Quote from: Jim on 01/18/2014 02:02 pmAFAIK, ABL intercepted during boost phase, not mid-course.Where both the aircraft and target are mostly about the atmosphere (ABL at 40k ft and target likely higher)
AFAIK, ABL intercepted during boost phase, not mid-course.
What metal? The casings are composite.
Where both the aircraft and target are mostly about the atmosphere (ABL at 40k ft and target likely higher)
That said, I'm unaware that anyone has tested a heat exchanger for either Kare's or Parkins' approach, and until their claimed efficiencies are validated, there's no reason to go further with either concept. But unlike Parkins' approach, Kare's could easily leverage any number of military lasers for this test -- I'm sure they'd appreciate having the target to shoot at.
It's also worth mentioning that the job of incoherently combining multiple beams on a heat exchanger to vaporize LH2 is actually much easier than the ABL's job of coherently focusing one large beam on a solid rocket casing long enough to punch through the metal.
I like Kare's approach a lot. It transfers much of the complexity of a launch vehicle to the ground, it's highly scalable, and it leverages Moore's Law-like trends in semiconductor lasers. It's the lazy man's (read "easiest") approach to power beaming and laser launch. The precision required of Myrabo's Lightcraft and the fact that the thinning atmosphere at higher altitudes worked against it always seemed like snake oil to me. And the inability to test a microwave launcher without installing a huge microwave array first makes Parkins' approach seem like a non-starter.
If Kare's heat exchanger was validated, sub-scale, 1-10kg payload testing would make sense. But proceeding to the full-scale, $2-3B array for 100kg payloads couldn't be justified unless there was a huge market, like unitized propellant (re)supply or massive swarmsat array deployment/replenishment.
Huge? Magnetrons run into the 100Kw-1MW range. The minimum spot size of a 3cm radar beam is not that serious an issue given the size of the target. They also tend to have high efficiency, 90%+ (IIRC).
...easier than persuading the military to runs such tests.
Getting that HX design validated would be a big step toward moving the concept along.
Since Skylon
Maybe I'm wrong, but my gut tells me that all that phase locking and phase controlling over multiple gyrotrons and antennas is going to be harder than demonstrating one lasing unit. I'm lazy -- I'd take the approach where I just have to point beams in the right direction over the approach where I have to phase lock/control beams on top of pointing them in the right direction. I think Table 2 in the joint Kare/Parkin paper, which shows that the microwave approach is at a lower TRL (2-3) than the laser approach (3-4), justifies my gut. (But I'll admit that the table is close.)
If I was a manager running a program in this area, I'd run with Kare's approach as the leader but throw enough resources at Parkin's approach to keep it in a follower position in the event that Kare's approach falters.
My experience is limited to a couple phone discussions a couple years ago, but I got the opposite reactions when bringing this up.
Agreed. WAG is probably less than a million or two to get it done. Would be great to see DARPA or someone fund it.
QuoteSince SkylonI profess to not understanding how Skylon will work technically or programmatically, but that doesn't mean that it won't.
Quote from: darkbluenine on 01/23/2014 10:02 pmMaybe I'm wrong, but my gut tells me that all that phase locking and phase controlling over multiple gyrotrons and antennas is going to be harder than demonstrating one lasing unit. I'm lazy -- I'd take the approach where I just have to point beams in the right direction over the approach where I have to phase lock/control beams on top of pointing them in the right direction. I think Table 2 in the joint Kare/Parkin paper, which shows that the microwave approach is at a lower TRL (2-3) than the laser approach (3-4), justifies my gut. (But I'll admit that the table is close.)This is the technology of satellite solar power. I'm not sure about Gyrotrons but it was solved for magnetrons in the late 1970's.
QuoteIf I was a manager running a program in this area, I'd run with Kare's approach as the leader but throw enough resources at Parkin's approach to keep it in a follower position in the event that Kare's approach falters.I'd guess the Gyrotrons are about 4x the electrical efficiency of the laser system. When you're talking a few watts from a wall socket that's not really an issue. But these systems are big. Note that 10MW gas turbines are feasible in 2 20" containers as well.
QuoteMy experience is limited to a couple phone discussions a couple years ago, but I got the opposite reactions when bringing this up. Now that does surprise me.
QuoteAgreed. WAG is probably less than a million or two to get it done. Would be great to see DARPA or someone fund it.There is sort of some work in this area. In 2003/4 there was a thesis written describing work on embedded heat pipes in Alumina substrates. I'm not talking about thin metal pipes, I mean out of the Alumina itself. This was in respect of cooling electronic components on hybrid devices. The basic work used water with slots of different widths to define edges the water would travel in due to surface tension. It was rated at 30w/cm^2, which is not bad given the Apollo heat shied was rated at 100w/cm^2.Hybrid substrates are processed in rolls up to 4 feet wide. Individual substrates are up to 4 sq inches.
It's also very close to "phased-array" and phase-locked-array radar systems. Lower "TRL" than actually making a "pointed" beam but then again we've been playing with MASERs for decades
Shouldn't really One of the issues with military test programs is that they tend to face on-again/off-again funding and interest levels. Any use generates some data and that's a good thing and a lot better than having to close everything up
Great where can I get some to start with?
Quote from: RanulfC on 01/24/2014 04:14 pmIt's also very close to "phased-array" and phase-locked-array radar systems. Lower "TRL" than actually making a "pointed" beam but then again we've been playing with MASERs for decades Very. In fact IIRC the Safeguard ABM radars were all phased array and in the MW range back in the early 70's.MASERS never really caught on except AFAIK in very low level signal amplification tasks. You just don't seem to see industrial cutting MASERs in the way that LASERs are (relatively) common place.
Quote from: john smith 19 on 01/24/2014 06:33 pmVery. In fact IIRC the Safeguard ABM radars were all phased array and in the MW range back in the early 70's.MASERS never really caught on except AFAIK in very low level signal amplification tasks. You just don't seem to see industrial cutting MASERs in the way that LASERs are (relatively) common place.I blame Laser's better PR department myself...And thanks for the refernce
Very. In fact IIRC the Safeguard ABM radars were all phased array and in the MW range back in the early 70's.MASERS never really caught on except AFAIK in very low level signal amplification tasks. You just don't seem to see industrial cutting MASERs in the way that LASERs are (relatively) common place.
They're cool concepts, with lots of interesting discussion on lasers, heat exchangers, etc.What's been glossed over is that for these concepts to work, they require very big & very light LH2 tanks. Most of these papers specifically don't get into 'vehicle design', just make general assumptions. Where they do specify tank mass ratios, they look aggressive, but plausible. Until you apply those same tank masses to vehicles using conventional rocket engines and high density propellants. We're talking much better than SSTO with LOX/RP1. If you can do that, why bother with lasers, etc?Much like STP and NTP, it has potential as an in space propulsion technology, where thrust (and hence heat exchanger mass) can be kept very low. Where also initial mass is expensive, so high Isp is valuable.
Both. the viability of a beamed vehicle making it to orbit is questionable.
The point with externally heated propulsion, and also with nuclear propulsion, isn't so much higher temperatures, but lower molecular weight of the exhaust gases. Hydrogen is the best option here, but unfortunately it has very low density. Ammonia comes second and is much denser. It's also storable, which is nice but not crucial for launch vehicles. Hydrazine is another option, but it's not very desirable for launch vehicles.
Actually the point of Beamed propulsin (along with NTR) IS higher temperature, up to the materials limit of the absober/HX. You get a much higher exhaust speed with lighter propellants but you CAN use just about anything.