Direct heating of air by alpha particles

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

Moderators: tonybarry, MSimon

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

Direct heating of air by alpha particles

Post by DeltaV »

Some questions for people better versed in particle beam physics than myself:

The range of 5 MeV alphas in air (1 atm) is about 3 cm. Polywell's 2-3 Mev alphas should be absorbed in less distance than that.

If the magrid field is strong enough to cause alphas to exit roughly along coil axes, as has been suggested elsewhere on this forum (and maybe also out the magrid corners, but no more than 14 crude "beams"), would it be possible to use something like electrostatic lenses (maybe with additional, smaller magnets) to further concentrate the alphas after they leave the magrid and have them exit the vacuum chamber into air through a small aperture?

Would the air (or maybe air+propellant) be heated sufficiently by the 2-3 MeV alphas to be used for propulsion? Or, would secondary particles steal the show? I'm thinking of use at low altitudes where ozone would be bad, precluding REB for QED/ARC.

This picture of a cyclotron beam in air shows that you can get an ion beam out of a vacuum chamber. May not be steady state, though, I don't know. Would any air molecule dare try to enter the vacuum chamber against such an ion outflow?

Image

kunkmiester
Posts: 883
Joined: Thu Mar 12, 2009 3:51 pm
Contact:

Post by kunkmiester »

There are other ways to keep the air out, so don't worry about that part. Tesla designed an open ended vacuum tube in the early 20th century, so that sort of thing is old news.

I can't help you with the air heating part. Probably be better to boil water and inject the steam into the airstream as a working fluid. You have to carry more mass, but it's an improvement on what we have now.
Evil is evil, no matter how small

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

I don't know if 3 MeV alphas would be sufficiently worse at ozone production than 1.5 MeV electrons to justify the effort. They're still ionizing radiation...

Besides, if I'm not mistaken, at high power levels beam self-spreading due to charge concentration starts to become significant, and guiding the beam out a narrow window like that may prove difficult or impossible.

Actually, I'm thinking ducted fans running on HVDC motors would probably be best for low altitudes. They would have higher efficiency than heat engines, even without regenerative cooling (dumping heat to a high-density non-hypersonic airflow is way easier than trying to radiate it in a vacuum, so full power shouldn't be a problem, and it probably isn't worth it to include equipment for dumping the heat into the engines without the main reactor power increasing the temperature ratio), and should be fine up to Mach 2 - Mach 3 or so at least, after which you really should be high enough for REB heating.

Now the question is: How feasible is a compact, relatively lightweight MV-range DC motor in the required power class? (The fact that it's GW-range should actually make it easier to do the HV thing, since it has to be bigger, and hence the distances involved are larger, making dielectric breakdown and arcing easier to guard against.)

Boiling water doesn't work all that well. Even with 20% waste heat at 1800ºC (Bussard's number for hydrogen, but for water it's probably a bad idea) + REB heating of the result, you get a vacuum Isp of around 700 seconds at best, with (naturally enough) higher performance in air due to the additional 'free' reaction mass. Using LH2, you can more than double that performance in vacuum, and you can do much better in air if the combustion potential of the hydrogen can be accessed (water kinda sucks re: heat of combustion in air).

I wonder if the cooling/recombining H2+air in the nozzle expansion section would produce a TAN effect?

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

Post by DeltaV »

93143 wrote:I don't know if 3 MeV alphas would be sufficiently worse at ozone production than 1.5 MeV electrons to justify the effort. They're still ionizing radiation...
Good point. I don't know either. Maybe there's a lucky break due to molecular resonance or something. I did find this, which suggests adding H20 would reduce ozone:

http://en.wikipedia.org/wiki/Ozone#Production
Temperature and humidity plays a large role in how much ozone is being produced. Any ozone machine, when operated in very humid ambient air, will produce up to 50% less ozone than when operated in very dry ambient air.
93143 wrote:Besides, if I'm not mistaken, at high power levels beam self-spreading due to charge concentration starts to become significant, and guiding the beam out a narrow window like that may prove difficult or impossible.
Good point. Cyclotron power levels and charge density are far below what would exist in a Polywell alpha stream. Maybe a series of "guide" coils spaced along the magrid coil axis, the last one being centered on the aperture at the chamber wall? For low altitude flight there should be plenty of spare power available. Pointless, though, if alphas are better ozone generators than electrons.
93143 wrote:Actually, I'm thinking ducted fans running on HVDC motors would probably be best for low altitudes. They would have higher efficiency than heat engines, even without regenerative cooling (dumping heat to a high-density non-hypersonic airflow is way easier than trying to radiate it in a vacuum, so full power shouldn't be a problem, and it probably isn't worth it to include equipment for dumping the heat into the engines without the main reactor power increasing the temperature ratio), and should be fine up to Mach 2 - Mach 3 or so at least, after which you really should be high enough for REB heating.

Now the question is: How feasible is a compact, relatively lightweight MV-range DC motor in the required power class? (The fact that it's GW-range should actually make it easier to do the HV thing, since it has to be bigger, and hence the distances involved are larger, making dielectric breakdown and arcing easier to guard against.)
I like the "more electric" approach also, but routing UHVDC to several external motors is extremely difficult. Hence my earlier posts about variable-twist hyperboloid electrostatic motors inside the vacuum chamber, aligned with magrid coil axes, with driveshaft rotary seals at the chamber walls to get the power out. Complicated. Bulkier. If Polywell can attain a "jet mode" for alphas, things get simpler. On the other hand, if there's a low-mass way to convert UHVDC... current state of the art is not promising.
93143 wrote:Boiling water doesn't work all that well. Even with 20% waste heat at 1800ºC (Bussard's number for hydrogen, but for water it's probably a bad idea) + REB heating of the result, you get a vacuum Isp of around 700 seconds at best, with (naturally enough) higher performance in air due to the additional 'free' reaction mass. Using LH2, you can more than double that performance in vacuum, and you can do much better in air if the combustion potential of the hydrogen can be accessed (water kinda sucks re: heat of combustion in air).
Maybe direct heating for steam propulsion would be efficient enough for low altitude. The peak efficiency is needed for the kick into orbit, right? I, however, really like the idea of indefinite lower atmosphere flight using a small amount of p-B11 fuel and only the surrounding air as reaction mass.
93143 wrote:I wonder if the cooling/recombining H2+air in the nozzle expansion section would produce a TAN effect?
And might it hinder low-altitude ozone production? Not my gig, chemistry.

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Now the question is: How feasible is a compact, relatively lightweight MV-range DC motor....
Motors operate on current. There is a trade off in winding density between more wire in a slot vs insulation. So you are probably going to want a voltage down converter. Adding weight and efficiency concerns.

Now a down converter with SC coils might work.
Engineering is the art of making what you want from what you can get at a profit.

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

DeltaV wrote:
93143 wrote:I wonder if the cooling/recombining H2+air in the nozzle expansion section would produce a TAN effect?
And might it hinder low-altitude ozone production? Not my gig, chemistry.
Just in case you haven't heard of it:

TAN = Thrust Augmented Nozzle. When stuff burns in the expansion section. Like an afterburner for a rocket. It lowers Isp, but allows use of a very large expansion ratio without flow separation issues at low altitude. Tests at Aerojet show nice clean combustion.

A hydrolox vacuum engine could potentially be used in an all-rocket SSTO with kerolox TAN injectors that shut off partway up, thus improving the T/W for liftoff and eliminating the need for an aerospike. I don't know if anyone has actually run any numbers for such a configuration, though...

As for the situation I described lowering ozone production, it might - the combustion product of H2 and O2 is H2O, and... yeah. Possibly...
I like the "more electric" approach also, but routing UHVDC to several external motors is extremely difficult. Hence my earlier posts about variable-twist hyperboloid electrostatic motors inside the vacuum chamber, aligned with magrid coil axes, with driveshaft rotary seals at the chamber walls to get the power out. Complicated. Bulkier. If Polywell can attain a "jet mode" for alphas, things get simpler. On the other hand, if there's a low-mass way to convert UHVDC... current state of the art is not promising.
Who says it has to be external motors? I was thinking a couple of big engine ducts integrated into the airframe, running right past the reactors on either side. The fans would have to be far enough forward to be capable of feeding deflector ducts for VTOL. (Unfortunately the logistics of rotating nacelles aren't very good; I've reluctantly decided to go with dedicated downward-thrusting rockets for lunar/Mars VTOL, at least for the forward thrust axis - it's possible the back end nozzles could gimbal enough to do double-duty... a vertically-oriented strut-landing rocket shape wouldn't have this problem, but it has other logistical issues...)

I can't imagine your rotary power transmission scheme being lighter or less bulky than a handful of well-isolated UHVDC power lines completely internal to the vehicle.

kunkmiester
Posts: 883
Joined: Thu Mar 12, 2009 3:51 pm
Contact:

Post by kunkmiester »

why bother with VTOL, especially at this point? I almost laughed when I saw Avatar and watched their shuttle make a vertical landing. It's more practical for a polywell powered system, in which you don't have to worry too much about running out of fuel, but horizontal take off will be much easier.
Maybe direct heating for steam propulsion would be efficient enough for low altitude. The peak efficiency is needed for the kick into orbit, right? I, however, really like the idea of indefinite lower atmosphere flight using a small amount of p-B11 fuel and only the surrounding air as reaction mass.
Perhaps water would serve as an "afterburner" fuel. It'd be used when taking off to provide extra thrust, and on the start to the orbital run, and in orbit where there's no air. You could do the same with H2 probably. Use it where you need it, but if you have to loiter at all, you'd make sure you'd have enough thrust with air to cruise.
Evil is evil, no matter how small

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

kunkmiester wrote:why bother with VTOL, especially at this point?
Because the moon and Mars don't have enough atmosphere to support HTOL. Titan does, but there are no airstrips.

Come to think of it, there are no airstrips on the moon or Mars either...

In fact, I'm not sure Earth has any airstrips that could handle a 3000-ton delta-wing spaceplane...

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

Post by DeltaV »

MSimon wrote:Motors operate on current. There is a trade off in winding density between more wire in a slot vs insulation. So you are probably going to want a voltage down converter. Adding weight and efficiency concerns.
Electromagnetic motors, yes. Jefimenko-style electrostatic motors? (No coils.)
MSimon wrote:Now a down converter with SC coils might work.
Please elucidate.
93143 wrote:Just in case you haven't heard of it:

TAN = Thrust Augmented Nozzle...
I have. I'm seeking a paradigm beyond chemical propulsion, but any assistance yielded by chemicals is still appreciated.
93143 wrote:As for the situation I described lowering ozone production, it might - the combustion product of H2 and O2 is H2O, and... yeah. Possibly...
Cool. Maybe just a little H2O would help here and there, but low atmosphere cruise should be done with air only to minimize propellant mass, if possible.
93143 wrote:Who says it has to be external motors? I was thinking a couple of big engine ducts integrated into the airframe, running right past the reactors on either side. The fans would have to be far enough forward to be capable of feeding deflector ducts for VTOL. (Unfortunately the logistics of rotating nacelles aren't very good; I've reluctantly decided to go with dedicated downward-thrusting rockets for lunar/Mars VTOL, at least for the forward thrust axis - it's possible the back end nozzles could gimbal enough to do double-duty... a vertically-oriented strut-landing rocket shape wouldn't have this problem, but it has other logistical issues...)
I don't want to think about SSTA until SSTO is whupped. By "external" I meant external to the vacuum chamber, not necessarily external to the fuselage. My vehicle baseline is sort of a blend of Pye Wacket

Image

and X-33

Image ,

with redundant, embedded vertical lift fans (8?) and forward-thrusting turbines (4?), whose inlets are maybe integrated with the intake ductwork for QED/ARC. All upper/lower surface intakes/exhausts sealable for reentry. Metallic TPS.
93143 wrote:I can't imagine your rotary power transmission scheme being lighter or less bulky than a handful of well-isolated UHVDC power lines completely internal to the vehicle.
"Complicated. Bulkier." That's sort of why I brought up direct heating via alphas. Here are some well-isolated UHVDC power lines for 800KV:

Image
kunkmiester wrote:why bother with VTOL, especially at this point?
Because Polywell, if it works, makes it possible (and I'm not getting any younger). Less ground-based infrastructure (runways, launch towers, cryo farms, ...) is very desirable. Hastens the day when peasants like me can own space hoppers.
kunkmiester wrote:Perhaps water would serve as an "afterburner" fuel. It'd be used when taking off to provide extra thrust, and on the start to the orbital run, and in orbit where there's no air. You could do the same with H2 probably. Use it where you need it, but if you have to loiter at all, you'd make sure you'd have enough thrust with air to cruise.
That's pretty much how I see it, if air by itself is not good enough.
93143 wrote:I'm not sure Earth has any airstrips that could handle a 3000-ton delta-wing spaceplane...
3000 tons -- assuming Polywell gamma emission is as bad as surmised and no improvements in shield technology. I'm hoping for a less gammas and better shields, but I'm just a dreamer.

(EDIT - Fixed dead link)
Last edited by DeltaV on Wed Sep 11, 2013 4:27 pm, edited 2 times in total.

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

MSimon wrote:
Now a down converter with SC coils might work.
Please elucidate.
A SC coil would lower the weight of the inductors. Same amount of insulation. Lower weight of current carrying conductors. esp if MgB is used.

As to electrostatic motors: I wonder if a macro motor (as opposed to a MEMS device) can give you decent power to weight ratios.

There has to be some reason industry doesn't use them.

Some technical details:

http://f3wm.free.fr/sciences/jefimenko.html
It operates at speeds of up to 12 000 revolutions per minute at an efficiency of substantially more than 50 percent.
Even 70% efficiency is pitiful compared to a magnetic motor, which can run to 92% or more in larger sizes.

And the motors depend on corona discharge. That is going to kill motor lifetime and radio communications in the vicinity.

http://www.engineeringtoolbox.com/elect ... d_655.html
Engineering is the art of making what you want from what you can get at a profit.

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

Post by DeltaV »

MSimon wrote:A SC coil would lower the weight of the inductors. Same amount of insulation. Lower weight of current carrying conductors. esp if MgB is used.
Would air-core work, no heavy laminated steel cores?
MSimon wrote:As to electrostatic motors: I wonder if a macro motor (as opposed to a MEMS device) can give you decent power to weight ratios.

There has to be some reason industry doesn't use them.
There is. Low voltage electromagnetic motors are much more practical and highly developed than high voltage electrostatic motors. There has simply been no incentive to develop HV ES motors. Jefimenko quote: "There are indications that compound motors of this type can develop up to 1000 horsepower for each cubic meter of their volume." That's not much different from a performance car IC engine. But that was probably just a SWAG on his part, based on his work at 7-30KV. Maybe they would yield less, or maybe yield more if run at higher voltages in vacuum (field emission instead of corona) or sulfur hexafluoride. Unexplored territory.
Notwithstanding the problem of handling potentials on the order of a million volts without effective insulation materials, Jefimenko foresees the possibility of at least limited application of corona power machines. In The Physics Teacher (March, 1971) he and David K. Walker wrote: "These motors could be very useful for direct operation from high-voltage d.c. transmission lines as, for example, the 800 kV Pacific Northwest-Southwest Intertie, which is now being constructed between the Columbia River basin and California. It is conceivable that such motors could replace the complex installations now needed for converting the high-voltage d.c. to low-voltage a.c. All that would be required if corona motors were used for this purpose would be to operate local low-voltage a.c. generators from corona motors powered directly from the high-voltage d.c. line."
MSimon wrote:Even 70% efficiency is pitiful compared to a magnetic motor, which can run to 92% or more in larger sizes.
Agreed. But for low altitude flight there should be power to spare (vs. QED/ARC), and no serious research to improve high-power ES motor efficiency has been published. I would much rather use efficient low voltage motors. I'm just looking for ways around the UHVDC step down mass penalty. Hey, maybe there's a way to combine voltage conversion mass with radiation shield mass...

I'm not thrilled with the ES motor idea either, which is why I brought up the direct heating by alphas. If by some miracle it produced an acceptable level of low altitude ozone, then that, combined with a REB for high altitude QED/ARC, would be about as simple as one could get. No need for UHVDC step down at all. Avionics/ECS/etc power could be obtained by other means.

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Would air-core work, no heavy laminated steel cores?
No. The iron cores actually reduce the weight. Ferrites would probably be used at high frequency (100KHz to 1 MHz) to reduce weight.

And don't forget the HV motors work by corona effect. i.e. lots of local ozone. That will kill motor life.
Engineering is the art of making what you want from what you can get at a profit.

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

Post by DeltaV »

My baseline concept for the ES motors has them inside the vacuum chamber. But I'm trying to get rid of them entirely with the direct alpha heating.

If alpha heating of airflow was acceptable ozone-wise (maybe via H2O injection) for low altitudes, I wonder if it could also replace REB for the high altitude phase.

Now that could really simplify/lighten things. One scheme used for all flight phases, no HV down-conversion or spinning HV whirlygigs necessary. Just use the copious alphas to heat the air, air + propellant, or propellant, depending on where you are in the flight/ascent envelope. Not as neat as Mach-Woodward Effect, but maybe the next best thing.

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

My baseline concept for the ES motors has them inside the vacuum chamber.
If the motors depend on corona for operation you are going to have a problem.
Engineering is the art of making what you want from what you can get at a profit.

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

Post by DeltaV »

MSimon wrote:If the motors depend on corona for operation you are going to have a problem.
As I mentioned above
...or maybe yield more if run at higher voltages in vacuum (field emission instead of corona) or sulfur hexafluoride. Unexplored territory.
Field electron emission if inside the vacuum chamber.
Corona discharge if inside a separate chamber filled with something like sulfur hexafluoride.

I like the direct alpha heating idea more, though.

If Polywell can be coaxed into an alpha "jet mode" by weakening one of the magrid coils, that would make things even simpler wrt cruise/orbit-ascent thrust, but distributed vertical lift ducts might be harder then. Jet mode could be toggled by varying current in one coil.

Post Reply