On 5/11/21 00:59, Jim Catchpole - jlcatchpole at googlemail.com (via tml list) wrote: > <snip> > > Anyway, having said all that, the major issue with Traveller ships is > that almost all of the design sequences don't give you a mass for > them, only a volume. When I was looking at it I went with a estimate > of about 5 tonnes/dton, which is almost certainly way too low (modern > commercial aircraft probably come in around 2-3 tones/dton empty, > although that involves a little guesstimating of their volumes). Given > that, a classic 100 dton scout would mass 500 tonnes, with 20 dtons of > LH2 for manouvering. It will need to generate 1e7 N thrust for a 2G > acceleration. Assuming an Isp of about 600,000 (there are *major* > problems with an Isp this high, but we'll cover those later), that > gives you a mass flow rate of about 17 kg/s - meaning your 20 dtons of > LH2 will last about 20 minutes. Water has a density of about 14 > tonnes/dton, so that would be about 1.5 dtons for your water propellant. > > Unfortunately, that Isp means the average velocity of your propellant > exiting your engine is 6000 km/s, or about 2% of the speed of light. > That is high enough to constitute a radiation hazard to unshielded > people (reasonable assumption, in ships/stations ok, in vacc suits, > not ok). > > It also means that with that mass flow rate the radiated heat from the > plasma will vapourise the ship just after you turn it on, barring > magic tech. > > Assuming you are willing to live with that, or handwave it, the short > version is that for 1G acceleration, 10% of your hull volume in LH2 > will get you about 10 minutes acceleration. A higher density > propellant will require less volume, but too much of it will raise the > mass of the ship requiring a recalc. <snip> > Also, a quick estimate of the energy output required to run a reaction > mass drive, or ground to orbit operations, suggests that a small > enough quantity of fuel to be handwaved into the reactor mass would > only last a couple of years in normal use, so I was thinking of > including topping up the fuel as part of the annual maintenance. > > Hope that helps. > > Cheers, > Jim Jim, Greg, Mind if I chime in with some rough numbers? IIRC, the mass-to-energy conversion efficiency of H fusion (since we have numbers on that) is on the rough order of 1% - so completely fusing 1 kg H gets you 990 g He + neutrons, and ~9e14 J of energy (m = 0.01 kg, plugged into E = mc ** 2). In somewhat more comprehensible terms, that kilo of hydrogen, before conversion losses, gets you 28.55 MW-years of energy. Taking Jim's figures as given (1e7 N thrust, 6e6 m/s exhaust velocity), I get a thrust power (thrust * Ve / 2) of 3e13 W - 30 terawatts. For comparison, total contemporary power consumption on Terra is (again, IIRC) ~15 TW. Assuming a 480 tonne dry mass for Das Sternenschiff, that gives a specific thrust power of 62.5 MW/kg. That power (again, assuming no losses), comes from fusing 3.3e-2 kg (or 33 g) of H per second your drive is running - approx 1/515 of your propellant flow rate. A dton of liquid hydrogen would supply enough power (and no propellant) for 30,300 seconds - approx 8.4 hours. Scaling back to provide enough juice to eject 20 dton of H as propellant would need ~ 40 kg of H for fusion. Those mass consumption figures will vary inversely with mass-conversion efficiency - for example, total-conversion, being 100% efficient at mass-to-energy, would need 400 g of mass per second. Jim, your estimate of "a couple of years in normal use" may have been a trifle optimistic. As for topping up powerplant fuel at annual maintenance, I've assumed that in both MTU even though starship powerplants ship with 200 years' fuel (as per GT). Alex
