To put 134 metric tonnes in perspective that's 2 SLS flights, 6 Delta IV Heavy flights or 9 Skylon flights, or 5 Atlas V heavy flights (if it ever gets funded).If the vehicle can be fully reused (which it seems it can) this is not ridiculously expensive (but it's not cheap).
Personally, I think speed is overrated.
Quote from: IRobot on 10/28/2013 05:00 pmOne question: if we wanted to double the average thrust, what would be the path? Scale up the engine? Shorten time between pulses? Putting two engines in tandem does not seem to be viable.I don't see why you couldn't put 2 engines in tandem. Four would be better, then fire them 2 at a time to prevent bending moments.Other than that, the lithium ring is the smallest they could make it, so it could be expanded. But I think increasing the firing rate would be easiest, and it doesn't need any increase in capacitor banks.
One question: if we wanted to double the average thrust, what would be the path? Scale up the engine? Shorten time between pulses? Putting two engines in tandem does not seem to be viable.
Quote from: john smith 19 on 10/28/2013 01:42 pmA lot of these problems can be worked out in sounding rocket or zero g plane flights long before a full vehicle is designed.Hmmm, I see the problem being the opposite: you cannot do representative testing of the reload mechanism on a small scale (sounding rocket), and/or small time-scale (aircraft). This is a large complex mechanism that includes multi-meter transport of components, cold-welding them with very fine-alignment, in vacuum zero g. Just two full cycles will take ~28 seconds.
A lot of these problems can be worked out in sounding rocket or zero g plane flights long before a full vehicle is designed.
Not to say it can't be done, but it was Elmar who suggested upthread we think of this mechanism as akin to placing a robot assembly line (i.e. well understood and highly-reliable) in orbit. I think that analogy is a good one.
To get confidence in (man-rate) a mechanism that is required to run for thousands of cycles to keep your crew safe, I think you need to launch a full-scale test article and run it for a representative period. While the mechanism is hoped to be just a few tonnes, a long-run test will require a lot of liner components, and that's going to be heavy.
As for reliability and recovering from a jam, given the number of failure points and the likelihood of in-flight repair being minimal, I think you just include two or three FDRs on the spacecraft to get full redundancy.
Quote from: alexterrell on 10/28/2013 05:35 pmPersonally, I think speed is overrated.I disagree. Speed is less relevant for cargo and unmanned science missions, but for manned missions, speed is the key to everything. More speed means less consumables needed on the flight. It means less exposure to radiation. It means less exposure to a low gravity environment and it means less risk overall. It means that you need a smaller habitat. More speed would also allow for more mission flexibility in regards to timing. Just to repeat that again. I am talking about manned missions.
Quote from: Elmar Moelzer on 10/28/2013 05:48 pmQuote from: alexterrell on 10/28/2013 05:35 pmPersonally, I think speed is overrated.I disagree. Speed is less relevant for cargo and unmanned science missions, but for manned missions, speed is the key to everything. More speed means less consumables needed on the flight. It means less exposure to radiation. It means less exposure to a low gravity environment and it means less risk overall. It means that you need a smaller habitat. More speed would also allow for more mission flexibility in regards to timing. Just to repeat that again. I am talking about manned missions.I agree on the advantages, but consider, for about 100 tons of Lithium fuel:Fast trajectory return, delta V 40,000m/s, inert mass = 80 tonsHohmann trajectory return, delta V 4000m/s, inert mass = 1,100 tonsNow, 1,100 tons is ambitious for early trips to Mars, but for Mars colonisation, that could be really useful. With that mass, consumables are trivial, you can use a torus to provide gravity, and you can have a shielded area in the middle for solar flares.
Quote from: john smith 19 on 10/28/2013 01:42 pmTo put 134 metric tonnes in perspective that's 2 SLS flights, 6 Delta IV Heavy flights or 9 Skylon flights, or 5 Atlas V heavy flights (if it ever gets funded).If the vehicle can be fully reused (which it seems it can) this is not ridiculously expensive (but it's not cheap). Or 3 Falcon Heavy flights, which would even leave some extra margin. For a reusable mars mission architecture that allows for 90 day transfers and if all goes well, even 30 day transfers, that is cheap, at least in my book.
That said I do believe a lot of the sub system can be tested as secondary payloads on various LV's.
Quote from: john smith 19 on 10/29/2013 06:41 pmThat said I do believe a lot of the sub system can be tested as secondary payloads on various LV's. I think they are planning that, actually. Sending mission using an untested engine on a trip to mars does not seem to make much sense
Quote from: Elmar Moelzer on 10/28/2013 05:48 pmQuote from: alexterrell on 10/28/2013 05:35 pmPersonally, I think speed is overrated.I disagree. Speed is less relevant for cargo and unmanned science missions, but for manned missions, speed is the key to everything. More speed means less consumables needed on the flight. It means less exposure to radiation. It means less exposure to a low gravity environment and it means less risk overall. It means that you need a smaller habitat. More speed would also allow for more mission flexibility in regards to timing. Just to repeat that again. I am talking about manned missions., and you can have a shielded area in the middle for solar flares.
Quote from: john smith 19 on 10/28/2013 01:42 pmTo put 134 metric tonnes in perspective that's 2 SLS flights, 6 Delta IV Heavy flights or 9 Skylon flights, or 5 Atlas V heavy flights (if it ever gets funded).If the vehicle can be fully reused (which it seems it can) this is not ridiculously expensive (but it's not cheap). Personally, I think speed is overrated.I like to see this engine take 1,000 tons from LEO to Mars on a Hohmann trajectory rather than 70 tons in 50 days or so.Maybe take the fast route back when it doesn't have all the landing modules and cargos etc.
Quote from: IRobot on 10/28/2013 05:00 pmOne question: if we wanted to double the average thrust, what would be the path? Scale up the engine? Shorten time between pulses? Putting two engines in tandem does not seem to be viable.Speeding up the pulse rate is not "easy," apparently not at all--if we are talking about fabricating a new half-kg lithium ring in the engine for every pulse, quite obviously not. But it's the approach that has the biggest payoff by far. Earlier press releases and published papers mention an ambition to get the rate up to once every 10 seconds (same ballpark as every 14) and I think they may have mentioned the possibility of once every second! If they did they may be regretting it now; OTOH if they can get a working system operational in space that pulses say once every 20 seconds, interest and funding to strive to improve the rate should be quite forthcoming, I'd think. Who knows what the limit would be then?The faster the pulse rate, the greater the thrust to weight ratio. Elmar Moelzer mentions the need for the exhaust products to completely clear the system as a limit, but with the main pulse exiting at 50,000 meters/sec it's hard for me to believe this can be a factor after say 1/100 of a second, even granting there will be a bell curve or something like that of particle speeds, so some small fraction will indeed be lingering about the chamber long after most of it has departed. More seriously, while I've found presentations where the developers predict that the overall mass of a rocket capable of the performance they are talking about (50 km sec exhaust, about half a kg per pulse, pulses in the range of 1 every 10 seconds) would be about 15 metric tons, I have no idea just how much of that is the ring-forming mechanism, or how the mass of that would scale with increasing pulse frequency (assuming the result can be accomplished at all). That it might wind up massing as the square of the frequency does not seem unreasonable to me; then the question is, how much would the baseline component capable of once every 10 seconds be? 100 kg? A tonne? That governs the point at which scaling up the pulse rate dominates the rising mass of the engine.However I see no reason why any other component of the system (except the FRC injector, obviously) should rise in mass with it. Each pulse is the same; a magnetic nozzle system that can handle a pulse every minute should be able to handle one every second just as well...Well, let me qualify--two things; one, the system proposed uses solar power input to drive the compression stage of the pulse--while I understand why they are going for that I've always felt the need to stress there's no fundamental reason the compression power has to come from outside the cycle! It could easily be drawn from the exhaust phase of the cycle, as a drain on the magnetic nozzle. With gains in power due to fusion in the range of 100 or more, it ought to be pretty doable. With power for the compression phase coming from outside the system, obviously that power source has to be scaled up in proportion to the pulse rate, and that's bad. (Unless we have onboard also the power generation system the same inventors are also working on, perhaps? But if the compression power is drawn from the previous exhaust pulse, the shorter the time scale the better, because that's power that has to be stored. (The same is true with solar power or other external power input--it has to be accumulated in capacitors; these same capacitors could simply store the full energy needed for the next pulse from the previous one, so either way the power storage system is required--and included in the 15 tonne estimate already).Two, obviously real world systems are never perfectly efficient; the magnetic systems will produce some waste heat, and the more cycles we get per minute, the greater the waste power is; that forms another practical limit and system that scales up in mass with the increased pulse rate.All of this is obviously voting to bell the cat; to consider the tradeoffs involved in pushing the pulse rate up we have to first be able achieve the upper end of the range considered at all! It all depends on the system used to form and position the rings, how reliable it is, and so forth.But until the scaling up of the formation mechanism, the FRC injector rate, and the heat rejection system starts to become a very significant fraction of the total engine mass, clearly the faster the pulse rate the better.----All that said, what if you can't increase the pulse rate beyond a certain point? Then I'd say that it would be better to keep the minimum scale of the engine system, and build many of them to increase the thrust, rather than to make the engine bigger in proportion to the mass of each pulse. It seems reasonable to me that it could be scaled up--that 5 kg of lithium can fuse 10 times the mass of fusible isotopes--but while there is little reason to think any component of the engine would scale up at a higher rate than linear, there is no reason to think any significant part of the mass would scale up at less than linear rate either. So we'd need a magnetic nozzle, including the compression component, 10 times the mass of the one proposed, and 10 times the mass of power storage capacitors, and so on. If compression is powered by solar power we need 10 times more panel area. Etc.
More seriously, while I've found presentations where the developers predict that the overall mass of a rocket capable of the performance they are talking about (50 km sec exhaust, about half a kg per pulse, pulses in the range of 1 every 10 seconds) would be about 15 metric tons, I have no idea just how much of that is the ring-forming mechanism, or how the mass of that would scale with increasing pulse frequency (assuming the result can be accomplished at all). That it might wind up massing as the square of the frequency does not seem unreasonable to me; then the question is, how much would the baseline component capable of once every 10 seconds be? 100 kg? A tonne? That governs the point at which scaling up the pulse rate dominates the rising mass of the engine.
Quote from: alexterrell on 10/28/2013 10:37 pmQuote from: Elmar Moelzer on 10/28/2013 05:48 pmQuote from: alexterrell on 10/28/2013 05:35 pmPersonally, I think speed is overrated.I disagree. Speed is less relevant for cargo and unmanned science missions, but for manned missions, speed is the key to everything. More speed means less consumables needed on the flight. It means less exposure to radiation. It means less exposure to a low gravity environment and it means less risk overall. It means that you need a smaller habitat. More speed would also allow for more mission flexibility in regards to timing. Just to repeat that again. I am talking about manned missions., and you can have a shielded area in the middle for solar flares. You are ignoring GCR radiation risk then.
off the wall idea here: How about getting the metal storm guys to design the injector system? Metal storm fires lots and lots of rounds per second. i know it has lots of barrels in parallel so the trajectory would be changing all the time but perhaps the design could be modified so that all the trajectories intersect at the laser focus spot.
well, perhaps the propellant charge could be hydrogen or something like that; that won't gum up the reactor with residue. otherwise yeah you would need a rail gun or similar effect.