Reverse hybrid rocket motor

[inonickname]

I've become interested in hybrid rocket motors lately (these use a liquid (or gas) and a solid as propulsive means- for example nitrous oxide and polypropylene). All of the hobbyist rockets I've found use a liquid or gaseous oxidizer with a solid fuel grain. While this is all well and good I'd like to try a reverse hybrid - with a solid oxidizer grain and a liquid fuel.

 

I'm thinking of make the oxidizer grain of ammonium nitrate- as most of my initial testing will be in small scale 30kg of it will last nearly forever, and using an alcohol or hydrocarbon (such as gasoline) as the fuel. The fuel will be pressurized to around 500 psi with a fairly inert gas such as nitrogen, and injected into the fuel chamber through a nozzle at high velocity. Plans are that it will then continue on as a regular hybrid.

 

Ammonium nitrate seems fairly suitable for this purpose- asides from it being used as an oxidizer it will also decompose into steam and nitrous oxide when heated. Granted, the decomposition can run away with undesirable results. However, the initial test motors will be very small and testing will be carried out with caution.

 

Does anyone have any experience, advice or thoughts on the subject?

 

(yes, I could make solid fuel rockets. yes, this is pointless. I'm doing it because I can)

[Miech]

Interesting concept. I think the first problem you'll need to tackle is getting the solid oxidizer to oxidize. In order to oxidize properly, there must be sufficient contact between the oxidizer and fuel. Further, the fuel must deliver enough energy to keep the ammonium nitrate above the temperature where it falls apart. The fact it melts before it decomposes may or may not be a problem here, I don't know what to expect. When you get this right, I don't see why it wouldn't work.

[inonickname]

Yeh, that will probably be the biggest problem. A gaseous or liquid oxidizer would be much more workable.

 

I have machining facilities, so I could make my own N2O and pressurize it.

 

NH₄NO₃ ---> N₂O + 2H₂O

 

The water will go to steam and the N2O may decompose. I was thinking simply heat it in a flask, condense off the water, capture the gas then pressurize it with a pump. 100g Ammonium nitrate should decompose into roughly 55 grams of nitrous oxide. Given that I have a lot of ammonium nitrate (and got it for free) this is probably a cheaper route than this:

 

http://cgi.ebay.com....=item2559589d2c

 

If it then works well I can up-size it and look into larger quantities or different varieties of oxidizer.

Edited April 4, 2010 by inonickname

[Miech]

That would turn it into a regular hybrid motor .

 

When making a regular hybrid, you could use the design found here: http://www.thefintels.com/aer/hybridmotor.htm It is still under development, but looks promising.

[Arthur]

The caution I'll offer is that ammonium nitrate has a crystal lattice phase transition in the 30 - 40 C temperature range, it expands! So the burning surface will be breaking away due to the expansion underneath. OR the grain could crack big time and the whole thing cato Edited April 4, 2010 by Arthur

[inonickname]

I experimented with decomposing some AN (in a test tube) and adding fuels such as ethanol etc. The results were fair, though in a rocket setup it would be difficult to implement. The amount of work to reliably get the AN hot enough (and keep it hot enough) is substantial. So I did a few small tests today with gaseous oxidizers. I'm using a "wet grain" instead of a regular fuel grain (eg. plastic). It's essentially a wick which is saturated with a liquid fuel (I used ethanol). As the pressurized gas passes over it at high speed the fuel evaporates quickly (aided by heat rather quickly).

 

I tried "highly absorbent" kitchen towels and old bits of cloth. Both seemed to work fine even with plain pressurized air. The chamber size was very small so it died down quickly, but still showed that the concept worked rather well.

 

With a better 'wick' and gaseous oxygen I should get impressive results. Hopefully I'll upscale my test tomorrow and get some video of it.