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Ivy Matt wrote: Perhaps, with sufficient advances in capacitor and other technologies, a FF device could fit into a lightsaber handle in the not-too-distant future.

…errrr… shielding?

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It’s interesting to speculate how far you can go with just compressor stages as essentially big ducted fans and MHD transfer and augmentation… with no heating stage per se…

… perhaps fast enough (mach 2+?) to go directly to a beam-heated ram duct for the slog from there to mach 8-10, which seems to be an optimal velocity to launch to orbit from…

… of course with FF one might well launch to orbit from mach 2-3 @ 60-70k ft…

edit: garbled “optimal”

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vansig wrote:

Query: what does the THz laser offer that REB or direct alpha heating doesn’t offer?

It *might* act at tunable distances and temperatures; and it *might* be possible to select wavelengths for strong absorption in propellant without absorbing strongly in tube walls.

for comparison,
if i recall correctly, VASIMR uses microwaves to heat propellant to plasma temperatures, at which point it accelerates the plasma magnetically. presently, VASIMR sizes are too bulky for a decent enough thrust/weight ratio for surface lift-off, *and* must be operated in vacuum; but what are the ultimate consequences of scaling it up, if amount of electrical energy required is not a factor and the propellant is atmospheric air?

this paper shows that the deepest absorption line for air is near 4.4 to 4.6 THz, with .001% transmission over a distance of 80cm; seems just right, but this would have to be studied at high temperatures also:
— http://act.nict.go.jp/thz/en/2/research2_e.html

… but that’s for em radiation, not particle beams.

As it turns out, most references to DPF propulsion assume it’s a vacuum-only thruster. Where REBs are mentioned it’s for the atmospheric stage of flight in the form of augmenting chemically-powered jets and heating additional propellant.

The odd thing seems to be that the plasma window and direct alpha heating are not mentioned even though some of the papers are as late as 2005… but then they were after high-performance military vehicles and the plasma window’s 9 atmosphere limit doesn’t lend itself to jackrabbit starts…

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Ah, it seems that some papers examining fusion propulsion combined DPFs with Bussard’s work…

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vansig wrote: the THz beam has two efficiencies to think about:

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Query: what does the THz laser offer that REB or direct alpha heating doesn’t offer?

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Strange, can’t find the Lerner DPF REB references… was I confabulating the DPF space work with Bussard’s space work?

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An FF jet engine:
A jet engine powered by an FF unit would be… different.

And it would operate by different rules.

No combustion. No need for supersonic combustion. No need to slow the incoming air to subsonic speeds.

The air would still need to be compressed while being heated. The engine geometry might have to change to adapt to the airflow going supersonic in the ducting, perhaps by taking the subsonic compressor out of the way at high mach numbers. But it would not need to slow the air down solely to get the airflow subsonic. And implementing the MHD trick will have different implications as well.

A “standard” FF unit emits three things that might be of interest to a jet engine designer: an energetic beam of helium nuclei (alpha particles), a burst of x-rays, and heat that is a byproduct of the fusion process.

In the standard FF setup the alpha beam is converted to electrical current and that is used to recharge the capacitors that triggered the fusion pulse as well as operating the vacuum pumps and other auxiliary gear. Any excess energy from this process is net power that can be used elsewhere. In addition a multi-layered photoelectric “onion” surrounding the FF unit captures the x-rays and converts them to electricity. This is also net power. The total net power of a “standard” FF unit is hoped to be about 5 MWe.

The heat from these two conversion processes and the heat from the fusion process itself is all considered waste heat and it is estimated that this will total about 7-8 MWt in a standard 5 MWe FF unit. Unfortunately for power plant operations this is regarded as “low grade” heat that might not even reach 600 degrees C in a single FF unit.

Transferring the energy:
So the waste heat is insufficient. Nor do the x-rays lend themselves to heating air very efficiently. If the “onion” is omitted and a sturdy shell of a material opaque to x-rays replaces it this could heat incoming air by contact… but to do this efficiently would require running the entire engine at very high temperatures.

Heating the propellant via a beam eliminates the need to heat the entire engine core. Indeed, it would be superior in that regard to the standard jet combustion process.

Professors Lerner and Bussard have both proposed using electrically-generated relativistic electron beams (REBs) to heat propellant in engines powered by their respective aneutronic fusion generators. It just so happens that such beams can be efficiently powered by the very high voltages that pB11 fusion is expected to produce.

Correction: it seems to have been just Bussard with REBs for the polywell, although others did propose applying REBs to craft powered by DPF.

But what about the alpha beam? Why generate an additional particle beam when one has already been generated as part of the fusion process? Could it heat the propellant directly… without the overhead of an intermediate REB?

The balance of power:
If the majority of the alpha beam is to be used to heat the air then the x-ray conversion “onion” must help take up the tasks of recharging the capacitors, running auxiliary gear and powering things such as intake fans and MHD generators. Or, alternatively, additional FF cores in the unit can help take up the load.

Under pressure.
One drawback to direct alpha heating is that current plasma windows can handle only 9 atmospheres of pressure against a vacuum. This is counterbalanced by a near 99%+ conversion rate… only ~8kWe is needed to maintain a 1cm diameter plasma window. And based on older jet aircraft performance that pressure should be sufficient for supersonic flight … as there is no intake decelaration drag and none of the energy is needed back to run the compressor etc. That’s all already been “paid for” up front.

So that would seem to leave us with seeing if the higher operational pressures afforded by the REB are offset by the energy overhead and added weight and complexity of the REB gear.

… to be continued…

edit: Lerner and REBs correction.

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As with my previous musings on spacecraft power generation and fusion-powered aircraft this will assume that the FF fusion power generator proves out and operates on aneutronic proton-Boron 11 fuel close to the currently hoped-for specs.

But unlike the propfan aircraft proposed earlier for cargo and passenger transport this time we want to see what what would be needed for jet-like engines to be implemented around an FF powerplant. Jet-like in order to enable supersonic flight as the first step to orbital flight.

It gets interesting as many of the assumptions that govern the design of current jet aircraft apply themselves very differently to a fusion-powered craft… or do not apply at all.

Also of interest is the fact that many of the standing assumptions about handling nuclear fission power plants in aircraft also turn out to have radically different implications when applied to aircraft powered by aneutronic fusion.

As a useful set of parameters to shoot for in this first cut I’m vaguely thinking of an aircraft that is between the SR-71 Blackbird and Reaction Engines’ Skylon spaceplane in size and capability.

If we can show that FF can get a few people and a few tons of cargo first supersonic and then into orbit that will be sufficient proof-of-concept for now. The single-stage-to-anywhere interplanetary cruisers can follow later 🙂

First up: safety.
Given the essentially unlimited power provided by FF I do not see any real advantage in trying to leave shielding off just to save a few tons in mass.

In fact design and implementation becomes much easier if one not only starts out with standard FF shielding but embraces it to the point where a flying FF unit is essentially as well-protected as a nuclear waste transportation container… only without being nearly so toxic and radioactive.

Then the FF craft can take off, fly amongst other aircraft, fly to orbit, dock with spacecraft and stations, undock, enter atmosphere, approach an airport, land, taxi, and enter a hangar… all without any concern at all as to where the engines are in relation to anyone else.

And not incidentally things are much cheaper that way 🙂

A standard jet engine:
In a standard jet engine a compressor stage pulls in air in front and feeds that air to a “burner” stage or “burner can” along with jet fuel. The fuel is ignited and the combustion and the combustion products heat the remaining air. All this heated material attempts to expand but finds it easier to escape to the rear of the engine. It becomes the jet exhaust. Along the way additional incoming air can be entrained with the exhaust, lowering the exhaust velocity but increasing the total thrust of the engine.

All this occurs at subsonic air velocities inside the engine. Even supersonic aircraft with ramjet engines must slow the incoming air to subsonic speeds at the engine intake before attempting to burn fuel in the airstream. This deceleration of air at engine intakes is a real drag (pun intended) and is a major source of unwanted heating in supersonic aircraft. And it is also why scramjets are seen as a way to true aerospacecraft. A scramjet (supersonic combustion ramjet) allows the engine to burn fuel in a supersonic airstream inside the engine. It’s not easy to do.

Another proposed solution to the engine airflow drag issue, and one that is very relevant to an FF-powered jet engine, is magnetohydrodynamics. MHD is hoped to enable conventional turbojets to reach hypersonic speeds… without needing supersonic combustion.

In these aircraft MHD generators are to be used to absorb the energy of incoming air, converting that energy to electrical power and slowing the air to speeds a conventional jet engine can handle. Once the air has been through the “burner” in the engine the MHD would take the electrical power it absorbed from the incoming air and accelerate the exhaust as it exits the engine. An energy transfer.

I believe that’s a recap of where we are starting from… might as well be as the forum chopped the rest of my post for length 🙂

… to be continued…

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QuantumDot wrote: Well the first question should be if you are going for Constant Pressure Combustion or Constant Volume Combustion.

… errrrrrr… I think we’re going for the No Combustion option 😉

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Returning to the fray after a severe bout of what was probably stomach flu… probably…

to recap: vansig wants fusion-powered jet engines because that can lead to supersonic flight and from there to spaceplanes.

That’s ok because currently projected FF units… even vansig’s massively ganged units… will not be able to compete economically with chemically fueled supersonic military combat units. And there still isn’t much of a market for supersonic cargo flights.

I believe that first subsonic and then high-subsonic passenger and cargo flights will be dominated by the fusion-powered propfans I have described.

In seeking ways to transfer energy from a notional array of FF units to the airstream vansig has proposed laser heating using a terahertz beam.

I have proposed using the alpha beam directly from the FF array directed through a plasma window into the airstream.

As a baseline Lerner-hakase proposed using a relativistic electron beam (REB) for heating propellant.

The first thing that must be addressed is the efficiency of the heating. A Thz laser is not very efficient. A free-electron laser tailored to operate in the precise Thz-range frequency desired might be as much as 40% percent efficient in transferring energy from its electron beam to its laser output…

… but then how are you ever going to catch up to the efficiency of just using a straight REB for heating? Not so good.

other issues…

Brian H wrote:

plasma window

Can handle up to 9 atmospheres; doesn’t seem like enough for the enviro of a jet engine.

Depends on the jet engine referenced. 10-40 bar is a common pressure range… 140 psi is respectable.

And the FF-powered unit has a huge advantage over traditional burners that might not be immediately apparent: the heated air does not have to give up any of its energy to power the intake compressors and auxiliary gear. Powering such takes upwards of 75% or more from the output of a standard jet engine.

75%.

FF don’t do dat 🙂

Past experience has proven that 140 psi can take us supersonic, given time, and scramjet design is all about maintaining combustion in a suprsonic airflow… and FF ain’t gots combustion either.

So it seems to me that the question now goes back to direct alpha heating vs REB heating… but did I miss anything?

I’ve taken a few preliminary cuts as to what might be involved in REB vs. direct alpha, but let’s make sure it’s on the right track to begin with 🙂

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vansig wrote:
i meant the THz laser beam

At what efficiency is the beam expected to operate?

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vansig wrote:
Yeah, microwave electrothermal thrusters use a propellant, like water; in space we’d still need a propellant, but this is a jet. i’d prefer to avoid noxious exhaust products, so maybe not raise it to plasma temperature.

Well, for the effect to work only a small portion of the exhaust needs to reach plasma… but it seems this might all be mooted by the invention of the plasma window.

Not only can this “force field” contain a vacuum against atmosphere at the cost of a few kilowatts while allowing a particle beam to exit that vacuum without having to vent the system… but it can focus an ion beam that happens to pass through it.

And alpha particles don’t travel far in atmosphere before being stopped.

vansig wrote:
of course there’s science to do, to make this go. i dont have all the answers, but tuning and focusing the beam seems to be the key

Just remember that the aloha beam is hoped to be used to recharge the caps with some juice left over.

If that balance is to be altered then the energy to recharge the caps must come from somewhere else… like an onion. Which might be workable, just something else to keep in mind.

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vansig wrote: Air has a strong absorption line near 1.3THz (230 µm). The idea is to superheat the air with a narrow-band, coherent beam that reflects off container surfaces.

I doubt heating air with x-ray can be done. x-rays are hard to focus,
though we’d want to carry away waste heat by convection, or radiation if possible.

X-rays can, however, efficiently heat some materials. My vague idea was that you’d place such a material around the FF core assembly to catch the x-rays and then while the air is entrained in your various manipulations you’d pass it around the heated FF containment.

… and weren’t relativistic electron beams the first option of Lerner-hakase for heating air and propellant since they are best driven by the extremely high voltages that FFs are expected to produce?

Starting with that premise my other idea was to just skip the conversion step and use the alphas directly.

As for ensuring the efficient transfer of beam energy to air… the people working on microwave electrothermal thrusters have already solved this by using focused energy to create a small spot of plasma in the propellant flow. The plasma is pretty much guaranteed to be opaque to infrared, electrons, alpha particles, microwaves and suchlike that you might want to throw at the propellant as long as you maintain the hotspot.
(you wouldn’t try to use this with the x-rays because as you noted the spherical x-ray flux couldn’t be easily focused)

So I’ve no objections per se to an FF-powered jet aircraft… it’s just that the energy density and conversion problems immediately bring up the question of how you would like to do this 🙂

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vansig wrote:

An FF transport would not even be as fast as a 747… mach 0.72 as opposed to mach 0.85…

What I still don’t get is, why is Mach 0.72 an upper bound?

That was just me using “default” 5MWe FF boxes for aircraft concepts equivalent to current commercial turboprops. A “go anywhere at any time” electric aircraft.

Turboprops such as the Tu-95 have gone as high as Mach 0.92 in tests.

vansig wrote:
We’re not limited by mass; 80 anodes tesselating a sphere yields 20 tonnes, not 160 tonnes.

We’re not limited to propeller or turbo-prop configuration.

Fusion powered jet does not require a turbine; we don’t have to slow the airstream down or compress it, to heat it, as a ramjet does, since we don’t have to inject chemical fuels.

Scramjet configuration seems to be the simplest engine architecture. and it’s highly efficient, *if* we have a way to heat air with electricity. which we do: i propose to build a high-wattage infrared laser, emitting at a wavelength that air strongly absorbs, yet engine materials do not absorb.

What remains, that could keep this below high Mach numbers and altitudes of 35km or more?

And that brings us back to the question of whether using the excess alpha output for direct heating of air would be more efficient. Your design is for 400 MWe, right? Are you omitting the 640 MWt “waste” heat from your calculations? What if there was no “onion” and you tried to use the x-rays as a heat source as well?

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MTd2 wrote: If it is being shown impossible to get decent focus cte. axial field why not trying a variable one. Make it spike with 30-100gauss 500ns before pinch. Thoughts?

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Did any news in particular bring this up?

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