|
|
Post by lennson on Jul 5, 2017 22:59:04 GMT
I was wondering whether at the scale of 100s of GWs of thermal radiation, which some designs give off, whether directed radiators could be used to produce noticeable thrust. According to Wikipedia the effect of thermal radiation was needed to account for the observed trajectory of the Pioneer spacecraft ( en.wikipedia.org/wiki/Pioneer_anomaly, arxiv.org/pdf/1204.2507v1.pdf). If we take that thermal radiation of ~50W achieved (8.74±1.33)×10^-10 m/s^2 acceleration of a ~250 kg craft then this seems to suggest that if we take apophys Barbecue Drone ( childrenofadeadearth.boards.net/post/23293/thread) which weights around 8,000 kg and puts out some where around ~1 GW of thermal radiation this appears to translate to an acceleration of around 5.4 ×10^-4 m/s^2, or around 55 micro g_0. Is this enough to be be useful for anything? Well it is comparable to the acceleration of the stock belt trawler, so maybe? The reason this would be of any value is of course because no propellant is used (unless you want to be pedantic and call the fission-ed mass transformed to energy the propellant). |
|
|
|
Post by Enderminion on Jul 5, 2017 23:15:32 GMT
Direction radiators have to be MUCH bigger
|
|
|
|
Post by treptoplax on Jul 6, 2017 2:31:26 GMT
|
|
|
|
Post by lennson on Jul 6, 2017 3:45:04 GMT
I agree that it looks essentially like a photon drive. The energy to force conversion of 1N per 300MW pretty much matches (that would give ~4.17 ×10^-4 m/s^2 as the acceleration for my example which rather similar to 5.4 ×10^-4 m/s^2). If one could direct the thermal radiation of radiators in a reasonable way (yes that appears to be an issue) it seems that would be a lot more reasonable than converting thermal to electrical and then electrical to laser since each conversion it quite inefficient. |
|
|
|
Post by treptoplax on Jul 6, 2017 14:50:43 GMT
So, taking the Barbecue drone and assuming a 5-year reactor life (is that the minimum?) and that you can somehow turn that heat into 100% directional thrust that 50 uG acceleration turns into 8.5Km/s dV. A bit disappointing given that the MPDs have 50 Km/s.
OTOH, if we can refuel the reactor... it has only 1 Kg Uranium. Replacing the HD fuel with uranium gives us a total dV of 850Km/s! Of course, it takes it 500 years to get up to that speed... that'll get you to Alpha Centauri in about 4000 years. You can increase mass fraction but you're thrusting the whole way before you get it below 2500 I think.
Working backward from rocket equation this gives an effective exhaust velocity of 67Mm/s, or 22% of C, if I've calculated correctly (probably better if you can use the nuclear waste for anything). That seems too good, honestly... am I missing something? That's a pretty plausible drive for generation ships if you can achieve anything like that performance.
|
|
|
|
Post by Rocket Witch on Jul 6, 2017 17:15:59 GMT
assuming a 5-year reactor life (is that the minimum?) 6 months is the game minimum. |
|
|
|
Post by apophys on Jul 6, 2017 17:23:40 GMT
assuming a 5-year reactor life (is that the minimum?) The minimum is 6 months (and the reactor is tuned to barely hit the minimum). Directional radiators would require double the radiator area, since they would only use one side. If you cut the laser away and double up the reactor radiators, it becomes 6 tons (of which half is fuel). So that's only 1500 years' worth of fuel. Increasing mass fraction to the 8 tons calculated with would be 2500 years. |
|
|
|
Post by treptoplax on Jul 6, 2017 17:24:03 GMT
assuming a 5-year reactor life (is that the minimum?) 6 months is the game minimum. Oh, too bad. That turns this from "possibly best option for interstellar drives" to "2x advantage in dV for 100x disadvantage in acceleration and horrendously bulky (and probably heavy)". Maybe you could make it better with an optimized reactor but that's turning from back-of-envelope calculations to some actual work. Maybe I'll go scan through Atomic Rockets and see if somebody's already done the maths on this one... |
|
|
|
Post by lennson on Jul 6, 2017 19:00:57 GMT
By the way, are you sure the initial dV calculation is correct?
I seem to get a similar value for the total dV of running an acceleration of 5.4 ×10^-4 m/s^2 for just 6 months (as opposed to 5 years):
5.4 ×10^-4 m/s^2 * 60 s/min * 60 min/hour * 24 hour/day * 30 day/month * 1/1,000 km/m * 6 months = 8.4 km/s
|
|
|
|
Post by lennson on Jul 7, 2017 2:39:36 GMT
Umm I think something is wrong with our nuclear reactors in term of the total energy they can output...
Outputting 1GW for 6 months is a total of ~15.5 PJ.
U233-Dioxide has a stated fission energy of 198MeV, so to produce the stated energy would require a total of 4.9 × 10^26 fission events.
Assuming complete fission of all fissile material this seems like it should require at least 210kg of U233-Dioxide...
(i.e. more than 100x what the game seems to be requiring)
The energy density of the nuclear reactors actually seems to be closer to that of an anti-matter reactor...
|
|
|
|
Post by apophys on Jul 7, 2017 3:35:49 GMT
Outputting 1GW for 6 months is a total of ~15.5 PJ. U233-Dioxide has a stated fission energy of 198MeV, so to produce the stated energy would require a total of 4.9 × 10^26 fission events. Assuming complete fission of all fissile material this seems like it should require at least 210kg of U233-Dioxide... Math checks out. Nice find! (the reactor makes ~700 MW of heat, not 1 GW, but that isn't enough to fix things) In this case it looks like our reactors can only last about a day... A nerf is not going to be pleasant for costs. (pokes qswitched ) |
|
|
|
Post by treptoplax on Jul 7, 2017 15:26:29 GMT
Coming at it from a different angle, Wikipedia lists effective energy density of U in a breeder reactor as 80TJ/Kg, which spread evenly over 5 years is just a bit over 500 Kw/Kg assuming I got my units right...
(Which agrees with lennson's calculation, now that I work through the numbers; 198MeV * Avagardro's Constant/233 * 1 Kg = 82 TJ/Kg.)
|
|
