Quote from: PurduesUSAFguy on 06/05/2010 05:21 pmI have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.For robotic missions, NTRs will lose horribly to other forms of propulsion simply on development costs. For human missions, they just lose badly instead of horribly.
I have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.
865 sec is not nearly enough Isp to justify putting an NTR on any deep space mission.
Quote from: kfsorensen on 06/06/2010 12:41 amQuote from: PurduesUSAFguy on 06/05/2010 05:21 pmI have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.For robotic missions, NTRs will lose horribly to other forms of propulsion simply on development costs. For human missions, they just lose badly instead of horribly.We differ here, Kirk. We've already spent the money on development costs where NTRs are concerned. It's not like we're starting from scratch; we already have well-documented NTR designs that have run at full power and have hours of testing. If you want better Isp than 865 seconds, the last-tested USA fuel, and you don't trust anything the Russians say, fine. Further fuel rod testing would be electric, first, and then probably progress to verification in a small NF-sized reactor in a scrubber, which is not an expensive proposition. The next step for NERVA-derived flight prototypes would be run-up to a few hundred degrees on the ground and then testing in space. That was the plan for the first flight prototype in 1969, and there's no need for the plan to change 40 years later. Quite frankly, NTRs have been tested far more than any other advanced option discussed on this forum (and are at a far more advanced level of development) when it comes to impulse levels high enough to take humans anywhere. Every other propulsion type I see on this forum is based on extrapolation and speculation.
It's not like we're starting from scratch;
Quote from: daveklingler on 06/06/2010 10:57 pmIt's not like we're starting from scratch;We are starting from scratch, because all the people who did that work are dead and gone,
and the work was done under different environmental and launch safety considerations than today.
We're starting at square one, and you can take five minutes (maybe 15 if you're slow) to look at the Isp and T/W of an NTR and figure out if it's worth the effort to develop in the first place.
It's not.
This has absolutely zero to do with thorium.Show me these "real numbers" you used and the results you got from them. If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.
Quote from: kfsorensen on 06/07/2010 03:00 amThis has absolutely zero to do with thorium.Show me these "real numbers" you used and the results you got from them. If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.NON IMPROVEMENT??? Over what, pray tell? Show me a single engine of more than 50,000 lbs thrust with an Isp that high that has gotten anywhere near being tested. Sorry but you are just out to lunch here.
Quote from: mlorrey on 06/07/2010 03:18 amQuote from: kfsorensen on 06/07/2010 03:00 amThis has absolutely zero to do with thorium.Show me these "real numbers" you used and the results you got from them. If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.NON IMPROVEMENT??? Over what, pray tell? Show me a single engine of more than 50,000 lbs thrust with an Isp that high that has gotten anywhere near being tested. Sorry but you are just out to lunch here.The problem is the thrust/weight ratio. At T/W of 4, your engine weighs 6.25 tons, compared to a few hundred pounds for an RL-10. Plugging the numbers into the spreadsheet, and the RL-10 actually has better payload mass fractions than the NTR for initial thrust/weights of 1.0. (RL-10: Isp=450; Mix ratio=5; T/W=59.5) For a delta v of 2.5 km/s, the mass frac is 0.53 for RL-10, and 0.48 for NTR).Even where the NTR has a better mass fraction, the difference is marginal, and so it's not worth billions in development costs to obtain an increase in performance that theory says can only be marginal.(I have a question: what is the significance of this "initial thrust/weight" factor? The NTR is much more sensitive to it than is the RL-10 if you run a spread from 0.2 to 1.0)?
So what? Payload mass fraction isn't the be all end all statistic to base everything on.
Quote from: mlorrey on 06/07/2010 04:32 amSo what? Payload mass fraction isn't the be all end all statistic to base everything on. Neither is Isp.
I think Kirk's point is that the be all and end all statistic to base everything on is the bang/bucks ratio. The marginal improvement isn't worth the many factors of $$$ required to obtain that.
Where a high thrust NTR with an Isp = ~950 seconds really excels in the bang per buck arena is in the reusable single stage to orbit (SSTO) applications.
There are a lot of highly qualified people remaining, several of them AAAS Fellows like Harry Finger, who feel the opposite. They're not stupid people, Kirk.
More tested? SEP is already operational! Has been for years.http://en.wikipedia.org/wiki/Dawn_(spacecraft)
Folks:If we can't build a NTR with a thrust to weight ratio of better than 25-to-1, I have to agree with KFS.