Boron availability

[texaslabrat]

Tulse wrote:

If the power density of the fusion power sources were sufficiently high (in terms of MW/m^3 and/or MW/kg) , that’s definitely feasible from at least a basic conceptual point of view (ignoring the complexities of the actual aircraft and propulsion system design). A lot of optimization and miniturization of the fusion source will be necessary to make this kind of system do-able for atmospheric flight propulsion.

One of the “advantages” of conventionally fueled craft is that they get lighter as they travel. With a FF powerplant instead, all the weight is constant. I presume there is some crossover point where a specific fixed weight fusion generator is more efficient than than a powerplant running on conventional fuel that gets lighter over time. But it is not immediately obvious that such crossover point can be easily reached — my guess is the fusion plant in such instance would have to be very light relative to its on-the-ground, fixed instantiation.

And I’m still not convinced that there would be any major advantage to such a system. As I noted earlier, the real problem with most launch systems isn’t lack of power per se (at least not power on the scale that an FF device would generate).

I’m not convinced it would be feasible in the near-term by any stretch…fitting probably a GW of power production into a space-worthy airframe (never mind the issues of power distribution within the craft) meant to take off from a runway presents issues that are non-trivial to say the least.

And while you say that an “advantage” of a conventional plane is that it gets lighter as it travels…I’ll counter by saying that a sufficiently miniaturized power source STARTS light so therefore has a huge advantage. Not having to carry the structural weight of a changing fuel load greatly simplifies the airframe design as well as makes the balance points and other dynamic issues easier to predict and control. That was the point I was trying to get across…that you would need to make the power source VERY small and VERY light to be advantageous over a conventially fueled craft…but if you could do so…it would revolutionize air (in addition to space) travel as you could then have non-range-limited aircraft in addition to the spacecraft previously discussed. All for a bit of boron…not too shabby. Very much “pie in the sky”…but an interesting thought experiment for sure.

In the mean time, I’ll still be putting my money on the space elevator tho 😉

[Phil’s Dad]

Could we have a ‘Quote of the Week’ thread;

I submit for an example this from Tulse 😉 who posted on 03 November 2009 at 03:05 PM

“…the only thing that FF would be doing is providing an alternate(sic) power source.”

The only thing!!! :-/

(Alternative – since you ask)

[Tulse]

Phil’s Dad wrote: Could we have a ‘Quote of the Week’ thread;

I submit for an example this from Tulse 😉 who posted on 03 November 2009 at 03:05 PM

“…the only thing that FF would be doing is providing an alternate(sic) power source.”

The only thing!!! :-/

(Alternative – since you ask)

I’ll certainly take the grammatical correction, but I think the point in context stands — the issue with scramjets is not how to power them, and using FF in a scramjet would not really solve the major obstacles with that technology.

[belbear]

Tulse wrote: belbear, in the system you describe the only thing that FF would be doing is providing an alternate power source. As I understand it, the problems with scramjets aren’t in that area, but instead in the engineering of the hypersonic air passage. Perhaps I’m wrong, though — someone else here may be more knowledgeable on this topic.

The problems of designing a scramjet are mainly in how to prevent a “flameout”, i.o.w. the need to maintain fuel combustion in a hypersonic airflow.

A FF powered scramjet does not do chemical combustion at all, so this problem does not pose itself, because it would work by simple heating of incoming air (using either electricity, X-rays or ion beam) And that always work.
Instead of fuel injection it could be using a tungsten heat exchanger, operating at very high temperature (2000°C) to transfer heat from the source to the airflow.

And in aviation and space flight the weight of the fuel to carry is of course also an important issue. For a FF aircraft, the landing weight is essentially identical to the takeoff weight. Although some kind of backup fuel could be used: Kerosene or even water injection could provide an extra low-speed boost during take-off. (afterburner)

There were experiments with water injection in turbojets, and it worked, but in the end it proved more efficient to simply carry the same weight in fuel.
In contrary to fuel, water injection also works reliably at hypersonic speeds. So water injection can give the upper rocket stage that extra final boost before separation.

[Aeronaut]

texaslabrat wrote:

If the power density of the fusion power sources were sufficiently high (in terms of MW/m^3 and/or MW/kg) , that’s definitely feasible from at least a basic conceptual point of view (ignoring the complexities of the actual aircraft and propulsion system design). A lot of optimization and miniturization of the fusion source will be necessary to make this kind of system do-able for atmospheric flight propulsion.

One of the “advantages” of conventionally fueled craft is that they get lighter as they travel. With a FF powerplant instead, all the weight is constant. I presume there is some crossover point where a specific fixed weight fusion generator is more efficient than than a powerplant running on conventional fuel that gets lighter over time. But it is not immediately obvious that such crossover point can be easily reached — my guess is the fusion plant in such instance would have to be very light relative to its on-the-ground, fixed instantiation.

And I’m still not convinced that there would be any major advantage to such a system. As I noted earlier, the real problem with most launch systems isn’t lack of power per se (at least not power on the scale that an FF device would generate).

I’m not convinced it would be feasible in the near-term by any stretch…fitting probably a GW of power production into a space-worthy airframe (never mind the issues of power distribution within the craft) meant to take off from a runway presents issues that are non-trivial to say the least.

And while you say that an “advantage” of a conventional plane is that it gets lighter as it travels…I’ll counter by saying that a sufficiently miniaturized power source STARTS light so therefore has a huge advantage. Not having to carry the structural weight of a changing fuel load greatly simplifies the airframe design as well as makes the balance points and other dynamic issues easier to predict and control. That was the point I was trying to get across…that you would need to make the power source VERY small and VERY light to be advantageous over a conventially fueled craft…but if you could do so…it would revolutionize air (in addition to space) travel as you could then have non-range-limited aircraft in addition to the spacecraft previously discussed. All for a bit of boron…not too shabby. Very much “pie in the sky”…but an interesting thought experiment for sure.

In the mean time, I’ll still be putting my money on the space elevator tho 😉

The rocket equation taxes the living daylights out of any mass, weather it’s needed or not. Imagine ditching not only the boosters, but redesigning the payload structure so that it has no aerodynamic or re-entry heat loads to deal with. Now the remaining questions should be

1. How much is NASA going to blow on the mars program?
2. Why not use those monies to build an elevator on both planets?

[belbear]

texaslabrat wrote: In the mean time, I’ll still be putting my money on the space elevator tho 😉

I totally agree it will be a formidable and lenghty engineering challenge to build a fusion powered first stage launcher, but so is a space elevator. Only think of all that orbiting space junk that needs to be cleaned up before you can even start building a space elevator. It may take as well a FF launcher to do just that and pave the way for space elevators.

Just because a FF launcher can take off lightweight, it doesn’t even need those outrageous power levels and brutal acceleration associated with Saturn V rockets or Space Shuttles, so much less than a GW may do. Neutron shielding doesn’t need to be as heavy too. The crew compartment must of course be safe, but leaking some neutrons in the atmosphere is not really a problem. Cosmic radiation does similar all the time.

Once we have FF as a proven technology in the next couple of years, thousands of engineers can bend over all the new applications that are opening up.
Then, around mid-21st century, those advanced FF concepts beyond simple power production can already be realized. (even before the time of currently projected practical DT fusion)

Fusion-powered commercial airliners are one of those must-have’s before the 21st century ends, at least if the airline sector has the ambition to survive the fossil fuel era. Spaceflight can follow that research and military flight may even lead it. (they’ll definitely want unlimited-range bombers and fighters)
It not only eliminates most environmental issues ralated to aviation (CO2, soot, contrails), but also two important causes of aircraft disaster: Fire after an otherwise intact crash landing and out-of-fuel emergencies.

Hydrogen, the only other alternative, just eliminates the CO2 and soot issue and causes even more fire hazard. (remember Hindenburg and Challenger)

Passenger acceptance may be a problem in the beginning (Nuclear planes? No thanks, I don’t wanna light up in the dark), but by that time some inevitable mishaps with focus fusion units (all without dangerous radiation issues) will quickly subside fears. And what that toxic decaborane concerns: In open air, these few grams involved will quickly disperse to harmless levels. Such “minor mishaps” with dangerous chemicals are quite common in the chemical industry.

[Tulse]

belbear wrote: A FF powered scramjet does not do chemical combustion at all, so this problem does not pose itself, because it would work by simple heating of incoming air (using either electricity, X-rays or ion beam) And that always work.
Instead of fuel injection it could be using a tungsten heat exchanger, operating at very high temperature (2000°C) to transfer heat from the source to the airflow.

Thanks for clarifying that. I wonder if instead of a heat exchanger if something like a plasma torch would be more efficient, as it would use the FF electricity directly.

I’m also still not clear on the power required, however. What kind of power density does one have to reach to match that of hydrocarbon fuel? Is such density practically achievable?

I also wonder if FF won’t be far more useful in deep space, where low thrust electric propulsion systems are more practical, and where one might not need a lot of the support gear on needs in an atmosphere. For example, I presume it wouldn’t be necessary to have a vacuum pump to keep the reaction chamber evacuated in space, or to have the reaction chamber heavily reinforced to fight against implosion from atmospheric pressure. I would think a space-based FF reactor could be much lighter and smaller in space. (The one issue that might be more of a problem is dealing with the waste heat, as it it more difficult to radiate it away in a vacuum.) I would also think that the inherent simplicity of the system would be attractive, relative to fission reactors that require complex systems to turn their heat into electricity.

[texaslabrat]

belbear wrote:

In the mean time, I’ll still be putting my money on the space elevator tho 😉

Just because a FF launcher can take off lightweight, it doesn’t even need those outrageous power levels and brutal acceleration associated with Saturn V rockets or Space Shuttles, so much less than a GW may do. Neutron shielding doesn’t need to be as heavy too. The crew compartment must of course be safe, but leaking some neutrons in the atmosphere is not really a problem. Cosmic radiation does similar all the time.

As I mentioned before, an F-16 (a small, single-engine, lightweight fighter) in full afterburner consumes the rough equivalent of 300 MW. The top speed of an F-16 in full afterburner is about Mach 2.0 at altitude. Believe me, you’re gonna need power in the GW scale to even think about a hypersonic high-altitude first stage or a hypersonic trans-continental aircraft. As a “fun fact” comparison, each of the 5 Saturn V first-stage engines consumed a heat-content equivalent of over 30GW. It’s hard to wrap your head around the shear power requirements needed for high-performance flight..but they are indeed quite steep.

[belbear]

Tulse wrote:
Thanks for clarifying that. I wonder if instead of a heat exchanger if something like a plasma torch would be more efficient, as it would use the FF electricity directly.

A plasma torch (some sort of spark inside the airstream) looks possible, but then only electricity can be used as input, not directly fusion energy. And I have no clue how sparks behave in a hypersonic airflow. Could be as problematic to keep the spark firing as combustion is. Electric discharges are doused in high-power high-voltage switches, using a jet of inert gas such as argon to “blow out” the spark like a candle flame. In a plasma welding torch, a relatively slow flow of gas is used.

I was rather dreaming of using fusion energy even more directly than electricity in a two-stage fusion engine.
My idea is to use an “externally powered” thrust-FF reactor in the engine nacelle, whose input capacitors are charged by the first-stage, ordinary electricity-producing FF unit inside the hull.
ALL of the energy output from this thrust-FF (ion beam + X-rays + electrode cooling) would then be converted directly into heat for engine thrust by releasing all them rays into a tungsten heatsink, thus multiplying the input energy directly by the achievable fusion Q-factor and greatly reducing the amount of on-board electric power needed.
Actually this second-stage FF reactor is like an over-unity plasma gun, but since FF needs a pure hydrogen-boron gas fill to work instead of air, heat needs to be exchanged.
Maybe hare-brained but I like the idea.

Tulse wrote:
I’m also still not clear on the power required, however. What kind of power density does one have to reach to match that of hydrocarbon fuel? Is such density practically achievable?

I’m not sure. The power of reaction engines is measured in thrust force (kilograms, pounds..), which is the punch you can expect, and specific impulse (in seconds), a measure for how efficiently your input energy is converted into that thrust. My guess is that specific impulse for fusion jets can be assumed equal to that of conventional jet engines, but I would need to look into how to translate electric or thermal kilowatts into thrust.
As for the power density of the actual borane fuel, so little of it will be needed to reach orbit (a few grams) that its mass is neglectable against tonnes of engine and airframe mass. When dealing with a fusion engine, the mass of fuel needed is totally irrelevant, unless you intend reaching a sizeable portion of light speed.

Tulse wrote:
I also wonder if FF won’t be far more useful in deep space, where low thrust electric propulsion systems are more practical, and where one might not need a lot of the support gear on needs in an atmosphere. For example, I presume it wouldn’t be necessary to have a vacuum pump to keep the reaction chamber evacuated in space, or to have the reaction chamber heavily reinforced to fight against implosion from atmospheric pressure. I would think a space-based FF reactor could be much lighter and smaller in space. (The one issue that might be more of a problem is dealing with the waste heat, as it it more difficult to radiate it away in a vacuum.) I would also think that the inherent simplicity of the system would be attractive, relative to fission reactors that require complex systems to turn their heat into electricity.

Of course it will. But the launcher I propose is not intended to leave the atmosphere, maybe ideally to fly into low orbit. A real deep-space engine is to be carried up as a payload.
As I mentioned in one of my very first posts here, a space-optimized FF engine could take us to the stars by beaming the alpha ion beam directly into space as reaction mass. (charge-neutralized by electron emissions, of course)
X-rays can be recuperated into electricity as usual and the eventual energy deficit of the direct-beam FF engine should be filled with energy from a FF electricity generator (also space-optimized) or by partly tapping into the beam energy. You indeed don’t need a vacuum pump, unless you find the loss of unfused reactants (especially boron) unacceptable. In that case you do need to pump them off and recycle them. Space vacuum can help with that, though.
Radiating waste heat into deep space isn’t so difficult, on condition you shield your radiator from the sun. After all, space shows us a 3 Kelvin black-body and that’s really cold.

[HermannH]

texaslabrat wrote:
As I mentioned before, an F-16 (a small, single-engine, lightweight fighter) in full afterburner consumes the rough equivalent of 300 MW. The top speed of an F-16 in full afterburner is about Mach 2.0 at altitude. Believe me, you’re gonna need power in the GW scale to even think about a hypersonic high-altitude first stage or a hypersonic trans-continental aircraft. As a “fun fact” comparison, each of the 5 Saturn V first-stage engines consumed a heat-content equivalent of over 30GW. It’s hard to wrap your head around the shear power requirements needed for high-performance flight..but they are indeed quite steep.

This is the crux of the story. The F-16 can generate power at a rate of 300 MW, but only for about an hour or two. After that the tank is empty.
A FF reactor will probably never produce enough power to lift itself into low earth orbit. The power to weight ratio is way too small.
Once you are in orbit and want to travel to distant planets or solar systems the story is different: you can let the engine burn for days or months and keep picking up speed. 5 MW for a couple of months beats 300 MW for a couple of hours.

[belbear]

texaslabrat wrote:
As I mentioned before, an F-16 (a small, single-engine, lightweight fighter) in full afterburner consumes the rough equivalent of 300 MW. The top speed of an F-16 in full afterburner is about Mach 2.0 at altitude. Believe me, you’re gonna need power in the GW scale to even think about a hypersonic high-altitude first stage or a hypersonic trans-continental aircraft. As a “fun fact” comparison, each of the 5 Saturn V first-stage engines consumed a heat-content equivalent of over 30GW. It’s hard to wrap your head around the shear power requirements needed for high-performance flight..but they are indeed quite steep.

Hmm, indeed quite steep. But that’s why I propose a two-stage approach (see my other post). The second fusion stage releases all its fusion energy as heat for thrust. No coils, layers or conversion losses, just pure punch. Count it out: For each p-B11 fusion 150keV goes in, 8,7MeV comes out.

It will be difficult to squeeze into an F-16, (MAYBE if you accept to expose the pilot’s balls to an unhealthy dose :sick: ) but a 100MW of electricity from stage 1 MAY be sufficient to provide the GW’s of thrust we need.
Of course it remains highly speculative until we can use data from an actual power-positive FF prototype.

[texaslabrat]

HermannH wrote:

As I mentioned before, an F-16 (a small, single-engine, lightweight fighter) in full afterburner consumes the rough equivalent of 300 MW. The top speed of an F-16 in full afterburner is about Mach 2.0 at altitude. Believe me, you’re gonna need power in the GW scale to even think about a hypersonic high-altitude first stage or a hypersonic trans-continental aircraft. As a “fun fact” comparison, each of the 5 Saturn V first-stage engines consumed a heat-content equivalent of over 30GW. It’s hard to wrap your head around the shear power requirements needed for high-performance flight..but they are indeed quite steep.

This is the crux of the story. The F-16 can generate power at a rate of 300 MW, but only for about an hour or two. After that the tank is empty.
A FF reactor will probably never produce enough power to lift itself into low earth orbit. The power to weight ratio is way too small.
Once you are in orbit and want to travel to distant planets or solar systems the story is different: you can let the engine burn for days or months and keep picking up speed. 5 MW for a couple of months beats 300 MW for a couple of hours.

Actually, an F-16 can produce 300MW for about 11 minutes before the tanks go dry so it’s far worse than even that scenario. But it’s apples-to-oranges since a FF-powered craft would, for all practical purposes, never run out of fuel and thus could take a longer path to achieve a given speed and altitude as long as it could get off the ground in the first place. My main point (as you’ve mentioned) is that the power-to-weight and power-to-volume of FF would have to be DRASTICALLY improved from today’s concepts/prototypes to be suitable for flight due to the enormous amounts of power required. Not saying it can’t happen (never bet against the ingenuity of motivated engineers)…just that there are probably easier technical avenues for achieving orbit and subsequent space flight (eg space elevator + FF-powered ion engine).

And by the way, the basic premise of the FF-powered aircraft (as envisioned by belbear in this thread) is not new. Look up “project Pluto” for a “blast from the past” 😉

[texaslabrat]

edit: nm…that post didn’t even make sense to me after I thought about it.

[Tulse]

belbear wrote:

Radiating waste heat into deep space isn’t so difficult, on condition you shield your radiator from the sun. After all, space shows us a 3 Kelvin black-body and that’s really cold.

Right, but that is purely radiative cooling, which as I understand it isn’t nearly as efficient as conductive or convective cooling — there’s a reason that thermos bottles use vacuum flasks. As “cold” as space may be, you can cool things far more efficiently on earth by, for example, dumping heat into a lower temperature fluid. (I’m sure that some one with way more technical expertise could clarify what sized radiator would be needed to dump 5MW of heat into space.)

[HermannH]

Tulse wrote:

Radiating waste heat into deep space isn’t so difficult, on condition you shield your radiator from the sun. After all, space shows us a 3 Kelvin black-body and that’s really cold.

Right, but that is purely radiative cooling, which as I understand it isn’t nearly as efficient as conductive or convective cooling — there’s a reason that thermos bottles use vacuum flasks. As “cold” as space may be, you can cool things far more efficiently on earth by, for example, dumping heat into a lower temperature fluid. (I’m sure that some one with way more technical expertise could clarify what sized radiator would be needed to dump 5MW of heat into space.)

I used this calculator to get the blackbody radiation at 300 degrees Celsius. It is a surprising 6 kW / square meter.

The radiation is highly sensitive to the operating temperature; at 200 degrees Celsius it is less than 3 kW / square meter.

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