Author Topic: Nuclear propulsion systems for DIRECT  (Read 25306 times)

Offline GraphGuy

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Re: Nuclear propulsion systems for DIRECT
« Reply #20 on: 12/11/2006 07:37 pm »
NEP or Nuclear driven plasma propulsion (VASIMR)?  Yes I know that VASIMR has yet to be flown in space, but we have a bit of time before we lock in those plans for the Mars rocket.

What is the timeframe for NEP (ion) to reach mars?

Offline GraphGuy

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Re: Nuclear propulsion systems for DIRECT
« Reply #21 on: 12/11/2006 07:44 pm »
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Joffan - 11/12/2006  12:01 PM

As I've mentioned elsewhere, the only way a never-powered-up space reactor can kill anyone is to fall on them. Radiation is way below harmful limiits (and those limits exist and are the basis of routine medical procedures). Even atmospheric burn-up of a used reactor in the atmosphere would almost certainly not cause any deaths, especially if it was shut down a few months beforehand. Using burnup as a routine method of reactor disposal might begin to cause problems.

You've obviously never met anyone who is against nuclear power.  Resistance has nothing to do with studies or analysis, but because it is nuclear.  There are people who think that any ionizing radiation in the athmosphere is bad no matter the ammount.  These people usually freak out and forsake atlantic salmon when you tell them about russian nuclear reactors on the deep ocean floor (which is fine with me, more salmon sashimi on my plate).

Bottom line: call something nuclear and you will get a crowd of paranoids mailing their congressman to cancel it.

Offline Joffan

Re: Nuclear propulsion systems for DIRECT
« Reply #22 on: 12/11/2006 07:53 pm »
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khallow - 11/12/2006  1:11 PM

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Joffan - 11/12/2006  10:01 AM

As I've mentioned elsewhere, the only way a never-powered-up space reactor can kill anyone is to fall on them. Radiation is way below harmful limiits (and those limits exist and are the basis of routine medical procedures). Even atmospheric burn-up of a used reactor would almost certainly not cause any deaths, especially if it was shut down a few months beforehand. Using burnup in the atmosphere as a routine method of reactor disposal might begin to cause problems.

Depends on whether they survive to landfall or not. Dropping one into a few thousand feet of ocean probably isn't a big deal. But having it burn up in atmosphere probably isn't a great idea. At the very least, it'll be detectable and hence, provide some ammunition for the anti-nuke side.
Yes, there could be political implications, but the health implications are as I said. Hopefully the realities of nuclear power and radioactive materials will start to overcome the hysteria in the next few years.
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Offline kfsorensen

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RE: Nuclear propulsion systems for DIRECT
« Reply #23 on: 12/11/2006 08:11 pm »
Let's stop talking about the reentry and burnup of spent nuclear cores--no one is suggesting that and no one would approve it.

Launching a cold core and a launch failure that results in a cold core going in the drink or being vaporized--that's what you have to explain to the public, and you tell them the truth:  that several hundred kilograms of vaporized uranium will fall in the ocean and mix with the millions of tonnes of uranium that's already in seawater.

If that scares you, stop swimming in the ocean, right now.  If vaporized uranium floating in the atmosphere scares you, then tell your congressman to stop allowing the burning of coal, since it is vaporizing many tonnes of uranium into the atmosphere every day.

Offline GraphGuy

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Re: Nuclear propulsion systems for DIRECT
« Reply #24 on: 12/11/2006 09:56 pm »
Vanilla I agree with you, but that isn't how people look at issues like this.  People hear about a cold reactor vaporizing and think that flipper is going to have two tails.  If you want to blame anyone, blame the environmentalists who irrationally hate nuclear power and their friends in the sensationalist news media.

Look at the protests for radioisotope generators on Cassini and New Horizons... and we lost one generator from Apollo13 with no contamination.

Offline kfsorensen

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RE: Nuclear propulsion systems for DIRECT
« Reply #25 on: 12/11/2006 10:44 pm »
RTGs are a totally different animal than a reactor.  They are very radioactive at launch, and a launch accident can conceivably spread radiation if the RTGs aren't properly protected.  That's why they have to be contained in units that can survive entry.

Offline meiza

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Re: Nuclear propulsion systems for DIRECT
« Reply #26 on: 12/11/2006 11:06 pm »
For some, I think the strawmen about "envirowhackos" and "dirty hippies" should stop and we should talk about real issues. Some nuclear materials really are dangerous. (Frex, the soviets spilled a satellite reactor over Canada in a launch failure, is there someone old enough on the board to remember it?) Some are not dangerous. We should talk about what the differences are, how and why, like Vanilla patiently does.
If you just take a contrarian stand, nobody is going to listen to you and make his/her mind on facts (which it should be based on).

Offline Joffan

Re: Nuclear propulsion systems for DIRECT
« Reply #27 on: 12/12/2006 12:16 am »
Real issues, absolutely, although since no-one had used the terms "envirowhackos" or "dirty hippies" on this thread, your post meiza is in fact a strawman itself.

The trick is to find out what is genuinely if slightly "dangerous", which is very hard in a world where radioactivity is legislated to be many magnitudes below those limits. For example, a working reactor which does not vaporise in the atmosphere completely but comes to ground I expect would pose a contamination risk and could cause disease or death for those unprotected and very close by. The advantage of stopping the nuclear reactions some weeks beforehand is that the iodine-131 would have decayed away, eliminating the thyroid cancer risk.

All a bit off-topic I know but my original impression of the thread was that it was to discuss the possibility of incorporating some nuclear drive into DIRECT launch system. While the discussion on carrying nuclear drive components to orbit for subsequent missions is also fascinating, it doesn't require the same intense look at what true dangers accompany the regular use of nuclear propulsion in close proximity to the Earth. However the discussion does make sense in another way since changing part of DIRECT to use nuclear would make it a completely different beast.
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Offline kfsorensen

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Re: Nuclear propulsion systems for DIRECT
« Reply #28 on: 12/12/2006 01:58 am »
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Joffan - 11/12/2006  6:59 PM

Real issues, absolutely, although since no-one had used the terms "envirowhackos" or "dirty hippies" on this thread, your post meiza is in fact a strawman itself.
I think language of that nature was used on the thread that spawned this thread.  I had thought this thread also included how DIRECT would launch systems that would then use nuclear propulsion to continue on in their missions.  If that is beyond the extent of this thread, my apologies for pursuing that topic.

Offline Joffan

Re: Nuclear propulsion systems for DIRECT
« Reply #29 on: 12/12/2006 02:57 am »
No apologies needed, it's all interesting stuff. I didn't read all the progentior thread, so I wasn't aware of any heated exchanges earlier.
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Offline publiusr

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Re: Nuclear propulsion systems for DIRECT
« Reply #30 on: 12/19/2006 11:11 pm »
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vanilla - 9/12/2006  3:00 PM

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kraisee - 9/12/2006  2:04 PM

Okay, from the point of view of the DIRECT LV, the best value configuration is actually the basic 70mT launcher.   What major physical parts of these example NEP & NTP systems could NOT be launched as modules in that mass category?
None.  The NEP vehicle breaks down into nice chunks.  The mass breakdown here indicates that three DIRECT launches would put the whole thing up there.  Throw in another one for the Mars lander and another one for the CEV taxi to L2 and you've got a five-launch Mars mission, where the launches can be spaced over about a year.  (a few months to assemble it, 6-9 months to spiral unmanned to EML2, and then the crew launch).

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kraisee - 9/12/2006  2:04 PM
Second - borrowing an idea I saw on another thread (sorry, I don't recall who posted it):   DIRECT could probably be used to launch the dangerous fuel separately - in a modified CEV.
The nuclear fuel isn't dangerous until the reactor has run at power.  Before then you can hold it in your hand, unshielded.


I must say, that looks do-able--a reasonable compromise? Ares V is better for NTRs and hydrogen--and carrying oxygen for high thrust 'afterburners' like what Stan wants...


Offline kfsorensen

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Re: Nuclear propulsion systems for DIRECT
« Reply #31 on: 12/19/2006 11:29 pm »
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publiusr - 19/12/2006  5:54 PM

I must say, that looks do-able--a reasonable compromise? Ares V is better for NTRs and hydrogen--and carrying oxygen for high thrust 'afterburners' like what Stan wants...
The system-level performance of LOX-augmented NTRs is easily surpassed by chemical engines.  By system-level, I mean accounting for development, boiloff, and astrodynamic issues that will arise with the use of NTRs.

Simply put, NTR isn't worth doing at 900 s Isp, and LOX-augmented NTR REALLY isn't worth doing at 500-600 s Isp.  LANTR is simply a Rube-Goldbergized chemical engine.

Offline khallow

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Re: Nuclear propulsion systems for DIRECT
« Reply #32 on: 12/20/2006 07:57 pm »
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vanilla - 19/12/2006  4:12 PM

The system-level performance of LOX-augmented NTRs is easily surpassed by chemical engines.  By system-level, I mean accounting for development, boiloff, and astrodynamic issues that will arise with the use of NTRs.

I think I understand what you mean by development and boiloff issues. But what do you mean by "astrodynamic" issues? Low thrust for the weight of the engine? Looking around, NTR's seems to have the same problem that any low thrust engine does when move cargo or people out of LEO, namely, it passes through the Van Allen belts multiple times.
Karl Hallowell

Offline kfsorensen

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Re: Nuclear propulsion systems for DIRECT
« Reply #33 on: 12/20/2006 08:10 pm »
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khallow - 20/12/2006  2:40 PM

I think I understand what you mean by development and boiloff issues. But what do you mean by "astrodynamic" issues? Low thrust for the weight of the engine? Looking around, NTR's seems to have the same problem that any low thrust engine does when move cargo or people out of LEO, namely, it passes through the Van Allen belts multiple times.
Yes, the thrust-to-weight of the engine is lower than a comparable chemical stage, which means that the vehicle will incur more "gravity losses" as it climbs out of the Earth's gravity well.  For most trajectories that use chemical engines, gravity losses are small, but the low T/W of an NTR means that they can be significant.  Even most of Borowski's NTR papers break the Mars injection burn into two parts: the first one lifts the spacecraft from LEO into a highly elliptical orbit, and after a full orbit, the spacecraft comes around perigee again and does another burn to transfer from the highly-elliptical orbit into a hyperbolic Earth escape orbit (positive C3).  This results in two more passes through the Van Allen belts for the crew.

But much more troubling than gravity losses is the potential system hit of having to begin from a so-called "nuclear safe-orbit".  No one seems to know exactly what an NSO is or what it should be, but I've seen them call anything from 800 km to 3000 km "NSO".  For high-thrust hyperbolic injection, you want the injection DV to be accomplished as deep in the gravity well as possible.  800 km is bad.  3000 km is REALLY bad.  Starting from these altitudes means it will take much more DV to achieve the same C3 (C3 is a measurement of orbital energy typically used for hyperbolic orbits).

Then there is the issue of getting the vehicle to the NSO in the first place if you can't use the NTR.  A dedicated chemical stage or a real performance hit from the launch vehicle will be required.  These are non-trivial issues that are typically ignored in NTR mission analysis because they degrade the ultimate performance of the vehicle tremendously.  Note that I'm not saying that the NSO concept is good, bad, right or wrong--I'm just saying that if you apply NSO to NTR things get a lot worse.

Offline publiusr

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Re: Nuclear propulsion systems for DIRECT
« Reply #34 on: 12/22/2006 08:00 pm »
Ares V is better for NTRs, and I wish Stan would come to the forum--because I am sure that NTRs do have strengths. Direct is perfect for NEP and would have made JIMO do-able.

Offline Scotty

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Re: Nuclear propulsion systems for DIRECT
« Reply #35 on: 12/23/2006 06:21 pm »
Do a word search on "timberwind".
Read everything you find, then we can talk about a real nuclear thermal propulsion system.
The Moon is the practical limit for manned flights using only chemical propulsion.
Mars can be done, but it will not be easy, and the time in transit between Earth and Mars will be long.
How would you like being stuck on a Grayhound Bus for 6 to 9 months, both ways?
NTP could cut that transit time to 6 weeks or less.
Again read up on the subject first, then we can disscuss it.

Offline kfsorensen

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Re: Nuclear propulsion systems for DIRECT
« Reply #36 on: 12/23/2006 07:07 pm »
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Scotty - 23/12/2006  1:04 PM

Do a word search on "timberwind".
Read everything you find, then we can talk about a real nuclear thermal propulsion system.
The Moon is the practical limit for manned flights using only chemical propulsion.
Mars can be done, but it will not be easy, and the time in transit between Earth and Mars will be long.
How would you like being stuck on a Grayhound Bus for 6 to 9 months, both ways?
NTP could cut that transit time to 6 weeks or less.
Again read up on the subject first, then we can disscuss it.
Timberwind is a particle-bed nuclear thermal reactor where surface area to volume ratio is enhanced tremendously by using particle fuel.  Unfortunately, it is dynamically unstable because local fuel melting welds particles together, blocking off cooling, which leads to further particle melting and ultimately core fusion.  (not the nuclear fusion kind, either)

Offline Joffan

Re: Nuclear propulsion systems for DIRECT
« Reply #37 on: 12/23/2006 07:26 pm »
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publiusr - 22/12/2006  1:43 PM

Ares V is better for NTRs, and I wish Stan would come to the forum--because I am sure that NTRs do have strengths. Direct is perfect for NEP and would have made JIMO do-able.
... will make JIMO do-able
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Offline Scotty

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Re: Nuclear propulsion systems for DIRECT
« Reply #38 on: 12/24/2006 02:32 am »
Vanilla - That is interesting, the bit about the pellets melting.
I worked on the LH2 feed systems on Timberwind, and I never heard of any such problem.

Offline kfsorensen

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Re: Nuclear propulsion systems for DIRECT
« Reply #39 on: 12/24/2006 03:28 am »
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Scotty - 23/12/2006  9:15 PM

Vanilla - That is interesting, the bit about the pellets melting.
I worked on the LH2 feed systems on Timberwind, and I never heard of any such problem.
I had heard of it and then confirmed it with one of the nuclear engineers who worked on Timberwind.  He said "yeah, it was a great idea other than the problem that the core always melts..."

Not the kind of reactor safety feature you advertise to the general public.   Then again, from what I've learned Timberwind was a bit of a doomsday device anyway.

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