Author Topic: Role of NTR/BNTR/NEP in future architectures  (Read 236350 times)

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #320 on: 05/28/2010 02:44 am »
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Sorry, but I've been trying to get my head around this the last couple of days while I made a feeble attempt to earn a living.  The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions. OTOH, w/o improvements in Isp, NEP currently makes more sense for longer missions, not shorter ones.  In other words, right now you'd stick to NEP for Jupiter out to Pluto, and NTP for Mars, asteroids and inner planets.  Bussard had a little table for NTRs; Isp of 1000 gets you lunar colonies and Mars missions, 1500 Mars colonies and asteroids, 2000 Jupiter, 2500+ the rest of the solar system.

So substitute solar for nuclear, and it seems to me that the inert fraction goes up, not down, with no commensurate increase in Isp.  What am I missing?

Offline Robotbeat

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #321 on: 05/28/2010 02:58 am »
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Sorry, but I've been trying to get my head around this the last couple of days while I made a feeble attempt to earn a living.  The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions. OTOH, w/o improvements in Isp, NEP currently makes more sense for longer missions, not shorter ones.  In other words, right now you'd stick to NEP for Jupiter out to Pluto, and NTP for Mars, asteroids and inner planets.  Bussard had a little table for NTRs; Isp of 1000 gets you lunar colonies and Mars missions, 1500 Mars colonies and asteroids, 2000 Jupiter, 2500+ the rest of the solar system.

So substitute solar for nuclear, and it seems to me that the inert fraction goes up, not down, with no commensurate increase in Isp.  What am I missing?
You're missing the fact that solar has a larger specific power than nuclear out to Mars and its minimum size can be smaller, whereas with nuclear, you need to at least lug around a shadow shield.

From what I've seen, optimistic nuclear power would be ~350W/kg, whereas we could easily build a solar array will 1000W/kg (500W/kg at Mars). From here: http://www.astronautix.com/craft/stcemsep.htm , the solar electric powered spacecraft for this opposition-class mission has the lowest IMLEO of any of the four architectures listed (NEP, NTR, SEP, or cryogenic chemical with aerocapture), and we can probably do better than the assumptions for that mission (specific power more like 250W/kg including structure for that architecture but we can do 1000W/kg).

Since a big solar array would certainly cost less to develop than a couple of big nuclear reactors (with liquid metal cooling, giant radiators, two reactors for redundancy), you could afford more IMLEO (even though you require less) so that translates into greater delta-v.
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Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #322 on: 05/28/2010 03:29 am »
From what I've seen, optimistic nuclear power would be ~350W/kg, whereas we could easily build a solar array will 1000W/kg (500W/kg at Mars).

Easily?

And I thought NTR's were hard to park.

Offline Robotbeat

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #323 on: 05/28/2010 03:54 am »
From what I've seen, optimistic nuclear power would be ~350W/kg, whereas we could easily build a solar array will 1000W/kg (500W/kg at Mars).

Easily?

And I thought NTR's were hard to park.
At least three SEP-propelled interplanetary spacecraft have flown, and some solar-electric rcs systems for comm sats are even commercially available. IKAROS is demonstrating high-specific-power thin-film solar power right now. "Easily" means it can easily make the leap from powerpoint to production if given money like the other SEP spacecraft like Deep Space 1 and Dawn. Only one US nuclear reactor has ever flown, and it had horrible performance. A high performing nuclear reactor could be demonstrated, but likely not without many billions of dollars... And who knows how many tens of billions for a megawatt-class system that's human rated. There's a lot of solar array you can buy for that much money.

Oh, and no additional political push-back either, as irrational as that is.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline 93143

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #324 on: 05/28/2010 06:55 am »
The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions.

...no, how does that follow?

Sure, you spend more time getting out of the gravity well, but the much higher Isp means you can add lots of delta-V and do a high-energy transfer, or even (if the Isp is high enough) do a continuous-thrust trajectory, where you accelerate halfway and then turn around and decelerate.

So electric propulsion can actually get you where you want to go in less overall time, if it's far enough away (Mars is, the moon isn't).

Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #325 on: 05/28/2010 07:24 am »
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Sorry, but I've been trying to get my head around this the last couple of days while I made a feeble attempt to earn a living.  The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions. OTOH, w/o improvements in Isp, NEP currently makes more sense for longer missions, not shorter ones.  In other words, right now you'd stick to NEP for Jupiter out to Pluto, and NTP for Mars, asteroids and inner planets.  Bussard had a little table for NTRs; Isp of 1000 gets you lunar colonies and Mars missions, 1500 Mars colonies and asteroids, 2000 Jupiter, 2500+ the rest of the solar system.

So substitute solar for nuclear, and it seems to me that the inert fraction goes up, not down, with no commensurate increase in Isp.  What am I missing?

You are missing VASIMR, which ranges from 5,000-30,000 sec Isp depending on the thrust level, while NTP is stuck around 800-1000 sec.

Yes, NTP is great for lunar missions, maybe even NEOs. But Mars and beyond, VASIMR is far more useful. While it is true that NTP allows for dV optimal Hohmann transfer orbits, and NEP has to spiral out of LEO, NEP, once out of earth orbit, can continue thrusting indefinitely and so can reach a higher velocity and go much further in less time.

A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

A NTP is going to get you out of Earth orbit and into an escape trajectory in a matter of minutes, but you'll be floating in that trajectory the rest of the way and will ultimately take a lot longer to get to Mars.

Since, at least as far as NASA missions are concerned, the moon is off the table for future missions, then it stands to reason that NEP is going to be the propulsion of choice.
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Offline isa_guy

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #326 on: 05/28/2010 09:23 am »
What could be the mass of current state of the art fission space reactors ( like molten-salt or liquid-metal ) that may power multi MW  electric thrusters ( like MPD and VASIMR )  that could cut the trip time to mars compare to NTR and chemical propulsion .

Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #327 on: 05/28/2010 12:35 pm »
A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

No it doesn't.  That's a fantasy, and repeating it doesn't make it any more true.

Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #328 on: 05/29/2010 06:56 pm »
A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

No it doesn't.  That's a fantasy, and repeating it doesn't make it any more true.

It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/2000_3756.pdf
http://www.physorg.com/news186397741.html
http://dma.ing.uniroma1.it/users/bruno/Petro.prn.pdf
http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/f_wsc02.pdf

Now, care to rescind your accusations?

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Offline Robotbeat

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #329 on: 05/29/2010 07:03 pm »
Come on, mlorrey. Do you really think a 200MW gas-core reactor is a reasonable proposition, when one-tenth that power would be plenty for a 450-day Mars mission?
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Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #330 on: 05/29/2010 07:21 pm »
Come on, mlorrey. Do you really think a 200MW gas-core reactor is a reasonable proposition, when one-tenth that power would be plenty for a 450-day Mars mission?

A 450 day mission to Mars when 400 of that 450 days is spent in zero g and subject to radiation hazards in interplanetary space is extremely risky to the lives of astronauts. Interplanetary radiation is far higher than in LEO.

Nor would that be in one single reactor. It would be in 2,3 or 4 reactors for redundancy and safety. You'd spend 1/5th the amount of time in space and thus have a hope of having kids and not dying of cancer.

It also allows you to spend a lot more time on Mars doing science. You can still do a 450 day mission, but spend 370 of those days doing actual science on Mars rather than a month or so.

A 450 day mission really is incredibly unrealistic beyond the radiation hazards, don't forget the logistical issues. Each astronaut needs about 10 kgs of consumables per day. So you need to plan not only for enough supplies storage space for 4500 kg of consumables per astronaut, but enough waste storage to handle a huge amount of sewage, recycling, etc. that makes that sort of a mission a much more complex undertaking than a much shorter duration mission using higher power propulsion.

You have the same environmental risks putting 200MW of nuke in orbit as you do putting 20MW in orbit, but you don't have all the logistical problems of supporting humans for 450 days. You could do the same 50 days of science on Mars with 80 days transit back and forth and have 1/4 the logistics issues.
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Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #331 on: 05/29/2010 08:01 pm »
The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions.

...no, how does that follow?

Sure, you spend more time getting out of the gravity well, but the much higher Isp means you can add lots of delta-V and do a high-energy transfer, or even (if the Isp is high enough) do a continuous-thrust trajectory, where you accelerate halfway and then turn around and decelerate.

So electric propulsion can actually get you where you want to go in less overall time, if it's far enough away (Mars is, the moon isn't).

From "Design and Optimization of Low-Thrust Gravity-Assist Trajectories to Selected Planets" (Debban, McConaghy, Longuski, 2002) and "The Application of Nuclear Power and Propulsion for Space Exploration Missions" (Zubrin, Sulmeisters, 1992):

                                           Mission         ∆V       TOF
                                                            (km/s)   (yrs)
Deban/McConaghy/Longuski   Pluto NEP      13.4       19
Zubrin/Sulmeisters                Pluto NTR       6.52      16
Zubrin/Sulmeisters                Pluto NTR      12.9       10

These studies favor NTRs for inner planet missions and NEPs for outer planet missions.  They illustrate the rule of thumb for mission planners that NTR's have approximately half the ∆V of NEP systems for the same time of flight.

The attraction of a VASIMR, should the plasma containment issues be smoothed out, is the variable impulse.  HIPEP, for instance, sits at 80% efficiency.  An NTR's efficiency, for perspective, is 98%.  Whatever's leftover has to be dumped some other way, usually via radiators.

By the way, just to get the numbers out there, a 75,000-pound-thrust NERVA, which I would characterize as a small-to-medium rocket engine, is approximately 1500 MW.  The 500MW Pewee derivatives now used in NTR reference missions are roughly the same thrust as an RL-10  (NTRs don't have a problem being clustered).  Looking at the enormous amount of power generated by a rocket engine, you can see why talking about making the power ratings of electric thrusters match those of rockets is somewhat unrealistic.  Rocket numbers are even more impressive when you look at how small and light they are for what they accomplish. 

Electric propulsion has its strengths and weaknesses, and so do NTRs, chemical rockets and everything else.  There are plenty of other options; slinging an NTR with a tether is a cool idea.  I saw a proposal the other day for a giant graphene trebuchet.  I hadn't thought of that one.

Offline kfsorensen

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #332 on: 05/29/2010 08:06 pm »
It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

I love it how when an engineer uses a number that another engineer thinks is hopelessly unrealistic, sideline observers like you (mlorrey) like to characterize it as a "liar" type thing, as if we're before a court of law or something.  When Franklin did this 2000 paper, I was on the phone with him regularly about the VASIMR study.  I was the one running the larger study of which VASIMR was a subcomponent.  So don't act like you're telling me something you know more than me about.  I know things about VASIMR that never have and never will get to see the light of day, precisely for the reason that FCD is an astronaut and politically powerful and says things that higher-ups like to hear.

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #333 on: 05/29/2010 08:24 pm »
A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

No it doesn't.  That's a fantasy, and repeating it doesn't make it any more true.

It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/2000_3756.pdf
http://www.physorg.com/news186397741.html
http://dma.ing.uniroma1.it/users/bruno/Petro.prn.pdf
http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/f_wsc02.pdf

Now, care to rescind your accusations?

I don't think accusing the main author of leaving some important bits out is at all an indictment of everyone listed on a paper.  The coauthors on these papers for the most part just had some of their calculations included.

The general consensus among EP guys is that VASIMR has gotten away from itself.

Offline mlorrey

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #334 on: 05/29/2010 10:26 pm »
It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

I love it how when an engineer uses a number that another engineer thinks is hopelessly unrealistic, sideline observers like you (mlorrey) like to characterize it as a "liar" type thing, as if we're before a court of law or something.  When Franklin did this 2000 paper, I was on the phone with him regularly about the VASIMR study.  I was the one running the larger study of which VASIMR was a subcomponent.  So don't act like you're telling me something you know more than me about.  I know things about VASIMR that never have and never will get to see the light of day, precisely for the reason that FCD is an astronaut and politically powerful and says things that higher-ups like to hear.

Well rather than acting obnoxious and arrogant, how about backing up your claims with some facts and some references? I don't know you from adam.
« Last Edit: 05/29/2010 10:29 pm by mlorrey »
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Offline isa_guy

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #335 on: 05/30/2010 11:14 am »
The former head (Professor Samim Anghaie) of the team at university of florida that worked on this concept of 200 MWe gas(vapor) core reactor   http://ams.cern.ch/AMS/ETB/Appendix%20D-Anghaie.pdf , which Chang diaz used for his study ( to power 200 MWe VASIMR thrusters ) has been accused of fraud  http://abcnews.go.com/Business/wireStory?id=8961162 :) .
« Last Edit: 05/30/2010 11:15 am by isa_guy »

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #336 on: 05/30/2010 02:58 pm »
The former head (Professor Samim Anghaie) of the team at university of florida that worked on this concept of 200 MWe gas(vapor) core reactor   http://ams.cern.ch/AMS/ETB/Appendix%20D-Anghaie.pdf , which Chang diaz used for his study ( to power 200 MWe VASIMR thrusters ) has been accused of fraud  http://abcnews.go.com/Business/wireStory?id=8961162 :) .

I read up a little bit on what's happening with Anghaie and I don't think the fraud charges affect his engineering work, for the most part.  There is some evidence that he illegally subcontracted some of the work he was supposed to have done himself, and my understanding is that in some cases he used Russian subcontractors.  I wonder whether he might not have appointed himself savior of the Russian nuclear rocket program.

Offline daveklingler

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #337 on: 06/02/2010 02:25 pm »
I'd like to explore something that Kirk might have some input on.

I think that forty years ago liquid core NTRs were basically too difficult.  Since then, materials technology has improved a great deal, especially in the field of carbon and its various allotropes.  I'm wondering about the concept of a thorium-based liquid NTR.  The two ideas I've kicked around while out walking dogs this morning both basically had the core material in the center with a few layers of carbon passages carrying liquid hydrogen on the exterior, surrounded in turn by a layer of beryllium.  One configuration is annular, the other linear.

Any thoughts?
« Last Edit: 06/02/2010 10:03 pm by daveklingler »

Offline Star-Drive

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #338 on: 06/02/2010 09:47 pm »
It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

I love it how when an engineer uses a number that another engineer thinks is hopelessly unrealistic, sideline observers like you (mlorrey) like to characterize it as a "liar" type thing, as if we're before a court of law or something.  When Franklin did this 2000 paper, I was on the phone with him regularly about the VASIMR study.  I was the one running the larger study of which VASIMR was a subcomponent.  So don't act like you're telling me something you know more than me about.  I know things about VASIMR that never have and never will get to see the light of day, precisely for the reason that FCD is an astronaut and politically powerful and says things that higher-ups like to hear.

Well rather than acting obnoxious and arrogant, how about backing up your claims with some facts and some references? I don't know you from adam.

Kirk & Mike:

Trying to hand-wave orbital mechanics without all the givens can be fraught with errors.  Here at JSC a small group of interested parties were curious about whether Franklin Chang Diaz (FCD)'s 200 MWe, 39-day transit to Mars VASIMR claims were correct, or him just blowing smoke.  So after running to ground Franklin’s mission assumptions for this 39-day example, we had two different programmers in our group run the orbital simulations for such a ONE-WAY mission, which required aerobraking at Mars for the small 22mT manned lander payload, assuming the use of FDR's stated 200 MWe VASIMR, (thruster eff.= 60% with a variable Isp= 3,000 sec to 30,000 sec), with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.  It did for both orbital programming approaches we used, so given THESE mission assumptions and limitations, including the crew being stranded on the Mars surface while their mothership is parked in Mars orbit under autopilot control with dry tanks ~45 days after the crew lands, Franklin's 39-day transit time claim is doable.  I wouldn’t want to fly it though, but hey that’s just me!

Paul March
Star-Drive

Offline A_M_Swallow

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Re: Role of NTR/BNTR/NEP in future architectures
« Reply #339 on: 06/02/2010 11:05 pm »
The return propellant can be sent using a slower tug.  The argon or hydrogen propellant could also be refined on Mars, the ascent stage being sent in advance.

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