Jon,
Ceramic with metal reinforcing mesh on the outside should do well, as long
as the opening size is much larger than the wavelength. That said, I'm not
sure that the absorber section needs to be all that high temperature,
because I think you can get away with extremely generous film cooling.
Envision a motor with three sections -- an absorber section, a mixing
section, and a nozzle. The flow entering the absorber has a core of doped
ammonia with a thick cooling layer of neat ammonia around it. The
microwaves pass through the wall of the absorber and the cooling layer and
superheat the doped ammonia. Given enough power density, you can get it
pretty much as hot as you'd like. Assuming that the cooling layer is thick
enough and residence time is short enough, the boundary layer should remain
cool and carry away any radiant heat deposited on the wall. The mixing
section has vortex generators to force the two streams to mix intimately.
The mixed hot gas then flows out the nozzle.
-p
On Wed, Dec 2, 2015 at 10:46 AM, Jonathan Goff <jongoff@xxxxxxxxx> wrote:
Pierce,
Interesting idea. Though making high-temperature, high-strength, microwave
transparent materials doesn't sound super easy either, but an gelled
ammonia engine using microwave absorbing particles does sound intriguing.
~Jon
On Wed, Dec 2, 2015 at 11:17 AM, Pierce Nichols <piercenichols@xxxxxxxxx>
wrote:
Jon,
Solid heat exchangers are a PITA -- they're heavy and their thermal
limits constrain ISP. I wonder if you could do the soot one better and mix
ammonia with nanoparticles of a variety of microwave absorbers (susceptors)
to get a desired temperature.* Then, instead of a heavy heat exchanger, you
just need a microwave-transparent plumbing section.
-p
*The reason you need more than one is each type of susceptor has a
temperature range. Ordinary glass is quite a good susceptor above red
heat... a fact which can be used to melt a glass bottle in your microwave.
On Tue, Dec 1, 2015 at 4:20 PM, Jonathan Goff <jongoff@xxxxxxxxx> wrote:
Doug,
Agreed on the disassociation benefits of Ammonia. The interesting thing
Kevin Parkin was mentioning on his site (
http://parkinresearch.com/microwave-thermal-rockets/) was that the soot
produced by methane decomposition at high temperatures is a good microwave
absorber, meaning it might actually enable heating the methane hotter than
if they had to transfer all the heat from the heat exchanger walls...
~Jon
On Tue, Dec 1, 2015 at 5:10 PM, Doug Jones <djones@xxxxxxxx> wrote:
In my opinion, ammonia is the unsung hero of non-combustion rockets. By
decomposing into N2 and H2 (no fouling), the molecular weight drops to 8.5.
lower than anything but helium and hydrogen. Yet at 200K & one bar the
density is 728 kg/m3, and at 2200K the vac Isp can hit about 3900 m/s. That
rho*isp is hard to beat.
Doug Jones
On 12/1/2015 3:43 PM, Jonathan Goff wrote:
On a note somewhat related to stable, high-performance monopropellants,
Kevin Parkin just announced the start of a San Francisco-based startup
to pursue his previous Microwave Thermal Rocket launch technology
(which
I think we discussed a bunch on aRocket back in the day). Hard to get
more stable than inert, and even with Methane or Ammonia, the
performance is still very respectable...
~Jon