Microwave Rockets

Robin shared with me that she met Kevin Parkin:
http://kevinparkin.org/index.asp

He has an interesting concept of Microwave Rockets:
http://monolith.caltech.edu/Papers/ParkinLauncher.pdf

I hope it will be good enough one day to counter gravity and create an even cheaper access to space, what do you think ?

I like it. For cargo only though. 19 Gs! Not for me!

You know, he could benefit from a DSS that could carry his power source and microwave generators! The obvious advantages are staying out of the 1-2km of altitude where water vapor is a problem, plus avoiding having to do the horizontal thrust in the thicker air at lower altitudes. Also, the beam range might be better up there (but I don't know what limits the range, can someone tell me?) I suppose you'd need a special DSS to float nearby and hold the launch vehicle, unmanned preferably (don't want to get nuked!).

Much has happened with the microwave thermal propulsion project after the initial two papers back in ‘03:

http://monolith.caltech.edu/html/Publications.html

Since then all of the writing effort has gone into proposals. Despite that we managed to build a small-scale setup in the lab and got initial results in April:

http://monolith.caltech.edu/Images/MTEarly.jpg

In that setup the microwaves didn’t heat the tube along a great enough length to raise the hydrogen temperature as much as we’d like (to ~ 2000 K), so we switched to a cylindrical cavity. Last week’s test was most encouraging:

http://monolith.caltech.edu/Blog/Archives/000201.html

In parallel with that I’ve been developing computational models:

http://monolith.caltech.edu/Blog/Archives/000199.html
http://monolith.caltech.edu/Blog/Archives/000194.html

It seems that the numerical work has overtaken the experiments, so it looks like I’ll have a simulation to show whether or not this thing will work over Xmas, and in January we should be running the new cavity with flowing hydrogen as well. Things are coming together at long last

In response to your comments:

- Yup, cargo is my priority, people come later. If you must launch people you can simply put them in water up to the neck and the g's aren't a problem. There are more extreme ways too: Ever seen those fluid breathing systems in the film The Abyss? They aren't sci-fi, and the rat they tested it on wasn't a special effect...

- What's a DSS?

Breathing with fluorocarbons at least is very unlikely to greatly help cope with high accelerations due to its fairly high relative density, also filling lungs completely makes breathing very nearly impossible even under good conditions. I am also not convinced these chemicals are completely harmless to the lungs.

If you want to try water immersion, you maybe want someone lying horizontally and approximately half-submerged. 19G might still be beyond the limit of endurance for a conscious person.

[There theoretically may be some insane way of suspending people in a magnetic field to apply a uniform acceleration, it might unfortunately involve a machine the size of the Earth. There may also be other better ways...]

DSS is the initialism of Dark Sky Station, a kind of near-space balloon platform envisioned by JP Aerospace.

Thanks Kevin for the more detailed information,

If nihiladrem is right about the limits of a concious person using water or breathing with fluorocarbons (anyone has more information about this ?), the largest cost of people in space will and I guess always will be cargo. So even if it can't fly people to space, it can be a very important milestone in manned space exploration. Especially if we want to achieve more than business only tourism in space. (without the too large costs)

For more information about DSS (Dark Sky Station), by JP Aerospace:
Their Website:
http://www.jpaerospace.com/

Their official forum hosted on this site:
http://www.spacefellowship.com/Forum/viewforum.php?f=25

The concept is also used (or possible copied) by others:
http://www.cruiser.ru/eng/releases.php

Of course, it's all work in progress.


From now on, I'll watch your project closely

I'm not sure there is necessarily a problem with people blacking out, so long as the living cargo is not actually damaged. Vibration might change the picture, but there's nothing that initially suggests it could not be reasonably smooth.

If I have to black out or inhale a special liquid, I will just stay on the ground. The floating on water would be OK, but I don't think it will reduce the stress of high acceleration as much as some may think. Has anybody done experiments on this? Test subjects floating in water in a centrifuge? I never heard of any.

Agreed. *** WARNING *** If you're an animal lover, cover your eyes now!

Apparently the Italians did some experiments where they fired pregnant rats from an air cannon into a wall. The foetuses survived hundreds of g's unharmed because they were in fluid. If anyone has papers for human research I'd be interested.

As I say, it's for cargo. With or without humans in space, most of what you're launching will be cargo. Once the cost of launch comes down enough, perhaps it may be worthwhile to build a beam facility in orbit that allows a much more leisurely ascent.

Being half-submerged lying flat isn't much different or better than lying on a seat that is *extremely* conformal to your body. (perhaps having fluid filled pockets to achieve that). You don't strictly need a liquid to manage much the same (and there have probably been human tests which are somewhat comparable, though with centrifuges there are effective limits on how short you can apply force for, and sled-tests a limit on how long forces can be applied for)

18g was once considered a lethal limit for jet-pilots, though Stapp may have had something to say about that. If you make a guestimate of the weight hanging otherwise unsupported off the ribcage lying down at 19g, you can get an idea of how much pressure is trying to collapse your lungs. For me, I think it would be somewhere between 100kgf (which I could maybe grudgingly handle) and 160kgf (which I'm not sure I could even manage for a second). There are other unpleasant things going on with blood-pressure gradients, but as a general rule you can always get away with things for very short periods that are completely unsustainable over longer periods. This even includes strength of materials, allowable elongation etc, forces muscles are capable of applying (usually while being back-driven).

Liquid breathing (look on Wiki) has some heritage, however I really do not envy the test-subjects lungs.

My main objection with filling even just the lungs with a liquid which is significantly denser than human tissue is that I think they become damaged with any real kind of internal pressure. I think the lungs get damaged easily, well before the rib-cage reaches its limits under the structural loads involved. Something else that came to mind was that holding these accelerations could cause inner-ear problems due to things shifting about. This additional kind of lasting disorientation could really ruin a trip to space.

Animal experiments:
The mechanical and physiological effects of acceleration are inherently scale dependant. This is why ants can withstand a thousand g apparently without any harm.

I'm not a fan of beamed power in general but if microwave thermal systems (very similar to laser-launch as they are) can deliver cargo/fuel to space cheaply, it is probably a very good thing.
Kevin - How much specific power are you hoping/expecting to get out of your heat-exchanger? Are hot spots a problem with microwaves or is this aspect a bit kinder than in a nuclear-thermal system?

All excellent points.

- Here's a link to that Wikipedia entry: http://en.wikipedia.org/wiki/Liquid_breathing

- Assuming that the problems of lungs are solved by some means, my understanding is that the limit to acceleration is determined by the small density difference between the brain and the cerebral fluid it floats in. It's the sort of thing you'd comtemplate with relativistic journeys.

- Averge power density is ~ 10-100 MW/m^2. The wavelength I'm designing for is ~ 2 mm (140 GHz). Millimeter waves with about double or quadruple that wavelength are used for industrial microwave heating of ceramics due to their heating uniformity.

- The objective here is to get the ceramic to absorb 100% of the incident energy from an incident plane wave and the layer we are heating is only ~ 1 mm thick. I expect it would be difficult to form a hotspot in this sort of configuration. The cylindrical resonant cavity configuration I'm testing in the lab is more akin to the nuclear thermal rocket (cylindrical neutron gas), and I do indeed get hotspots which I am trying to carefully iron out by changing the field configuration. This is a very different situation to the free-space one above though.

Oh man, after reading that liquid breathing stuff I am even more determined to never submit to that!

It seems the high G load is required because the vehicle would fly over the horizon before reaching orbital speed, and if ground stations could be spread out along the flight path, a lower acceleration could be used. But finding a string of sites with high transparency at the desired wavelength is a problem.

So...

What about launching directly to escape velocity, to send people to the Moon or whatever. The vehicle could fly almost directly up from the launch site and stay in view of the same ground station much longer. The only problem would be keeping the beam focused on the vehicle at greater distances. Does that sound workable?

*LOL* Well, as I say if the launcher meets its goals of reducing launch costs (*) then it might be cost effective to put the power source in orbit and launch people on a much more leisurely trajectory.

The main objective though is to solve the economic and logistical problem of the launch bottleneck. That priority overrides all else as far as space is concerned. I wouldn't want to launch on a system that's flown less than 1000 times.

(*) Typing Isp=800 sec into current cost models spits out a price on the order of ~$200/kg as opposed to $5,000/kg for conventional expendable launchers. This doesn't include the cost of the beam facility. However I've been working with Jordin Kare on this end of the problem and I'm getting numbers on the order of $100/kg extra if you include that. That work will be published shortly, but it's only preliminary numers, and these numbers always bounce around as engineering setbacks interplay with optimizing things and having more smart ideas.

Still, a single ground station could be used to send payloads to escape velocity with lower peak acceleration, couldn't it? Would the increased distance as it neared escape velocity be a problem?

I was getting a estimate of needing ~2.5MW/kg to get a thrust to weight of 50:1 from a pump/heat-exchanger/nozzle combination at 800ISP, that's pretty bleeding edge, probably becoming tougher if you scale things up.

This really seems like a hard problem, even without needing the additional mass fraction and possible specific power needed for escape.

BTW. Your incoming power density seems to be (back of they eyelid estimate) about what a tungsten filament puts out. Though the temperature won't be that high, are radiation losses from the heat exchanger expected to be serious or just a few percent of the incoming power?

I will be very interested to see what you and Jordin can come up with.