Post your designs here!

*cough*radon*cough*

I recommend amorphous carbon over silicon nitride, as its better in every way.

This isn't my idea. Ignition!(again!) pages 177-180:
"Why not use the densest known substance which is liquid at room temperature — mercury itself? Just squirt it into the chamber of a motor burning, say, acid-UDMH. It would evaporate into a monatomic gas (with a low Cp, which would help performance), and would go out the nozzle with the combustion products. That technique should give Phil all the density he wanted! Charmed by the delightful nuttiness of the idea, I reached for the calculator.


    For my calculations I used the monopropellant Cavea A, not only because it had a good density by itself (1.5) but because it would be simpler to handle two liquids than three in the wildly improbable event that things ever got as far as motor work. I calculated the performance of Cavea A with various proportions of mercury —up to six times the mass of the primary propellant. (It was easy to fit mercury into the NQD calculation method.) As expected, the specific impulse dropped outrageously as mercury was added to the system, but the density impulse (specific impulse X propellant density) rose spectacularly, to peak at 50 percent above that of the neat monopropellant with a mercury/propellant ratio of about 4.8. The next thing was to set up the boost velocity equation: cb =c In (1 + <pd), and to plug in the results of the performance calculations. I did this for various values of <p* plotting the percentage increase in boost velocity over that produced by the neat propellant against the percentage of the (fixed) tank volume filled with mercury rather than propellant. The result was spectacular. With cp = 0.1, and 27.5 percent of the tank volume filled with mercury instead of propellant, the bulk density was 4.9 and the boost velocity was about 31 percent above that of the neat propellant; at cp = 0.2 there was a 20 percent increase with 21 volume percent of mercury. At <p = 1.0, on the other hand, the best you could get was a 2 percent increase in boost velocity with 5 volume percent of mercury. Obviously, a missile with a low <p, such as an air-to-air job, was where this system belonged— if anywhere.

* <p, as you may remember, is a loading factor: the propellant tank volume divided by the dry mass (all propellants gone) of the missile. If there are ten kilograms of dry mass per liter of tank volume, <p = 1/10, or 0.1.

[Removed paragraphs]


    At NOTS, Dean Couch and D. G. Nyberg took over the job, and by March 1960 had completed their experiments. They used a 250-pound thrust RFNA-UDMH motor, and injected mercury through a tap in the chamber wall. And the thing did work. They used up to 31 volume percent of mercury in their runs, and found that at 20 percent they got a 40 percent increase in density impulse. (I had calculated 43.) As they were firing in the middle of the desert, they didn't bother with the scrubber. And they didn't poison a single rattlesnake. Technically, the system was a complete success. Practically—that was something else again."

name an engine that used radon propellant theholyinquisition

exactly

Indeed. Radon has two problems: it is quite radioactive, and it has a half-life of a few days.

Still, I'm glad it is a (totally non-viable) option for the lulz.

You mean as the nuclear fuel in an NTR?