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Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Ok, so I lied about this thing being tested soon, but alas it has now been tested. I made a new spike which is longer and nicer. If I were to do it again (which I will) I would use a composition with much less oxidizer (I mixed this stochiometric with both the epoxy and the magnesium fuel, but it burned very dirty and slowly). There is a video here (terrible sound, it was really windy), the flame was roughly 8-10 inches long. The rocket was pretty loud and intense, but since it burned so long and slowly I doubt the thrust was very impressive. No shock diamonds were apparent. We did not have time to set up a thrust measurement device in the field for this test.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Shock diamonds from a solid propellant rocket, is that does happen? I thought it was more of a liquid fuel kind of thing, if only because all the smoke makes it impossible to see.
Do you have an explanation for the slow burn-rate? I'd imagine that Mg in the fuel wold speed up the burn rate compared to "ordinary" Nitrate / organic fuel compositions. I suppose you have a ball mill to grind the nitrate before you mix the fuel?
Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
Cool rocket, good luck with your testing :)
I'll have to readup on aerospike rockets... I don't really understand what's going on with the tube down the middle - is that for the thrust or cooling or what?
Curious tho, why doesn't the heat from the fuel melt the copper nozzle part? I can see the walls standing up, but the constricted part doesn't melt away?
Would a thin walled stainless steel tube work better? Check out an auto parts store for stainless exausht tubing - at the very least it would reduce your weight I'd think.
And a word on graphite: Glassblowers (lampworkers in particular) use lots of graphite for our tools. There are good suppliers on ebay, but be careful about buying surplus graphite from labs as I've been told that some iof it can be fairly radioactive, and the dust from tooling may be quite toxic. (honestly, I've always wondered if that was true, or if the guy was just trying to sell more virgin graphite - comments?)
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Do you have an explanation for the slow burn-rate? I'd imagine that Mg in the fuel wold speed up the burn rate compared to "ordinary" Nitrate / organic fuel compositions.
Just a poor composition. The nozzle may not have been restrictive enough, too, so not enough pressure built to increase the burn rate significantly. I used 10% epoxy, 10.8% magnesium, and the rest potassium nitrate. That is stochiometric with both the magnesium and the epoxy, but since the epoxy is not very reactive, I guess it works out to being way too much oxidizer. This would explain the white buildup on the nozzle. Next time I can try something stochiometric with only the magnesium, or somewhere in between, so a higher magnesium content. Also I have ammonium nitrate to use now, and I have been synthesizing lithium nitrate which should also be promising. I am working on a magnesium atomizer to make more controlled burning fine spherical powder. Perchlorates, perfluorates, and nitronium compounds thereof are next on the list of oxidizers. Reactive organic fuels like hexamine and trioxane may show promise, and in the future I will experiment with the highly active light metal hydrides and laminated propellants to control the burn rate.
I don't really understand what's going on with the tube down the middle - is that for the thrust or cooling or what?
Blowing compressed air (or air from a scoop at the tip of the rocket) out the end of a truncated aerospike simulates a more or less infinite spike length, which is more optimal than a short spike. Having the air flow through a central tube helps to cool the core, so it retains strength and does not melt. In this test however, there was no core aitflow.
Curious tho, why doesn't the heat from the fuel melt the copper nozzle part? I can see the walls standing up, but the constricted part doesn't melt away?
Copper is very conductive, so it conducts the heat away from the hottest portions. Rest assured though, the rocket was very hot for a very long time after the burn (you could light firecrackers by touching them to the spike for like 10 minutes afterward, and the casing was very hot). Stainless steel would probably overheat and melt easily due to its very low thermal conductivity. My larger rockets will be aluminum with a titanium core tube. A shorter burn would undoubtedly result in less heating on the parts, even though it is more intense.
And a word on graphite: Glassblowers (lampworkers in particular) use lots of graphite for our tools. There are good suppliers on ebay, but be careful about buying surplus graphite from labs as I've been told that some iof it can be fairly radioactive, and the dust from tooling may be quite toxic. (honestly, I've always wondered if that was true, or if the guy was just trying to sell more virgin graphite - comments?)
I usually wear a respirator when cutting and maching graphite, but I didn't for this small job. Maybe I should have though.
Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
Very cool stuff, I just spent a couple hours going over rocketry stuff on wikiopedia and such. I now grasp the basics of an annular aerospike rocket....
so my next question.... would thrust be increased if you put a high velocity blower in front of your test bech? As I understand it the infinate air spike is only half the rockt, the other being the flow off the main body of the craft, which creates a low pressure zone right outside the nozle.... or something
Oh, and another note on graphite. Very fine steel wool then move to wet paper towels - makes for a great polishing cloth on graphite. a smoother cone may allow for better efficiency.
If you made enough of those spikes you could sell them to lampworkers as mini soffiettas (a tool to puff air into molten glass)
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Yes, something like a supersonic wind tunnel in front of the rocket would enable a more comprehensive test of the engine. There's also a FEA-type program out there specifically for designing and modeling aerospikes. I want it bad, but its $50 and I can't find it elsewheres.
These spikes are hand shaped and it takes many hours of work even for a smallish one. I doubt I would be very well off to sell them, unless people would to pay like $50 or something for one. That shape is really only good for aerospike nozzles too, but if someone wants to build an aerospike and doesn't have a good way to turn their own, I could possibly do it for them. I would think lampworkers would use a more arbitrary cone for shaping things.
Vigilatny Registered Member #17
Joined: Thu Feb 02 2006, 02:47PM
Location: NL
Posts: 158
joe wrote ...
Shock diamonds from a solid propellant rocket, is that does happen? I thought it was more of a liquid fuel kind of thing, if only because all the smoke makes it impossible to see.
I don't know of any reason for solid or liquid fuels causing shock diamonds, perhaps one is more visible than the other, but the actual formation should be unaffected.
That being said, you should never see shock diamonds with an aerospike nozzle. That's the whole point of an aerospike nozzle. Shock diamonds are a sign of the exhaust being over-expanded. With a properly designed aerospike nozzle you should always be optimally expanded. I'm pretty sure it is practically impossible to design an aerospike nozzle that would ever over-expand.
Vigilatny Registered Member #17
Joined: Thu Feb 02 2006, 02:47PM
Location: NL
Posts: 158
Well I stand corrected, apparently shock diamonds can occur with under-expansion as well as over-expansion. Still seems like there's something wrong witht the design to see such pronounced diamonds.
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Solid Fuel Annular Aerospike
