Rocket thrust

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

Moderators: tonybarry, MSimon

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Nov 16, 2009 7:40 am

DeltaV wrote:OK, speaking only of lower altitude, low-to-moderate speed (non QED-ARC) operation, let's assume that we have a heat exchanger that hermetically separates a REB, in vacuum, from the flowing air (+ any propellant), but still transfers enough thermal energy into the flow to provide sufficient thrust for lift, hover and forward acceleration until QED-ARC territory is reached. Also assume that the REB is magnetically or electrostatically diffused or defocused and that the intercepted power per unit area is low enough to avoid disintegrating the heat exchanger.


No good. Relativistic electrons are going to spall atoms off the surface no matter how diffuse they are. You have to thermalize the energy somehow, for instance by firing the beam at a fluid rather than at a solid.

Also, the heat exchanger is going to consist of a lot of little tubes and/or fin-type structures, in order to maximize surface area. Directly REB-heating the inside of the metal structure evenly is probably close to impossible.

While I've not done any calculations (it's been many, many years since thermodynamics class, and I've never used it since), intuitively it seems like such a heat exchanger would have to be, at a minimum, white hot, more likely blue (UV?) hot, to get sufficient energy into the flow to lift/propel a massive vehicle.


If you want huge power and thrust output, you have to make the engine bigger. The temperature won't be that high - remember that the turbine blades have to be able to take it.

The catch is that while the turbine blades can be cooled, it's silly to try to cool a heat exchanger. On the other hand, the turbine is under more stress, so creep is a bigger problem...

So I'm wondering, what kind of fluid are you considering for the closed loop, a liquid metal such as sodium, or something else? Maybe my intuition is failing to give me a clear picture of just how much energy needs to be transferred to the flow to be practical for flight.


I don't know, actually. Maybe helium would do it? It has almost six times the thermal conductivity of air, so the heat exchanger shouldn't get that much bigger relative to a liquid metal one... REB-heating liquid metal could be problematic. On the other hand, you might be able to use resistance heating in a ceramic-coated element rather than a REB... heck, you could forget the exchanger loop and just run a resistance heater through the engine. But that wipes out the regen cooling advantage...

Megavolt-range resistance heating? Why doesn't that strike me as a bad idea? Maybe it's WELL PAST TIME I WENT TO BED...

93143 wrote:500,000 lb? A fully shielded 6 GW BFR weighs twice that on its own.

A million pound shielded BFR? (I think less than 6 GW might do, but let's go with it.) Is this a generally accepted value? That's about the max takeoff weight of a 747-8. Polywells are basically just big spherical vacuum tubes. I know, it's mostly shielding and cooling system weight. MSimon and other nukes -- is this a reasonable weight number for full shielding?


I calculated it upthread. >350 tonnes for just shielding - 5 metre vessel radius, 6" water followed by a thin layer of boron-10 followed by 3½" of lead.

Now that I've been reminded about the high levels of gamma radiation (600 kW at up to 16 MeV), the lead doesn't really seem thick enough, so that number might go up...

Actually it might go up a lot... this might be less feasible than I thought. Drop the gamma intensity by 7 orders of magnitude, and the dose from sitting next to it for a year is about 30 rem. According to the data on Wikipedia's "Gamma_ray" page, that takes about ten inches of lead.

(By the way, this will completely obliterate the 300 MW of bremsstrahlung coming off the core; X-rays are easy to stop... and you probably won't need to bother with the water and boron-10 either, since 10" of lead will handle the neutrons nicely too...)

So a fully-shielded 6 GW BFR weighs maybe twice what I said it did. Two million pounds.

Please remember that the power-to-weight ratio goes down VERY FAST if you go for a less powerful reactor, because aside from the obvious fact that the reactor itself isn't that much smaller, you still need almost the same shield thickness. So the huge reactor is about as good as it gets, unless you want to go to 10 GW (which people are apparently dubious about)...

Maybe the shielding thickness can be varied a bit depending on how likely it is that someone's going to be standing next to the vehicle in that particular direction...

Note that even with the new shielding calculations, using the thrust-to-power ratio of a high-bypass turbofan, the generated power is still sufficient to lift the reactor straight up off the ground, along with more than a million pounds of additional structure and payload...

IntLibber
Posts: 747
Joined: Wed Sep 24, 2008 3:28 pm

Postby IntLibber » Mon Nov 16, 2009 9:53 am

Why aren't we just using a big tank o water for the shield and fuel? Run the water between some big hafnium plasma electrodes and you separate the water into hydrogen and oxygen which then recombines further on down the expansion. You've got a 450+ sec LH2/LOX rocket (there will be lots of additional heat dumped into the water, but your fuel tankage is the size typical of kerolox cause the water density is 1.0 g/cm^3 rather than the 0.08 g/cm^3 for LH2. You eliminate weight needed to deal with cryogenics.

kunkmiester
Posts: 866
Joined: Thu Mar 12, 2009 3:51 pm
Contact:

Postby kunkmiester » Mon Nov 16, 2009 5:44 pm

You still need to deal with cryogeneics though, because you need to cool the magnets in the reactor--superconductors aren't high temp yet. You have a good idea though.

Is there a reason to not just dump the REB working fluid into the engine airstream. Not as efficient sure, and you have to use it up even in lower altitude flight, but the mass saved might be worth it.
Evil is evil, no matter how small

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Nov 16, 2009 6:12 pm

Uh...

One SSME at 104.5% develops about 5 GW of jet power in vacuum. The combustion power (and thus the energy per second needed to generate the H2 and O2) is more.

That gives you 488,800 lbs of thrust, or roughly enough to lift a 10 m diameter lead shadow shield capable of protecting the astronauts from gamma rays during the burn... and nothing else. Forget the reactor itself. Forget the electrolysis system. Forget the structure of the vehicle, and the payload, never mind the huge tank of water...

Unshielded, the gamma rays deliver a potentially fatal dose in under a minute. From 50 m away. You'd need about three metres of water (at a guess) to offer the same protection as the 10" lead shield; maybe more (lead is slightly better at gamma shielding than its mass indicates). Reserve propellant might do it, but this time it'd really better be an emergency if you dip into the reserve...

Need I remind you that the 10% propellant reserve in this example weighs more by itself than the thrust available from the rocket?

The upshot is that it's not even close to worth it, for the tankage mass you'd save... 450 seconds isn't really all that good; there's a reason we're having so much trouble getting into orbit with hydrolox rockets...


I suppose I should redo the analysis for a large reactor cluster, to see where the breakeven point is (if there is one). Either way, it doesn't look good for Polywell-powered launch vehicles that take off vertically on internal propellant - to get more thrust you'd have to decrease the Isp...

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Nov 16, 2009 6:23 pm

It would be really awesome if rnebel showed up right about now and told us that the gamma-emitting branch probability is an artifact of the thermal plasma assumption, and that a Polywell is expected to produce gammas at a rate five orders of magnitude lower...

But I doubt he will...


And really, even if he did, a vehicle powered by a single reactor would still not be remotely capable of vertical liftoff on internal propellant at a useful Isp...

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Nov 16, 2009 6:39 pm

kunkmiester wrote:Is there a reason to not just dump the REB working fluid into the engine airstream. Not as efficient sure, and you have to use it up even in lower altitude flight, but the mass saved might be worth it.


That's essentially an RBCC QED engine. It could be useful for launch vehicles, and I am a proponent of the idea. If the REB working fluid is the hydrogen coolant, it should burn in the airstream for extra thrust. The question is, is it better than just using kerosene-fueled turbojets for the first couple of Mach numbers? It's possible that it is, and Bussard was just being conservative with his engine design (I believe he used kerosene-fueled turbojets to Mach 2.5, then switched to a QED rocket)...

The fusion-powered turbojet with closed heat exchanger is not for a launch vehicle. It's for an airplane. Weight saved is secondary; the big question is "how long can it fly?". In this case, the answer is "as long as the fusion fuel holds out; ie: indefinitely."... It might be possible to fly well past Mach 2 with this system.

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

Postby DeltaV » Mon Nov 16, 2009 7:47 pm

93143 wrote:The fusion-powered turbojet with closed heat exchanger is not for a launch vehicle. It's for an airplane.

I think we're too ingrained with the notion that we have to get to orbit in a hurry, before the fuel runs out, a legacy of decades spent on chemical rockets. Fusion opens up the possibilty of a more leisurely ascent out of the lower atmosphere.

Are you saying that a blended system, using fusion-turbines for lower altitudes and QED-ARC for higher altitudes, is not practical for SSTO? Assume here that the radiation/shielding numbers discussed above are overly pessimistic, otherwise the required shield mass swamps all other considerations.

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Nov 16, 2009 9:04 pm

DeltaV wrote:Are you saying that a blended system, using fusion-turbines for lower altitudes and QED-ARC for higher altitudes, is not practical for SSTO?


No, but the large mass does tell against it. The requirement for a launcher is not to stay at Mach 2.5, it's to get to Mach 2.5 (on the way to higher speeds), and it's entirely possible - even likely - that kerosene-fueled turbojets do that while weighing much less (including fuel) than fusion-heated ones (including the exchanger loop and related systems), thus improving payload substantially. They also have the advantage that you don't need to start the reactor until you're at altitude and well away from anyone you could fry with gamma rays. Thus you could get away with shadow shielding without requiring bunkers for the ground crew. Ground handling of conventional turbojets isn't all that onerous, really - it's not like we don't know how to deal with jet fuel, and the liquid hydrogen is going to be the dominant factor in propellant cost anyway...

Then there's the RBCC option, which should be a lot lighter because you can use the same engine all the way to orbit, and remain airbreathing well into the hypersonic regime. I don't know about hydrogen consumption during takeoff; hopefully the ejector-jet effect combined with hydrogen/air combustion will help out a fair bit...

On the other hand, the idea of a multipurpose vehicle that can go to orbit, while still retaining unlimited supersonic flight range in atmosphere, is very appealing... perhaps I should try to run some numbers...

Assume here that the radiation/shielding numbers discussed above are overly pessimistic, otherwise the required shield mass swamps all other considerations.


For an SSTO, you need the power anyway, so shadow shielding at least is a given. You could use propellant reserve, but it's slightly less efficient mass-wise at blocking gammas than lead is, so it becomes a trade - do you want to accept marginally less nominal performance in exchange for the ability to run the engines a bit longer in an emergency at the cost of the crew's health?

I just thought of something. Shielding the crew cabin with lead could end up weighing a lot less than using reserve propellant, because the shield could be smaller... I suppose it depends on whether you want to shield a large payload bay, or just the cockpit, or maybe just the acceleration couches...

The extremely pessimistic conclusions of the analysis above, with the SSME example, apply only to a non-airbreathing vertical takeoff of a vehicle powered by a small number of reactors. Horizontal takeoff with airbreathing engines should still be fine - but you do have to do the final leg of the trip to orbit on rocket propulsion, so you still have to be careful with mass allocation...

Fusion is good, but it doesn't allow us to throw everything we know about rocket science out the window like high-thrust Mach-effect propulsion would... As a space travel enabler, high-thrust M-E is much further advanced over Polywell fusion than Polywell fusion is over von Braun's primitive LOX/alcohol rockets... too bad it's also a significantly longer shot scientifically...
Last edited by 93143 on Mon Nov 30, 2009 7:53 pm, edited 1 time in total.

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

Postby DeltaV » Mon Nov 16, 2009 9:29 pm

93143 wrote:On the other hand, the idea of a multipurpose vehicle that can go to orbit, while still retaining unlimited supersonic flight range in atmosphere, is very appealing... perhaps I should try to run some numbers...

Please do. Given the realities of air (hopefully soon, aerospace) traffic control, airport (spaceport) takeoff/landing fees, orbital mechanics and so on, a mid altitude loiter/excursion capability might have a lot of economic appeal for operators, e.g. when the departure/destination points are not well placed with respect to the required orbit, or when orbit insertion has to be delayed to avoid debris threats or traffic. [Edit] I would also consider the possibility of suborbital or "skipping stone" point-to-point mission profiles.

93143 wrote:I just thought of something. Shielding the crew cabin with lead could end up weighing a lot less than using reserve propellant, because the shield could be smaller... I suppose it depends on whether you want to shield a large payload bay, or just the cockpit, or maybe just the acceleration couches...

I think this is what MSimon was alluding to earlier.

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Nov 16, 2009 9:54 pm

DeltaV wrote:
93143 wrote:I just thought of something. Shielding the crew cabin with lead could end up weighing a lot less than using reserve propellant, because the shield could be smaller... I suppose it depends on whether you want to shield a large payload bay, or just the cockpit, or maybe just the acceleration couches...

I think this is what MSimon was alluding to earlier.


Hey, it was! I missed that somehow...

Maybe I oughta be more careful, considering how I tend to rip into people who respond without reading my posts...

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Sat Nov 28, 2009 4:45 pm

Well, after doing a dose calculation, it seems to me that 1½" of lead should suffice for the neutron absorption gammas. However, this will probably not suffice for the bremsstrahlung, which may require ~2-5" depending on energy spectrum (based on attenuation of 300 MW of ~200+ keV X-rays to 2 rem/year at 5 m from the core, or 11 orders of magnitude). If the gamma rays are as frequent as the Wikipedia article on aneutronic fusion says, they still dominate, requiring about a foot of lead. I can imagine the rate being distribution-dependent, though...

I'm going to try to conceptualize an SSTA with unlimited supersonic cruise, using optimistic assumptions. I'll assume POPS and/or strong magnets can get the 6 GW core down to 3 metres in radius, and that it only needs 3" of lead shielding on top of 6" of water and a thin layer of boron-10. This yields a shield mass of about 115 tonnes.

The vehicle might be designed to take off vertically using rotated jet engines, Osprey-style - but with four of them rather than two. I wonder if it's possible to have the blades of a very large turbofan retract, pivoting into slots in the annular wall (or the shaft, but that might be an even worse idea structurally), the idea being to clear the bypass for QED ramscram operation... almost certainly the fan would have to stop before attempting this, which requires controllable blade angle at the very least...

...naw, that's a dumb idea. To land, you'd have to re-extend them and lock them in place while flying at a substantial forward speed... darn it, that's a perfectly good bypass going to waste there...

The wings could double as high-temperature radiators. I will assume that only about 700 MW of waste heat (~10%) needs to be dealt with, but since the thing needs to operate in space it needs to be capable of dumping that in the absence of atmosphere, while in high-Isp mode.

There's a major tradeoff between radiator size and reactor efficiency, because pumping the heat up to a higher temperature uses a lot of power that is then unavailable for anything useful. Given a 350°C primary reactor cooling loop, a radiator system at 1200 K would require (I think) over 1 GW of refrigeration power per core, in the form of shaft power to high-temperature compressors. The use of turbines on the cool side could improve this somewhat, but they'd need to be capable of dealing with a saturated two-phase flow of liquid metal...

I don't know how feasible it is to significantly increase the temperature of the primary cooling loop, but it would sure help... I'd really like to use lithium as the refrigerant (it's lighter and can carry more energy), but its boiling point at 1 bar is over 1600 K - great for radiator size, less great for refrigeration power...

Question: Given that we're talking hundreds of tonnes here anyway, how much exactly would it weigh to have a gigawatt of voltage step-down capacity from whatever final voltage the direct conversion and conditioning system puts out?

Or would it be better to try to produce an electric motor that can run on 1.5 MV?

This is what happens when I watch soft sci-fi. Maybe I should keep SSTO in mind as a fallback in case I can't make this work...

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

Postby DeltaV » Sun Nov 29, 2009 12:16 am

93143 wrote:Question: Given that we're talking hundreds of tonnes here anyway, how much exactly would it weigh to have a gigawatt of voltage step-down capacity from whatever final voltage the direct conversion and conditioning system puts out?

Recently I bought a used book titled "Similarities in Physics", 1982, Shive and Weber. In Chapter 12, "Transformers and Impedance Matching", they show a high-voltage DC "transformer" consisting of a rotary variable capacitor and some brushes. The capacitor plates (rotor and stator) are sectors of a disk, symmetrical about the rotation center. The stator is at ground potential. The rotor alternately contacts a "charging" brush and a "takeoff" brush, separated by 90 deg of rotation. For a step-up transformer, as shown in the book, the rotor and stator overlap (max. capacitance) when the low-voltage charging brush is contacted, and are orthogonal (min. capacitance) when the high-voltage takeoff brush is contacted. Between brush contacts the rotor charge is constant, and voltage varies according to V=Q/C. I think this would also work as a step-down transformer if you reverse the rotation direction so that the high-voltage brush is contacted 90 deg before the low-voltage brush.

93143 wrote:Or would it be better to try to produce an electric motor that can run on 1.5 MV?

Maybe something like this can be scaled up (but kept in a near-vacuum to avoid ozone): http://www.arcsandsparks.com/tourbillion.html
Last edited by DeltaV on Sun Nov 29, 2009 3:22 am, edited 1 time in total.

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Postby D Tibbets » Sun Nov 29, 2009 1:03 am

Concerning shielding from gammas (and x-rays and neutrons). Assuming the 5 meter diameter of the shield represents a 5 meter magrid, how much is needed to block the radiation from the core. Assuming the wiffleball diameter is significantly less than the magrid diameter,and the further central confluence might result in perhaps 99 % of the gamma radiation (fusion) coming from a core of only 2-3 meters. A shileld slightly wider than this would cast an expanding shadow protecting structure and personel ahead of it. Adding some protection from perhaps 20-30 meters of standoff distance could substantially reduce the shielding weight. eg: instead of a 5 meter diameter lead shield, a 3 meter diameter shield might do. With the same thickness, this would weigh only ~1/3rd as much.

Dan Tibbets
To error is human... and I'm very human.

ANTIcarrot
Posts: 62
Joined: Wed Aug 15, 2007 11:47 pm
Contact:

Postby ANTIcarrot » Thu Dec 03, 2009 2:12 am

93143 wrote:I'm going to try to conceptualize an SSTA with unlimited supersonic cruise, using optimistic assumptions. I'll assume POPS and/or strong magnets can get the 6 GW core down to 3 metres in radius, and that it only needs 3" of lead shielding on top of 6" of water and a thin layer of boron-10. This yields a shield mass of about 115 tonnes.


I've been doing a little research on arc-jets recently. (Since more engineering research is available for them.) Specifically the 100kw HIPARC design. If you scale up the results of that experiment, a 1GW polywell driven arc jet would deliver around 50 tons of thrust at a specific impulse of 1400 seconds. Which has a vague similarity to the LANTR concept...
http://sgc.engin.umich.edu/erps/IEPC_19 ... 93-221.pdf
http://www.nss.org/settlement/moon/LANTR.html
If the LANTR numbers translate (and I'm reading them correctly) then we could use the same arc-jet (this time with LOX afterburner) to generate ~200 tons of thrust, at an Isp of ~700s. A VTOL SSTO with these engines would require an MR of about 2.36.

With those numbers, a 6GW polywell could create 1200 tons of thrust, and a 115 ton shield might not be a major problem Though 1200 tons of thust creates its own issues...
Some light reading material: Half Way To Anywhere, The Rocket Company, Space Technology, The High Fronter, Of Wolves And Men, Light On Shattered Water, The Ultimate Weapon, any Janes Guide, GURPS Bio-Tech, ALIENS Technical Manual, The God Delusion.

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Thu Dec 03, 2009 5:17 pm

You can calculate the minimum power required for a given thrust/Isp combo by simply computing the jet power:

(F [kgf])/(Isp [kgf/kg·s]) = mdot [kg/s]

0.5*mdot*(g*Isp)^2 = P [kg·m²/s³]

For F = 50,000 kg and Isp = 1400 s, this gives a jet power of 3.37 GW.

At the quoted efficiency of 28% (from your first linked paper), this requires almost exactly the total output of two 6 GWe BFRs.

This is why I want to go airbreathing for most of the ascent. Maybe carry LOX for orbital insertion, if the subsequent REB heating doesn't just dissociate the H2O...

It would be nice to at least be able to land on the Moon. This requires a minimum vacuum T/W of maybe 0.2, which is pretty stringent for a fusion vehicle. I don't want to have to go to shadow-shielding, but I may have no choice...


Return to “Implications”

Who is online

Users browsing this forum: No registered users and 1 guest