A beam window design for a 10's of MW continuous proton beam (thin, hemispherical metal shell): http://www.wpi.edu/Pubs/E-project/Avail ... SI0603.pdf
Other designs use nested hemispheres with coolant flowing between them. Convex towards the vacuum so that outside pressure prevents collapse/buckling.
For simplicity's sake, I still like the idea of using the alpha beams/cones/fans to directly heat airflow passing around the vacuum chamber, eliminating the HW mass for direct conversion and REB, but it's probably not doable unless a pressure-tolerant array of plasma windows can be developed. Material windows for alphas would have to be some super-thin yet strong and thermally conductive material (diamond?). Graphene doesn't seem to offer much hope: http://www.materialstoday.com/view/1606 ... -pressure/
...a single sheet of graphene is impermeable to helium gas atoms...
but maybe He nuclei could squeeze through without shredding it if the graphene lattice was positively charged.
The best route to simplify things might be Tom's idea to use electric arcs to heat the airflow... a grid of isolated, hermetically-sealed electrodes passing through the vacuum chamber wall. The alphas charge the electrodes and cause multiple arcs outside the vacuum chamber (inside the air ducts) as electrons from ground jump across multiple spark gaps. This would get rid of the "window" problem. Inject something downstream to reduce ozone for low altitude flight.
The facilities utilize a high-voltage, d-c electric arc discharge to heat air to temperatures up to 13,000 degrees Rankine. High-pressure test flows are achieved by confining the electrical arc discharge to a water-cooled plenum section capable of withstanding high chamber pressures above 100 atm.
Tom Ligon wrote:I remember bringing up the point about the e-gun needing to be in a high vacuum because we were having fits getting dispenser cathodes to work well. The bright discharges that sometimes occurred in our machines tended to kill cathode activity, so we were sensitized to the problems of deactivation and poisoning.
But at the same time we were looking at alternatives. Among these are diamond films and the ends of Buckytubes, both of which can be coaxed into emitting copious electrons, and they may resist deactivaton.
We also found tungsten filaments to be relatively robust, and I think they are the preferred emitters at EMC2 today. What I saw on my visit was nearly identical to some I made, possibly just copies of my design. I used halogen bulbs from a common model of car headlight, because they were readily adapted and I knew I could get more.
I don't think commercial electron beam welders use a super hard vacuum. Presumably one for a scram jet engine wouldn't need a super hard vacuum either. The polywell would, but I don't think the beam emitter would. I had always envisioned the concept as having a reactor and a beam emitter. Are you suggesting that the reactor be a combination reactor and beam emitter?
Quite right, the common plasma cutter has a high temp copper cutting head and skirt with the electrode having a hafnium tip/needle going through the axis. Hafnium is about the best at thermal tolerance, but even a plasma cutter in operation has the hafnium operating in a semi molten state at about 50,000 degrees in the plasma. Both Hypertherm and Thermocut brand cutting heads uses these technologies, and they are used in rather dirty industrial environments, typically shooting a plasma beam into metal that is awash in cooling fluid.
Tom Ligon wrote:Obviously, to be economical the craft would have to be reusable. The practical design will have to last orders of magnitude longer than 200 seconds.
The thermal equilibrium achieved allows the engine to operate indefinitely, for as long as there is fuel.
The X-51A engine was not designed to melt, unlike X-43A. They still let the test vehicle crash into the ocean afterwards... not even room for a tiny little parachute. I'll bet future versions will be recoverable just so that predicted thermal wear can be verified, but right now they don't have the budget for that.
For a Polywell flyer, chamber-protecting magnetic fields (mentioned by Dr. B for REB) and use of liquid propellent for cooling chamber walls are both possible.
The amazing thing about hand-held plasma cutters is that you can cut patterns in 1/4 inch metal with one, then apply it to your thumb with no obvious damage incurred.
Tom Ligon wrote:Ah, I see the X-51 is an ARC (All Regeneratively Cooled) design.
Yeah its quite innovative, the jet fuel is heated by the skin cooling system until it cracks into its constituent elements, then is injected into the airstream, so its technically burning hydrogen and carbon fuel with the o2 in the air without relying on the airflow compression to crack the fuel.
I suspect that the under performance issues are related to carbon coking up the fuel system.
It is very hard to test something like this in a wind tunnel. NASA has or had one that could hit high Mach, but not for this duration. It basically discharged a huge pressure bottle into a huge vacuum chamber thru a test section about nine inches across, for a few seconds. Ames supposedly still has hypersonic capability fast enough to run a scramjet, but probably not for 300 seconds.
In a cubbyhole within arms length of me is some Fortran code I worked on in college, modeling shock tubes. Plenty high Mach numbers, but just for microseconds. The temperature rise is impressive, but a shock tube is so fast it rarely takes a reaction to completion.
I think right now they would love to recover one of these. It is just a range safety thing ... they probably need a clear range for a thousand miles or more, and some way of floating the thing once it comes down. You could wait months for a combination of good weather and a clear range. Expensive.
Ground tests of the X-51A began in late 2006. A preliminary version of the X-51, the "Ground Demonstrator Engine No. 2", completed wind tunnel tests at the Langley Research Center on 27 July 2006.[10] In April 2007 an entire flight was simulated. The result was positive.
