Author Topic: Role of NTR/BNTR/NEP in future architectures  (Read 236356 times)

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #380 on: 06/06/2010 10:57 pm »
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.

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.

Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #381 on: 06/07/2010 12:26 am »
865 sec is not nearly enough Isp to justify putting an NTR on any deep space mission.

Offline mmeijeri

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #382 on: 06/07/2010 12:32 am »
865 sec is not nearly enough Isp to justify putting an NTR on any deep space mission.

What if you throw time to initial operational capability into the mix? I think that chemical + prepositioning propellant with SEP is the solution that could be operational soonest. That could be used soon, say for landing probes on Mars in areas that are currently inaccessible. Might NTR be faster than NEP for manned spacecraft? It would only have to be competitive with chemical in that case. I'm specifically thinking of water as a working fluid: it could give you slightly higher Isp than LOX/LH2 and much greater density. As an added bonus it doesn't even require cryogenic propellant transfer, though that may not be much of an argument if it requires NTR. Would this help with T/W?
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Offline Robotbeat

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #383 on: 06/07/2010 12:34 am »
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.

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.
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft)
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Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #384 on: 06/07/2010 12:44 am »
It'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.

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #385 on: 06/07/2010 02:54 am »
It'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,


No they're not, surprisingly enough.  I talked to Harry Finger a few days ago, and I'll talk to Frank Durham this week.  I'll probably talk to Paul Wagner in the next couple of weeks.  Stan Gunn's still around.  There are plenty of people still alive.  They're in their eighties and nineties, but they're still kicking.

But moreover, there's an active group of people up at Los Alamos who have carefully archived the Rover documentation and maintain relationships with the remaining Rover folks.  A lot of it has reappeared on the DOE website over the past few months.

Quote
and the work was done under different environmental and launch safety considerations than today. 

The environmental and launch safety considerations during the testing have nothing to do with the data.  Physics is physics, data is data.  Examining what was actually developed, most current launch safety considerations have not changed substantially since the period when NRX or Pewee were designed.  Further, both were designed and tested under more stringent conditions than most chemical rockets ever had to meet.  Only one rocket program has ever been asked to meet a goal of 10 hours of operation and 70 restarts.  NRX met that goal.

Quote
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.

I used your methods, but real numbers, and published the numbers right here in this forum.  It was obviously worth the effort. 

Quote
It's not.

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. 

I don't understand your emotional problem with NTRs.  There are reasonable and unreasonable objections, and yours are obviously unreasonable.  I think you should get over it and figure out how to do it with thorium.  That would be a real contribution.

Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #386 on: 06/07/2010 03:00 am »
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.
« Last Edit: 06/07/2010 03:02 am by kfsorensen »

Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #387 on: 06/07/2010 03:18 am »
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.

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.
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Offline Warren Platts

Re: Role of NTR/BNTR/NEP in future architectures
« Reply #388 on: 06/07/2010 03:42 am »
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.

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)?
« Last Edit: 06/07/2010 03:50 am by Warren Platts »
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Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #389 on: 06/07/2010 04:32 am »
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.

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.
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Offline Star-Drive

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #390 on: 06/07/2010 04:41 am »
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.  However from the early work on the DUMBO NTRs, the particle bed reactors performed by the USAF back in the late 1980s and early 90s under the Timberwind program, and the current tungsten ceremt NTR work espeically as applied to the high power Dumbo reactors, a viable NTR with T/W ratios of greater than 25-to-1 should be buildable.  Especially if we pursue thrust levels of 50 kLb-f or greater.  Of course there is the negative politics surrounding anything "nuclear" in this country to contend with, and at the rate the USA is going broke under the current administration, this discussion may be moot to begin with.
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Offline Warren Platts

Re: Role of NTR/BNTR/NEP in future architectures
« Reply #391 on: 06/07/2010 07:16 am »
So what? Payload mass fraction isn't the be all end all statistic to base everything on.
Neither is Isp.
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Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #392 on: 06/07/2010 08:03 am »
So what? Payload mass fraction isn't the be all end all statistic to base everything on.
Neither is Isp.

When it comes to in-space operations, Isp is far more important than, say, launching into orbit.

Rather than a straight T/W, I would rate different propulsion systems by T*Isp/W along with a coefficient that involves the density of the propellant and resulting tank mass requirements along with a factor regarding the ease of refuelling (i.e. cracking water is a lot easier than finding xenon or kerosene out in space), as well as factoring trip time. Any propulsion using LH2 alone will have serious payload mass fraction performance issues, but punishing it for that when an interplanetary vessel using LH2 can rather conveniently be refuelled is another matter entirely. Likewise punishing a poor mass fraction on a vehicle that can get somewhere twice as fast as a vehicle with a better mass frac is not smart.
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Offline Warren Platts

Re: Role of NTR/BNTR/NEP in future architectures
« Reply #393 on: 06/07/2010 08:25 am »
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.
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Offline Star-Drive

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #394 on: 06/07/2010 11:54 am »
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.  Too bad no one has the intestinal fortitude to pursue it.  I guess that is a tribute to the fact that humanity has yet to decide that human space flight (HSF) is really that important, yet.  In the meanwhile we will have to wait for the aneutronic fusion reactors/rockets to come on line, if they ever do...
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Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #395 on: 06/07/2010 01:24 pm »
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.

Not even close.  This was one of the first things we looked at when I came to NASA ten years ago.  The engine had nowhere near the thrust-to-weight for the earth-to-orbit application.

Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #396 on: 06/07/2010 01:39 pm »
Is it possible to build an NTR with a >900 s Isp and >25 T/W?

Perhaps--I don't know how, but I can't exclude the possibility.

What I can exclude is that nothing that was tested back in the 1960s has performance anywhere close to that, and the stuff that was tested doesn't have sufficient performance to "buy" its way onto any deep space mission.

Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #397 on: 06/07/2010 02:14 pm »
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.

I never said that they were.  Good engineers bring their calculations to the table so that they can see each others' work and assess whether they started from the same initial numbers or if there were flaws in the calculation strategy that led to different answers.  I've showed my work and invite you and anyone else to show yours.

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #398 on: 06/07/2010 03:39 pm »
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft)

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #399 on: 06/07/2010 03:41 pm »
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.

Eh?  Why?

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