Big Polywell size.

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

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Aero
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Post by Aero »

MSimon wrote:Doesn't engine power required depend on the mass?

I was thinking of reducing the mass. First off - no LOX (except for minor amounts) second off LH2 for reaction mass.

I did some BOE a while back an IIRC 50 ton 6 GW engine. 600 tons of reaction mass. 100 tons of payload.
I did what I could to verify your numbers. Its difficult because there isn't much data available on the Internet (for free) giving the thermal characteristics of LH2. That leaves me to guess at the specific heats. I ended up using 28.836 kJ/kg-K. Best I could come up with is 77.8528746 M J/kg or 0.076412418 M BTU/kg to raise the temperature of LH2 from 14 K to 3000 K. I had to assume that chamber pressure was high enough, pumped of course. (Assume a configuration like used for the SSME hydrogen pumps where energy extracted to cool the nozzle drives the pumps.) I chose 3000 K temperature because hydrogen starts to dissociate at that point. More heat starts to increase the engine ISP from 1251.083382 seconds to 1771.053473 fully dissociated, but it takes a lot of energy to dissociate hence your mass flow drops, reducing thrust. 3000 K LH2 temperature and the energy available from your 6 GW engine gives power limited mass flow of 74.42 kg/sec and a thrust of 791 (not 913.41 as I first wrote) tonnes with a burn time of 8061.94 seconds. That will get you into space with a lot of reaction mass left over.

I didn't run the acceleration profile, but use something like 9.7 km/sec for delta V required to LEO. That accounts for LEO orbital velocity and the extra aerodynamic drag of the HUGE hydrogen tank, and gravity drag. Of course, with this low acceleration and a near vertical climb-out, aerodynamic drag might not be so bad. However, with the low acceleration, gravity losses will operate for a longer time so the delta V required might be higher.

Again, the big uncertainty is in the specific heat of LH2 at the required pressure, throughout the temperature profile.

Edit: I originally used 4000 K to calculate ISP, using a separate spread sheet. Using 3000 K reduces ISP hence thrust to 791 tonnes.
Aero

MSimon
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Post by MSimon »

Obviously I'm not keeping up.

What is TPS?
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Post by MSimon »

Aero wrote:
KitemanSA wrote:
Aero wrote: Yes, your math is correct. Do you know where the 100 MW at 2 meter radius came from? If that is a scale-up from WB-6, then WB-6 should have generated 2.4 Watts. ??? or maybe 240 Watts ??? or maybe 1.4 Watts scaled r^7. ???
I believe I first heard this in Dr. B's Google talk. 1.5m for D-D, 2m for pB11, IIRC.
Well, when I run the scaling backwards, 7 meter down to 15 cm, I get more power than WB-6 delivered. Maybe there is justification for that, I just don't know what it is.
Higher B field?
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KitemanSA
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Post by KitemanSA »

MSimon wrote:Obviously I'm not keeping up.

What is TPS?
Thermal Protection System.

MSimon
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Post by MSimon »

KitemanSA wrote:
Heath_h49008 wrote:High energy photons from the reaction, and from Bremsstrahlung still have to go somewhere. And they can still heat up what they hit.
True, but that wasn't Chris's point. His comment was re: "charged particles", not EM.

Yes, the Em will hit the MaGrid. Yes it will heat it up. That is what the TPS is for. At this point, the TPS is expected to be much smaller than was expected when we falsely believed that the alphas radiate straight out and would also impact the MaGrid. Whole different order of magnitude, I think.
If Dr. B was correct 10% of the output would be EMR (photons). The deal is: most of the internals would be semitransparent to that depending on frequency. So the heat load shouldn't be too bad. I'd say a reduction of between 10 and 100 in expected heat load. However, the SCs will directly receive some of that. Increasing the load on the 2K (to get the B field up) coils. Naturally once we get a device that is operational measurements can be made and device 2 will be better.
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Post by MSimon »

Aero,

Thanks! That gives the option of bigger engines (mass/power ratio higher than estimated), more payload, more airframe, and other such disconnects.

What it does is give a reasonable estimate of size.

BTW I was figuring shuttle temps. More like 2,400 K. And a maglev kick off to about 600 mph. It would be nice to do that on some equatorial plateau. Of course you then have a logistics problem. But about 20 or 50 BFRs should provide the major resources required.

But over ocean might be better for safety. And just take the speed hit. Of course a maglev boost works out to the good. At 600 mph in 1 minute you are 10 miles high. So you are intrinsically accelerating slowly where the atmosphere is thickest.
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Post by Aero »

Any initial velocity is good, but as an engineer you know that simplicity has its merits, too. Did you note my correction to the post? Thrust is only about 790 tonnes at 3000 K. (My ISP calculator is on a different spreadsheet and it was set to 4000 K initially.) 3000 K still gives a thrust to weight greater than one, which is well within the range of a lifting body. The vehicle can reach flight speed for reasonable runway length, then use aerodynamic forces (lift) to counter gravity drag while in the atmosphere as well as carry reaction mass to fly back for a landing. The simplicity of using infrastructure developed for aircraft is a strong argument.

Some will argue that LH2 is not the best "fuel" even though it does give the highest thrust of all liquids. Ammonia is a strong candidate because it easily dissociates into N + H + H + H for a reasonable ISP with much much smaller volume/mass tanks, simpler pumps and smaller, lighter "inter tank" structure. Structure holding the tank off of the engine, in this case. Supercritical water is also good, if the temperature can be raised high enough to dissociate it, somewhere short of 6000 K. But containing that temperature is an issue.

Using hydrogen for reaction mass might be frowned on, considering that the exhaust would be extremely hot pure gaseous hydrogen in an oxygen atmosphere. Imagine the trail of fire from engine ignition at the ramp, all the way to high altitude. As for Ammonia, I don't know what the environmentalist would say. They'd need a study proving that the hot dissociated nitrogen did not recombine with oxygen to form NOx pollution. In any case, it would smell to high heaven. That brings us back to the problem of containing supercritical water steam at 6000 K. At that temperature and 25 MPa, what started as cold water contains only about 6% H2O. The rest is 2% O2, 9.5% H2, 25% O, 48% H and 9.5% OH. You can calculate the mole weight hence ISP. Of course it will take a lot more power using water than using hydrogen so the answer is anyone's guess.
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Post by Aero »

I took a break, and it occurs to me that actually, all of the exhausts would be cold after expanding through the nozzle of the engines. I don't know whether or not the constituents have recombined into their original fluids though. I do know that cold gaseous hydrogen would still present an explosive hazard, and ammonia would still smell to high heaven. Water, I don't know, but I'm pretty sure that none of these engines would be allowed to operate out of Lindbergh Field in San Diego.

Maybe its best to use the maglev on some equatorial plateau and kick it up to altitude before starting the engines.
Aero

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Post by Heath_h49008 »

So burn the hydrogen as you expel it.

Have you guys ever seen the catalyzed hydrogen peroxide rockets? One of the byproducts was oxygen (and water) They created a second stage in the engine by injecting kerosene into the O2 rich exhaust.

We could use the hydrogen in a similar manner. Probably mixed with atmospheric O2.

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Post by MSimon »

Using hydrogen for reaction mass might be frowned on, considering that the exhaust would be extremely hot pure gaseous hydrogen in an oxygen atmosphere.
One of the reasons for using a maglev sled. H2O is not nearly as good.
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Post by Aero »

H20 is not nearly as good when considering ISP in isolation. H2O still has many advantages resulting from density, more than 14 times as dense hence less than 7% the volume, it does not need tank pressurization or insulation. This reduces the tank mass by a very large factor. It also has inherent advantages resulting from its long term use and detailed study. It is much easier to pump to high pressure with smaller, lighter pumps and its behavior can be accurately predicted by current science. Little additional research required except in the area of cooling the pressure chamber and engine throat. Not to mention solar system wide availability. Of course where there is water, hydrogen can be made.

Studies have shown that LOX/LH2 has little advantage over LOX/kerosene as a first stage engine. This is because of the extra structural mass required in order to use Hydrogen. Of course, first stage is what we are discussing. Hydrogen's advantage shines for use in higher stages, where it allows the fuel mass launched by the first stage to be much less.

Burn the hydrogen with atmospheric oxygen. That might work. Might even get some usable thrust from it with the right design. Ram Jet, SCRAM Jet, anyone?

If Polywell fusors work as we hope, power will no longer be the limiting factor. If you're short on power, add a few more pounds of boron. Instead, factors such as those addressed by the airlines (or the Navy) will be the main considerations. Operability, maintainability and capital costs to name three.
Aero

Heath_h49008
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Post by Heath_h49008 »

The design is simple enough. How reactive is the hydrogen after a pass through a VASIMR?

Providing we keep the hydrogen in a usable form, bypassing air from a compressor or ram scoop directly into the hot hydrogen gas/plasma stream coming out of the thruster should make for one nice self-lighting candle.


Hell, it would let us reduce the reactor output requirements if we can produce additional thrust with the ram. And we might save some weight if we used MaGrid coolant liquid hydrogen once and dumped it into the thruster/ram combination. Just substitute liq. hydrogen for liq nitrogen in the secondary cooler.

The magnets can stay energized for months if we short them and keep them cool. You can turn the whole reaction on and off as quickly as you can control ion flow, and, unless I'm missing something, we should be able to throttle this thing over a pretty large range of outputs.

I need a cad/cam and not my laptop... but this compound VASIMR doesn't really strike me as rocket surgery.
:wink:

EDIT: substitute MPD for VASIMR... we can work on better electrodes and longevity, but it has to be able to operate in atmosphere. If we can create some kind of "check valve" on the business end of VASIMR, so be it. If not, we have other ways of heating up and accelerating gas with electricity.

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Post by Heath_h49008 »

All of the MPD thrusters use very small quantities of gas, accelerated to very high velocities to give us that wonderful thrust....

But we can make them bigger.

VASMIR looks great, because you short the SC control magnets and they no longer need much energy, and you simply power the antennae to expand liquid to plasma. Whoooosh! (Yes, I like onomatopoeia. Blame Stan Lee.) Very little contact with gas or plasma, and few moving parts. But it only works in a vacuum.

The other MPDs are basically rail guns with a gaseous projectile. But that means we have to power up the electrodes to create that arc, the electrodes erode and become brittle, but they operate in atmosphere.

Why can't we build the MPD out of concentric ring electrodes, and feed it either atmosphere, LH2, or a combination that we ignite in a glorified afterburner?

Really, the MPD is operationally no different than a Kerosene fueled jet engine. We are converting energy to heat/combust liquid and accelerate it. We are just using electrodes as the compressor, and the arc is our "combustion" heating and expanding our working fluid.

Am I missing something basic? It would seem possible to scale this up as large as we see fit with ring electrode, after ring electrode, in a disk/pipe configuration. (Each ring being two electrodes back to back.)

I'm trying to imagine a blending of the two... MPD and VASIMR... But I'm just not seeing it.


I swear this isn't thread drift. If we know what our thruster will be, we'll know how big the reactor has to be to power it. It would seem to be the direction I would work the problem, anyway.

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Post by D Tibbets »

Without digging up Bussards discriptions of SSTO boosters, I recall that a required power of ~ 7-8 GW was needed. The power needed is proportional to the specific impulse of the exaust. As the efficiency goes up the amount of needed fuel carried goes down so the takeoff weight and average weight goes down a corresponding amount. The spacecraft could be smaller and lighter because it would not need nearly the volume and structural weight nessisary to carry all of the required fuel. Also, the described SSTO is a space plane, it utilizes arodynamic lift to support it for a significantly longer time (higher speed) than the shuttle that essentially punches up through most of the atmasphere then accelerates to orbital speed. A space plane using atmospheric lift would have to use less of it's thrust to keep it from falling, at the cost of increased drag. To figure out the net benifits/ costs would take ...well... a rocket scientist.

Dan Tibbets
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Aero
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Post by Aero »

If you're going to talk about an SSTO then I think that wings are a given. I don't see much benefit with an SSTO that comes back by parachute. The space shuttle configuration with the external tank and boosters isn't very aerodynamic (can't fly) but, with the power levels under discussion an SSTO could.

The wings give lift to counter gravity, reducing the vertical thrust required by F = ma, or mg. That is a lot of benefit. Their is some extra drag induced because the lift force vector is tilted backwards by an amount equal to the aerodynamic angle of attack, but that is small relative to mass times gravity. As the SSTO increases its angle of attack, the vertical lift component exceeds mg, so the vehicle lifts away from Earth instead of thrusting away. Because the SSTO has enough power to reach orbit, it can continue to accelerate in the very thin high altitude atmosphere with wings generating lift up almost to the edge of space at 62 miles. One of the things we discovered with the X-15 is that there is a distinct edge to the atmosphere. Air pools in the gravity well in a way akin to water pooling in a pond so once the edge is reached, lift stops and the vehicle vertical thrust and speed over Earth curvature combined must keep it out of the atmosphere as it continues to accelerate to orbital velocity. And of course you can't stay in the atmosphere at to high a speed or the vehicle will get to hot.
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