[vansig]

exit beam alphas should be roughly from 600 to 2900 keV, which is *not* relativistic.

at the upper end of this, 2900 keV = 4.646e-13 joule, mass ~ 4u = 6.64e-27 kg
and kinetic energy, E_k = .5 m v² so

v = sqrt( 2 E_k / m) = sqrt ( 2 x 4.646e-13 / 6.64e-27 ) = .04 c

the bulk of them are probably closer to 600 keV = 9.613e-14 joule,
v = sqrt ( 2 x 9.613e-14 / 6.64e-27 ) = 0.018 c

*after* extracting 80% of their energy with the capture coil, they’ll still be traveling at 2400 km/s, which is 42x the relative momentum of any given particle in the compressed airstream at mach-24. but even a small number of collisions with any air particles will change that, in the same way as shooting a break at a pool table slows the cue ball.

by the way, for each 5 MW anode, i estimate ~ 10^17 alphas in a pulse, 1000 pulses/sec.

for a 400 MW engine, that’s 8 x 10^21 alphas exiting each second. (about 53 mg)

in other news, industrial physicist has a good article by Dean Andreadis, showing scramjet engine design
http://www.aip.org/tip/INPHFA/vol-10/iss-4/p24.pdf

[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

[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.

[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.

[vansig]

jamesr wrote:
For example the copper peak in the SEM-EDAX graph (p176) is tiny – look at the number of counts on the scale
Poisson error on N counts is sqrt(N) so peak is barely above noise level. To me that graph just looks like the sample was badly cleaned and contaminated from other sources.

I’ve lately been reading up on metal electroplating processes, and my opinion after studying this graph is,
that we are looking at nickel-plated aluminum electrodes as they would appear after routine wear.

Lots of aluminum alloys contain silicon and magnesium; further, copper plating is often applied before the final nickel finish.

[vansig]

Interesting. I’m reading NUBASE data. here’s its help file…

NUBASE is a “horizontal” nuclear evaluation designed by
G. Audi, O. Bersillon, J. Blachot and A.H. Wapstra.

NUBASE has integrated results from 2 evaluations:
a) The Ame (Atomic Mass Evaluation);
b) The Ensdf (Evaluated Nuclear Structure Data File)
It has been completed and updated from literature. Full
references are given for all experimental information.

If no experimental data exist for a nuclide, in many cases a
value could be estimated from systematic trends. Such values
are clearly labeled with the `#’ symbol.

The most recent published version of NUBASE is Nubase’97:
Nuclear Physics A624 (1997) 1.
The present data are from Nubase’97 plus updates from literature.
Corrections are gratefully received (audi@csnsm.in2p3.fr).

[vansig]

Brian H wrote: No, they’re quite specific about a small amount of nickel being converted to copper. One claim is that running continuously for 6 mo. converts over 50% of the sample.

if there’s *any* copper produced whatsoever, then it would be altogether too surprising if there weren’t also quite a lot of radioactive Ni-59 produced; (half-life 101 ky), formed by the reaction
Ni-58 + p -> Cu-59 -> Ni-59 + e+.

A test for Ni-59 that would seem to be conclusive.

[vansig]

MTd2 wrote: 30KJ of net energy at each firing x 180 ~ 5MW
Alright, you say 1000Hz, so he expects 5KJ converted for each firing. But, why this value, then? Why not 500Hz or 2000Hz or whatever?

not 5kJ converted; but 5kJ net output after charging for the next shot

[vansig]

MTd2 wrote: Considering a net surplus of 30KJ, as it is intended by the team, why the pulse frequency is 180Hz? Why not 10Hz or 100Hz or 500Hz or 1000Hz?

i dont know where you get 180 Hz, but a few things factor into this:
1. conversion efficiency; and
2. maintaining the chamber at plasma temperatures for the next shot

(1) is not 100%; and for (2), if i recall correctly, Eric said that at least 1000 pulse/s is preferred.

[vansig]

Aeronaut wrote:
Sorry, I’m still not seeing how the compression is achieved without the compressor stage fans. A Blended Wing Body (BWB) architecture may be easier to engineer for mach 24…

BWB is even-more like the manta ray shape, true.

Well, SR-71 design is clear, but tops out at about Mach 3.2. How’s compression achieved in existing scramjet designs?
Isn’t this done by manipulation of shock waves?

By the way, this book has a section on scramjets, that mentions how electricity assists present designs:
Low temperature plasmas: fundamentals, technologies, and techniques edited by R. Hippler
— http://tinyurl.com/6d2vcxw

What we need right now is a team of Maxwell’s demons, to open the window for particles traveling in preferred directions only. 😉

I’m brainstorming, that the doppler effect will help: the emitters will be carefully tuned and focused, so that particles moving the wrong way through the engine are slowed, and particles moving the right way are accelerated.

Oh, and I like vortices, too. If we use them, then this should be a twin-engine design, for stability.

[vansig]

Brian H wrote:

plasma window

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

Do remember that 9 atmospheres is 0.9 million newtons per square metre. 🙂

That’s like supporting a 90 tonne vehicle on top of a 1m² window.

Seen head-on, a vehicle of this type might be reminiscent of a manta ray. With a huge mouth, its cross-section is mostly engine, in comparison to its airfoil and payload.

Attached files

[vansig]

zapkitty wrote:

i meant the THz laser beam

At what efficiency is the beam expected to operate?

To be determined. I don’t have detailed-enough spectral charts to assist with the task of optimizing it, and spectroscopy guys tell me to be prepared for the possibility that the data might not yet exist — so there’s science to do. The emitters would likely be a large array of nano-klystrons.

For comparison, quartz heaters are 85% efficient at heating objects *without* heating the air, but they operate as blackbody radiators, with bulge around 100 THz.

We’ll be looking for strong absorption lines for air, in the 1 to 6 THz spectrum, that remain strong over a range of temperatures from about 240 to 2000 kelvin.

Atmospheric pressure is ~0.01 bar at 35 km. We’ll want to know the path length for 100% absorption, here.

240 K is the inlet temperature at 35km altitude; and 2000 K is the upper bound to avoid production of NOx. Keeping this squeeky-clean on the environment sure would help to sell it.

2000 from 240 K is an 8:1 increase in pressure, as gas flows through the expansion tube. the scramjet design does not slow the gases. for low speeds, the air inlet shape might have to change to avoid back-pressure in the tube. there are several ways to accomplish this (including the plasma window).

Yes, this could go into a surface-to-orbit vehicle. Being an air-breathing engine while flying through atmosphere, it will really save on propellant. Mach-24 horizontal flight at high altitude is the magic threshold that will make it possible. We can expect, if this works at all, to see the classic shock diamonds characteristic of rocket thrusters and scramjets.

[vansig]

zapkitty 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.

i meant the THz laser beam

[vansig]

zapkitty wrote:
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.

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.

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

[vansig]

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.

The engine design is so simple that i’m not really seeing a minimum speed for this.
Hot air simply flows out from one end of a large tube, while cooler air gets sucked in the other end.

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