could significantly decrease performance.

The motors available to us have a wide range of operating conditions, from the initial thrust spike through the rest of the burn. Even the 'sustainer' burn rarely occurs at constant temperature, constant mass flow, and constant pressure. The same problem with optimization of bell nozzles for NASA-sized launch vehicles applies here: even if you get the nozzle shape right for one part of the burn, it will be wrong (and probably badly wrong) for all the rest.

You also run into the proverbial Krushnik effect in your model or sport rocket. The thrust is killed by swirling flow outside the motor nozzle, inside your added bell nozzle. You end up with less instead of more.

Even if our motors burned at a nominal constant thrust, we still have the problem of sputtering and spitting combustion debris through the nozzle; this problem is proportionally bigger for our motors than for the 'big' ones. These events cause momentary pressure pulses that further complicate nozzle design.

Finally, if you really do somehow achieve a useful containment of exhaust flow, you will have grabbed hold of some pretty hot, high pressure flow. What structural material will you use to build your bell nozzle? (a cardboard model of a nozzle won't get you far)

These big bell nozzles can certainly be modeled for static display, but there are many reasons why they are removed for flight. Anyway, from 500 feet below, where we are watching your rocket, no one can see the nozzle anyway?

 

[rstaff3]

Whether this is a good idea for HPR is being tested now. Aerotech is playing with an aerospike, and Mike Fisher has been discussing his efforts on ROL. (bad wording..these are 2 separate efforts)

 

[wyldbill]

Originally posted by rstaff3
...Mike Fisher has been discussing his efforts on ROL....

Here's the site....

FWIW,
-bill

 

[Zippy]

Those are all very good reasons for not adding a bell, but of course I'll probably end up doing it any way sooner or later. I'll just position the end of the motor at the end of the bell. Any less than perfect aerodynamics is not a problem since I don't mind if a scale model only goes 750' instead of 1000'.

 

[powderburner]

Originally posted by ZippyOgiveHead
I'll just position the end of the motor at the end of the bell.

That will work just fine. This effectively makes the bell just another part of the body-tube, from the perspective of form and function. The bell may get dinged up once in a while, but it won't hurt motor performance at all.

It's when you position the motor nozzle and the HEAD of the bell nozzle, and try to achieve extra thrust with an add-on nozzle, that you run into trouble.

 

[Ryan S.]

interesting I have found some more information but not much. I understand the concept of the Aerospike now, how it works, etc. However, there is one problem I keep coming across; how dows the thing stay in there? Is there a rod running from the back up to the forward closure? or is there an aft closure, but if there was the pressure would just push the nozzle agianst it and the motor would probably overpressurize. You guys no anything?

 

[rstaff3]

On this subject, you could pop over to ROL and just ask Mike how he is doing this. If AT releases such a nozzle it will most likely be held in like all their other ones.

 

[benjarvis]

Originally posted by Ryan S.
interesting I have found some more information but not much. I understand the concept of the Aerospike now, how it works, etc. However, there is one problem I keep coming across; how dows the thing stay in there? Is there a rod running from the back up to the forward closure? or is there an aft closure, but if there was the pressure would just push the nozzle agianst it and the motor would probably overpressurize. You guys no anything?

The ones I've seen are either supported by a rod up the center and/or vanes between the cone and the outer edge of the chamber....

A true toroidal aerospike (bi-prop) would tend to have a donut shaped combustion chamber around the base of the spike... think of it more as a tube ringed around a cone rather than a cone mounted in the end of a tube.


Ben

 

[Ryan S.]

ah that makes so much more sense, but would they still be able to be used with composite propellant if one were to make a case like that?

I found a picture just the other day and the spike part did have a rod coming up through the middle and there was also a ring that attatched right above what would be the convergance section in a regular nozzle, but I guess in thise case it would be the divergance section. I have seen a few other pictures and these include an aft closure. What is the point of the aft closure in this case? I know it cannot hold the Aerospike in because that would also hold the gases in.

 

[rstaff3]

The only thing I've now seen about aerospike nozzles is in regard to liquid motors, and that some of their performance gains are due to a small amount of secondary flow through the center of the nozzle. The solid motor nozzles I've seen have a full cone in the center and no apparent secondary flow.

Are these true aerospike nozzles? Do they have much performance gain without a secondary flow? Am I mistaken?

 

[bobkrech]

rstaff3

Yes, they are true aerospike nozzles.

Conventional expansion nozzles are a compromise since they are optimized for a specific external pressure, and most of the time the exhaust flow is either underexpanded or overexpanded. The aerospike nozzle is an attempt to increase the Isp of rocket motor over a large range of external pressures by using the atmosphere itself as the external element.

You won't gain anything for a model rocket motor since most flights only go a few thousand feet and the pressure variation over that altitude range is small. On the other hand, engines like the shuttle's operate for 8 minutes from sea level to the vacuum of space. If you want high efficiency at lift off you want a smaller area ratio than in space where you really want a large area ratio. The SSME engine has a nozzle area ratio of 76:1 which is a compromise. A longer nozzle with a larger area ratio would enhance performance at altitude but the extra mass would hinder overall system performance.

The aerospike design can have much less mass compared with a bell nozzle and always works at the optimum pressure ratio. The reason why you don't see them is that the cooling problems have not been solved sufficiently to insure long burn operations.

Bob Krech

 

[rstaff3]

Thanks, Bob. The stuff I saw was on liquid motors and like I said all the Aerospikes shown, had an open truncated cone. But there was no formal definition. In fact the ones that had long center cones were called Spike nozzles and the ones with short center cones were Aerospikes. Then there are the Linear Aeospikes.

I guess I could read up on them, but..naw..I'll let the TRF edikate me

 

[bobkrech]

You can get a lot of pictures, technical information, references and links on aerospike engines at https://www.aerospaceweb.org/design/aerospike/

Bob Krech

 

[rstaff3]

It still may just be semantics, but this reference agrees with the one I saw, showing how an Aerospike has a truncated cone with a center feed.

 

[bobkrech]

rstaff3

Yes. You are correct. There are some semantics since there are several different types of spike nozzles: the full-length isentropic airspike, and truncated aerospike nozzles with and without base bleed. In all cases the main hot gas injection occurs annularly around the widest part of the spike.

https://www.aerospaceweb.org/design/aerospike/inflow.shtml shows a cut-away of a full-length airspike nozzle. This type of spike nozzle is the most efficient, but is the longest.

The lower portion of the spike can be truncated and completed aerodynamically (thus the name aerospike) as shown on https://www.aerospaceweb.org/design/aerospike/x33.shtml to save length however you get some aerodynamic losses and create a recircultation zone of hot gases as shown in https://www.aerospaceweb.org/design/aerospike/aerodynamics.shtml which makes cooling a problem. To reduce the drag you can add some base-bleed gas which will also provide some cooling of the spike. The advantage of base-bleed is at the 2-3% of total flow level.

Bob Krech

 

[rstaff3]

Thanks, the latter description again agrees with what I'd read earlier.

 

[aksarben10]

Originally posted by bobkrech
rstaff3

Yes, they are true aerospike nozzles.

Conventional expansion nozzles are a compromise since they are optimized for a specific external pressure, and most of the time the exhaust flow is either underexpanded or overexpanded. The aerospike nozzle is an attempt to increase the Isp of rocket motor over a large range of external pressures by using the atmosphere itself as the external element.

(SNIP)

The aerospike design can have much less mass compared with a bell nozzle and always works at the optimum pressure ratio. The reason why you don't see them is that the cooling problems have not been solved sufficiently to insure long burn operations.

Bob Krech

I am still not conviced that the Fisher nozzles are real aerospikes or are just plug nozzles.

As I understand it, for it to be an aero-spike, the spike must be truncated and after the wake closes, there must be some thrust contribution from base pressure...don't you have to inject gas into the base to maintain the pressure? Where is this gas injection coming from?

From the pictures on the Fisher web site of the nozzle it does not look truncated at all.

Take a look at the nozzle for the California State's rocket, their nozzle is truncated. I am not sure if they have a gas injection but it could be getting it from the exhaust gas expanding against the virtual spike formed by the trapped gas at the truncated end.



https://www.csulb.edu/colleges/coe/ae/rockets/


Scott

 

[rstaff3]

Originally posted by aksarben10
I am still not conviced that the Fisher nozzles are real aerospikes or are just plug nozzles.

From these references it is clear they are not true aerospike nozzles. It will be interesting to see experimental analysis of these nozzles when applied to solids hovever.