FF for Jet Engines?

[vansig]

zapkitty wrote:
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.
[…]
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.
[…]
So it seems to me that the question now goes back to direct alpha heating vs REB heating… but did I miss anything?

No market for supersonic cargo flights? there goes my delivery service, then. (“world-wide 90-minutes or it’s free”) 😉

Re efficiency.. that’s actually not as important as thrust:weight ratio, heat management, specific power, overall energy density, and specific impulse.
Nuclear wins hands-down on overall energy density.. even if it’s a larger reactor. Everything else is a matter of engineering.

greater efficiency does help these other things; but hypersonic speeds are already like exposing yourself to high temperatures.
7.8 km/s through atmosphere is about like having a 7800 kelvin head-wind; i’m glad it will be at low pressure.

[zapkitty]

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.

[zapkitty]

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

[vansig]

the THz beam has two efficiencies to think about:
1. is electrical efficiency of the THz emitters; this is a nano-klystron array consisting of thousands of cells, focusing on the air in the reaction chamber and expansion tubes;
2. is absorption of THz energy into propellant. we’d want to pick an absorption line in air that is wide enough to absorb well, through delta-v of ±7.8 km/s or more and temperatures from 240 kelvin and up. like other scramjet designs, we’d consider a 5-species model consisting of N2, O2, NO, O+, N+. If absorption tends to be low, long expansion tubes may help.

I tried to keep the temperatures down to under 2000 K, to avoid NOx, but couldn’t.

Using plasma windows on ducting could have an advantage of fewer moving parts.

[zapkitty]

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

?

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

[zapkitty]

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

[vansig]

zapkitty 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

[zapkitty]

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…

[zapkitty]

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”

[zapkitty]

Edit jan 30: I screwed this up royally the first time around mixing feet and km. Shouldn’t have started doing stream-of-consciousness jotting until I was sure I was actually conscious…

spaceplane sanity check: wishlist

4 tons payload in a
3 meter x 4 meter payload module
(most space payloads are light for their bulk)
2 crew
2 mission specialists/passengers
and/or
4 more ms/p in a passenger module

2 power modules with 10 FF cores each for 100 MWe

2 compressor/MHD engine nacelles providing 250 kn each

air breathing to mach 3 @ 10km
air breathing w/DPF augment from 10km to 100km
straight DPF from 100km to orbit.

[zapkitty]

Oooh… even with a 1cm channel through a meter of shielding the radiation is kinda fierce… x20 channels is not as friendly a neighbor as I’d hoped for. To maintain the safety margin either closable ports w/ the added weight and complexity or use beam heating all the way and onboard propellant for the last stage… ouch… needs more MWe…

… but as the MHD units have to have beams anyhow for their work…

[Tulse]

zapkitty wrote: mach 8-10, which seems to be an optimal velocity to launch to orbit from…

Perhaps I’m not following, but can you clarify what you mean here? Isn’t orbital velocity ~ Mach 25?

[Aeronaut]

zapkitty wrote: Oooh… even with a 1cm channel through a meter of shielding the radiation is kinda fierce… x20 channels is not as friendly a neighbor as I’d hoped for. To maintain the safety margin either closable ports w/ the added weight and complexity or use beam heating all the way and onboard propellant for the last stage… ouch… needs more MWe…

… but as the MHD units have to have beams anyhow for their work…

Why not try BWB with 1 to 5 thrusters in the fuselage, along with the 10 to 20 cores? And assume desert spaceports, of course.

[zapkitty]

Tulse wrote:

mach 8-10, which seems to be an optimal velocity to launch to orbit from…

Perhaps I’m not following, but can you clarify what you mean here? Isn’t orbital velocity ~ Mach 25?

“… optimal velocity to launch to orbit from…”

Spacecraft of all types want to get above the sensible atmosphere first before seriously putting on speed. Chemically-powered spaceplanes scoop air to supplement their fuel while in atmosphere but they have to carry all of the fuel and oxidizer for that final sprint to orbit when the air runs out… as well as the fuel for the air-breathing portion of the flight.

So spaceplanes want to get as high and as fast as is feasible before heading for orbit… but pushing that too far ends up hurting more than it helps as they have to carry and burn up fuel that could be better used on the climb to orbit. Different concepts take off for orbit between mach 8 and mach 12 depending on the design.

I just got carried away with the fuel-free aspect of DPF flight without realizing I was literally poking holes in my “friendly neighbor” paradigm. Shiny objects, y’know 🙂

(for something often compared to an ion drive the FF version of a DPF can actually pack quite a kick for something that can be treated in the short term as an essentially fuelless thruster)

[Aeronaut]

I thought ‘less than friendly neighbor’ meant drastically limiting shielding mass…

[Author]

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