Orbitec's VCCW thrust chamber and Polywell

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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DeltaV
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Joined: Mon Oct 12, 2009 5:05 am

Orbitec's VCCW thrust chamber and Polywell

Postby DeltaV » Fri Jun 04, 2010 10:23 pm

Another long-shot weight-reduction scheme for a flying Polywell (SSTO with unlimited atmospheric cruise, ozone be damned). Trying to get rid of the UHVDC down-conversion mass, REB mass and thrust chamber magnetic shield mass.

Orbitec's Vortex Combustion Cold-Wall (VCCW) thrust chamber uses a coaxial, reverse-flowing vortex to obtain very low heat loads on thrust chamber sidewalls.

http://www.orbitec.com/propulsion.html

http://maji.utsi.edu/publications/pdf/AIAA20034473.pdf

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I'm thinking alpha-charged (no down-conversion) electric-arc airflow heaters* with coaxial anodes and cathodes, flow-shaped, with the heater assembly coaxial with and internal to a VCCW thrust chamber. A crude working assumption is that electric arc heating would give similar behavior to chemical combustion. Auxiliary compressors needed to force air through the (larger for air) swirl injectors until a high enough airspeed is reached for ram pressure to work.

AEDC claims up to 120 atm for their arc heaters operating at dozens of MW, but they use lower voltage and higher current:
Normal operating conditions for the heater are about 20,000 volts and 1,200 amps, providing heater chamber pressures up to 120 atm at high stagnation enthalpies.

6GW Polywell, 14 cusps, (429MW/cusp)/(1.5MV)=286A/cusp ignoring losses.

* [Edit] Arc heaters electrically in series so that the voltage drop across an individual heater allows reasonable dimensions without risking arc-over outside the working zone.

DeltaV
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Joined: Mon Oct 12, 2009 5:05 am

Postby DeltaV » Sun Jun 06, 2010 6:40 am


zapkitty
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Postby zapkitty » Sun Jun 06, 2010 10:27 am

If you're using air augmentation then, while you may use the vortex to keep the plasma from eating your nozzles, you are still going to be moving a lot of air that while hotter than jet exhaust won't be heated to anywhere near plasma temperatures... so could that cooler air be used to cool the nozzle walls instead while the plasma is heating the air?

And outside of the nozzles is the ducting. The primary drawback to air augmentation is the weight of the ducting but designers were willing to work with that even for the chemically-powered Nova rockets...

Now, this design is oversized for the task, 500 tons to LEO, but it's a VTVL design that I think, in a smaller version with 2 polywells stacked where the LH2 tank is, might be closer to the results you want:

Image

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... okay, I admit it...I've just always liked this design... :)

Stoney3K
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Joined: Sun Jun 28, 2009 9:24 pm

Re: Orbitec's VCCW thrust chamber and Polywell

Postby Stoney3K » Sun Jun 06, 2010 11:10 am

DeltaV wrote:I'm thinking alpha-charged (no down-conversion) electric-arc airflow heaters* with coaxial anodes and cathodes, flow-shaped, with the heater assembly coaxial with and internal to a VCCW thrust chamber. A crude working assumption is that electric arc heating would give similar behavior to chemical combustion. Auxiliary compressors needed to force air through the (larger for air) swirl injectors until a high enough airspeed is reached for ram pressure to work.


That sounds almost like using the electric arc heating in a Brayton cycle gas turbine engine for low-speed operation.

Looking more and more like this....

Image

Slap on a Mach drive, shove the Polywell in the rear and get ready to roam the black.

:mrgreen:
Because we can.

DeltaV
Posts: 2245
Joined: Mon Oct 12, 2009 5:05 am

Postby DeltaV » Sun Jun 06, 2010 8:54 pm

zapkitty wrote:If you're using air augmentation then, while you may use the vortex to keep the plasma from eating your nozzles, you are still going to be moving a lot of air that while hotter than jet exhaust won't be heated to anywhere near plasma temperatures... so could that cooler air be used to cool the nozzle walls instead while the plasma is heating the air?:)

The basic idea of VCCW is a reversed-flow vortex, and it's more about keeping the thrust chamber cool than keeping the nozzle cool. In Orbitec's version the cold outer vortex is mostly oxidizer and the hot inner vortex, moving axially in the opposite direction, is where the combustion takes place after fuel is injected at the top. It works so well that for some tests involving H and O combustion they were able to use a clear acrylic cylinder for the outer wall without melting it, so they could photograph the combustion zone. This behavior is opposite that of the well-known Ranque-Hilsch vortex tube, which has a hot outer vortex and a cold reverse-flow inner vortex.

After reading more about segmented arc heaters being the most efficient type, I think that heater design considerations should overrule VCCW design considerations, but there might be some potential there for combining the two concepts, say, to eliminate the water cooling typically needed for arc heaters.

zapkitty wrote:And outside of the nozzles is the ducting. The primary drawback to air augmentation is the weight of the ducting but designers were willing to work with that even for the chemically-powered Nova rockets...

Now, this design is oversized for the task, 500 tons to LEO, but it's a VTVL design that I think, in a smaller version with 2 polywells stacked where the LH2 tank is, might be closer to the results you want:

That's another possible arc heating application.

The vehicle configuration I'm considering here (another Plan B... Plan A is still Mach Effect) is more along the lines of a slightly flattened, more rounded X-33, maybe intermediate between X-33 and Lenticular Reentry Vehicle. The flattened, large-planform shape makes VTVL a little easier when you consider Polywell cusp locations, and provides lots of area for thermal radiators as well as making the vehicle fluffier for reentry (lower ballistic coefficient) so that metallic TPS might be used. It would operate as VTVL for lighter payloads, but could also do HTHL for heavy payloads. In either case, vertical and horizontal thrust would be obtained by electric arc heating of airflow at low to medium altitudes (no additives, unless something is absolutely needed to reduce ozone near the ground), transitioning to heating onboard propellant as the air thins out on the way to orbit. Able to cruise indefinitely in atmosphere prior to orbit boost and after reentry, since the main consumables in that mode are small amounts of H and B11. Intakes and exhausts generally within the aeroshell envelope if possible (maybe pivoting or retractable for VTVL like Excalibur, but with all of the lower surface sealed for reentry).

DeltaV
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Re: Orbitec's VCCW thrust chamber and Polywell

Postby DeltaV » Sun Jun 06, 2010 9:06 pm

Stoney3K wrote:That sounds almost like using the electric arc heating in a Brayton cycle gas turbine engine for low-speed operation.

Yeah, it might end up like that. Or, maybe use Polywell waste heat to spin a separate turbo-compressor for takeoff and acceleration. The ram air inlets would be closed to build up the pressure for the air swirl injectors and then opened at speed.

DeltaV
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Joined: Mon Oct 12, 2009 5:05 am

Postby DeltaV » Mon Jun 07, 2010 7:26 am

Grrrr... further research shows that arcjets used for satellite thrusters have dismal peak thrust (Newtons), and efficiencies only around 30-40% (though above we're talking about 3-5 orders of magnitude more electrical power). I got overly excited when I saw test facility arc heaters with chamber pressures about 1/2 of the SSME's chamber pressure. It'd be interesting to know what effect "fractalizing" the arc discharges would have on energy transfer, electrical to airflow. In any event, the reverse-flow vortex design might still be useful for thermally managing other heating methods, like REB. Maybe a REB self-fractalizes as it interacts with air.

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