Author Topic: Geoffrey Landis Nuclear Thermal Rocket presentation  (Read 42581 times)

Offline Elmar Moelzer

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Not sure if this has been posted here before, but this is an interesting presentation on the use of NTRs for the exploration of the solar system by Geoffrey Landis at last years Starship Century symposium.

Offline AlanSE

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #1 on: 03/12/2014 05:24 pm »
A lot of this is pretty universal material for nuclear thermal rockets.  I was surprised by some of the temperatures the USSR program was said to get to.  I think he mentioned something like 3700 K.  Yikes.

It's interesting to hear him come out in favor of a Phobos mission.  With a nuclear thermal rocket, is aerobraking in the Martian atmosphere out of the question?  It seems like the obvious implication, but no one states this directly.

The video was from June 2013.  But NextBigFuture just picked it up.  Maybe it had just been in the author's watch queue.

Offline JasonAW3

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #2 on: 03/12/2014 05:49 pm »
A lot of this is pretty universal material for nuclear thermal rockets.  I was surprised by some of the temperatures the USSR program was said to get to.  I think he mentioned something like 3700 K.  Yikes.

It's interesting to hear him come out in favor of a Phobos mission.  With a nuclear thermal rocket, is aerobraking in the Martian atmosphere out of the question?  It seems like the obvious implication, but no one states this directly.

The video was from June 2013.  But NextBigFuture just picked it up.  Maybe it had just been in the author's watch queue.

Aerobraking with an NTR?  Not likely.  You'd need to slow WAY down before the Martian atmosphere would have enough effect for an aerobraking maneuver.
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Offline RanulfC

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #3 on: 03/12/2014 06:19 pm »
Aerobraking with an NTR?  Not likely.  You'd need to slow WAY down before the Martian atmosphere would have enough effect for an aerobraking maneuver.

You probably wouldn't NEED to, (most NTR missions I've seen have plenty of Delta-V for propulsive breaking) but there is no real reason you couldn't if you wanted to. Some of your short-transit chemical missions have about the same un-breaked Delta-V's incoming as your averge NTR mission and they HAVE to use aerobreaking. You usually need more than one pass though.

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Offline Dalhousie

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #4 on: 03/12/2014 08:29 pm »


Aerobraking with an NTR?  Not likely.  You'd need to slow WAY down before the Martian atmosphere would have enough effect for an aerobraking maneuver.

Most NTR studies don't having the payload arriving at Mars faster the chemical propulsion
Apologies in advance for any lack of civility - it's unintended

Offline Dalhousie

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #5 on: 03/12/2014 08:33 pm »

You probably wouldn't NEED to, (most NTR missions I've seen have plenty of Delta-V for propulsive breaking) but there is no real reason you couldn't if you wanted to. Some of your short-transit chemical missions have about the same un-breaked Delta-V's incoming as your averge NTR mission and they HAVE to use aerobreaking. You usually need more than one pass though.


I have never seen the reason discussed but I can't recall a Mars mission study that has used both aerocapture and NTR.

I speculate that this is because an already used NTR needs ongoing shielding which can't be performed within an aeroshell.

Aerobraking is a much slower process and is probably quite possible but takes to long to be useful on a crewed missions.

Apologies in advance for any lack of civility - it's unintended

Offline Robotbeat

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #6 on: 03/12/2014 08:49 pm »
There's room between aerocapture and typical slow aerobraking. Aerocapture is merely aerobraking in a single pass and with an initially hyperbolic "orbit."
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Offline daveklingler

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #7 on: 03/12/2014 10:26 pm »
A lot of this is pretty universal material for nuclear thermal rockets.  I was surprised by some of the temperatures the USSR program was said to get to.  I think he mentioned something like 3700 K.  Yikes.

That's actually 3100K, right around where a lot of high-temperature materials (like carbon) start to melt.  As it happens, that's also just a tad above where hydrogen dissociates, which means a big increase in Isp.

Offline daveklingler

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #8 on: 03/12/2014 10:48 pm »
As a primary developer of SEP, Landis points out something that's very important and frequently gets dropped in discussions of NTRs in these forums. 

High-Isp, low-impulse rockets are great for transporting almost everything around the solar system except humans.  When humans are involved, high thrust is important, to minimize radiation exposure, to save mass on logistical supplies, and because we humans have finite lifetimes.  1960's-vintage NTRs could open the solar system out to roughly Martian orbits for humans; 1990's-vintage NTRs would get us to the asteroids, and we need something more advanced to get beyond that. 

But if we're willing to wait, i.e. humans aren't going, we can do most missions with low-thrust rockets.

Offline Elmar Moelzer

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #9 on: 03/12/2014 10:58 pm »
I really want to hear more about this MITEE NTRs. The specs sound almost too good to be true. 1000 Isp, 28,000 Newtons thrust from an engine with a total weight of 140 kg. This is a T/W of 20, quite exceptional for such a small nuclear thermal engine. Something like this would be a game changer for solar system exploration. I might even see applications for launch (but politics wont allow that).
http://adsabs.harvard.edu/abs/2001iaop.work...66P

Offline AlanSE

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #10 on: 03/13/2014 12:26 am »
A lot of this is pretty universal material for nuclear thermal rockets.  I was surprised by some of the temperatures the USSR program was said to get to.  I think he mentioned something like 3700 K.  Yikes.

That's actually 3100K, right around where a lot of high-temperature materials (like carbon) start to melt.  As it happens, that's also just a tad above where hydrogen dissociates, which means a big increase in Isp.

I've had trouble accepting this argument.  I delved into the subject here:

http://space.stackexchange.com/questions/2021/nuclear-thermal-rocket-specific-impulse-calculation-uses-1-amu-is-that-wrong

It seems that a lot of designs are crediting this dissociation.  However, I can't convince myself of it, even though I've read it several times.  The disassociation energy is easily found as 4.52 eV.

http://en.wikipedia.org/wiki/Bond-dissociation_energy

The RMS molecular energy would be equal to this when 3/2 kT=E.  The temperature that gets you there is 78,700 Kelvin.  Not possible.  I frankly don't know what the resolution of this is.  Perhaps it reflects my weak background in chemistry, although I can't see how.  If breaking the chemical bond requires that much energy, I fail to see how it could be done with thermal energy any less than that.  I could be noobing all over myself now, so I'll stop.

Offline JasonAW3

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #11 on: 03/13/2014 12:27 am »
Aerobraking with an NTR?  Not likely.  You'd need to slow WAY down before the Martian atmosphere would have enough effect for an aerobraking maneuver.

You probably wouldn't NEED to, (most NTR missions I've seen have plenty of Delta-V for propulsive breaking) but there is no real reason you couldn't if you wanted to. Some of your short-transit chemical missions have about the same un-breaked Delta-V's incoming as your averge NTR mission and they HAVE to use aerobreaking. You usually need more than one pass though.

Randy

That's pretty much what they did with the Mars Observer, if memory serves.
My God!  It's full of universes!

Offline Robotbeat

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #12 on: 03/13/2014 01:45 am »
As a primary developer of SEP, Landis points out something that's very important and frequently gets dropped in discussions of NTRs in these forums. 

High-Isp, low-impulse rockets are great for transporting almost everything around the solar system except humans.  When humans are involved, high thrust is important, to minimize radiation exposure, to save mass on logistical supplies, and because we humans have finite lifetimes.  1960's-vintage NTRs could open the solar system out to roughly Martian orbits for humans; 1990's-vintage NTRs would get us to the asteroids, and we need something more advanced to get beyond that. 

But if we're willing to wait, i.e. humans aren't going, we can do most missions with low-thrust rockets.
I disagree with Landis. Chemical plus advanced SEP opens up the solar system all the way to Jupiter, and NEP opens it up beyond that. And actually, for the longer missions like to Mars and the asteroids, advanced electric propulsion can actually speed past NTR because NTR is still pinned down by the exponential rocket equation. Electric propulsion isn't limited in the same way rocket engines are by material constraints. You can and should send cargo ahead of human craft anyway. NTR is neither needed nor really much of a big boost compared to high-performance hydrolox (that dry mass is a killer).

If you can build a solar sail, you can build a SEP propulsion system that makes NTR almost irrelevant.

EDIT:I'm allowed to disagree with Dr. Landis, he was my research mentor last summer. ;)
« Last Edit: 03/13/2014 02:01 am by Robotbeat »
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Offline grondilu

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #13 on: 03/13/2014 02:21 am »
That was a very nice presentation.  The design for a Callisto mission was very appealing.   Also, I learnt that Ve =  V is the optimal way to pick your exhaust velocity.  I often wondered and I'm glad I know now.   Thanks for posting.

Offline Robotbeat

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #14 on: 03/13/2014 02:36 am »
There's a more optimal way of picking your exhaust velocity, but... If your delta-v required is much less than your characteristic velocity, then V=Ve works just as well.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Dalhousie

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #15 on: 03/13/2014 06:33 am »
There's room between aerocapture and typical slow aerobraking. Aerocapture is merely aerobraking in a single pass and with an initially hyperbolic "orbit."

Do you think then that there is a feasibility window for NTR between aerocapture (which can include 2-3 passes, I think) and historic aerobraking (which has required months)?
Apologies in advance for any lack of civility - it's unintended

Offline Elmar Moelzer

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #16 on: 03/13/2014 12:21 pm »
TBH, my biggest worry is still getting into orbit. This still sucks and even if SpaceX achieves a fully reusable F9, it still sucks. I would like to use NTRs for that but it is politically impossible.

Offline Nilof

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #17 on: 03/13/2014 01:34 pm »
Most of the cost involved in BLEO exploration missions won't be hauling freight into orbit, it'll be development costs. The cost of sending the hardware into orbit will be small in comparison, assuming you use already available launchers. This won't change untill a fairly late stage of colonization is reached.

Of course, having cheap and flexible access to orbit is helpful for developing some hardware since it could allow for fast iteration of hardware in realistic conditions. But cheap access to orbit while helpful isn't the immediate solution to everything.

For exploration, the most important requirements imposed on a launch infrastructure is that:
1) It exists.
2) That it will keep existing and won't be spontaneously canceled.
3) That it can reliably deliver payloads large enough for the mission to be feasible using it.

with "low operating costs" being an afterthought unless you're doing the same thing over and over again. Which leads us to fuel depots, since they significantly expand what is practical using a given set of available launchers, and once they're there they are very hard to pull the plug on since they are reusable. No matter what political decisions are made they stay in orbit and allow for a large portfolio of BLEO missions to be done without waiting for a new BFR to be developed.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Elmar Moelzer

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #18 on: 03/13/2014 01:40 pm »
I don't agree. If you have cheap access to orbit, the things like fuel depots become a no brainer and many problems that drive up cost can be easily solved by throwing more weight at it (which currently drives up cost also).

Offline gbaikie

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Re: Geoffrey Landis Nuclear Thermal Rocket presentation
« Reply #19 on: 03/13/2014 08:00 pm »
Quote
Most of the cost involved in BLEO exploration missions won't be hauling freight into orbit, it'll be development costs. The cost of sending the hardware into orbit will be small in comparison, assuming you use already available launchers. This won't change untill a fairly late stage of colonization is reached.
I don't agree. If you have cheap access to orbit, the things like fuel depots become a no brainer and many problems that drive up cost can be easily solved by throwing more weight at it (which currently drives up cost also).

NASA should spend the money needed for development fuel depots.
And effort to develop fuel depots should done at low costs.
And what does low cost mean?
Well a depot which cost 10 billion dollars is not low cost, whereas entire multiyear "program"
to develop fuel depots may total more than 10 billion dollar.
So developing fuel depot may be process that takes about 10 years to get to point of
depots to get beyond experimental, to stage where they are operational.

As comparison the only US launcher now in operational phase of cargo to ISS is SpaceX. Or SpaceX
completed it's demonstration phase of this development.
So COTs program in total to get cargo and crew to ISS may cost over billion dollar- in terms
"developing" US launch. Whereas delivery of crew and cargo, total cost depends how long ISS
stays in orbit.
So for fuel depot development it could cost more than 10 billion, whereas in comparison COTs will be probably be over 1 billion. 
So the development cost portion of depots, does involves transferring rocket fuel to other craft before
the point in which it becomes operational. And not just cost making one depot on earth and shipping it
to orbit.
So not trying to make perfect depot [as it's impossible], rather you want test systems with prototypes to get to point where get something can be more than experimental- where risks involved can be more or less understood.
« Last Edit: 03/13/2014 08:03 pm by gbaikie »

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