Author Topic: NASA's new nuclear engine  (Read 41137 times)

Offline Robotbeat

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Re: NASA's new nuclear engine
« Reply #40 on: 02/17/2016 09:43 pm »
If T:W is the killer for NTR then it should be the killer for SEP even more-so. I think there's another killer: cost.
SEP basically stops trying to get benefit from the Oberth Effect, which is about a factor of 2, and it can afford to do so because it has Isp in the range of 2000-5000+. Nuclear thermal can't afford to do so since half of 900s is 450s, the same as chemical propulsion.
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Offline RanulfC

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Re: NASA's new nuclear engine
« Reply #41 on: 02/18/2016 04:32 pm »
If T:W is the killer for NTR then it should be the killer for SEP even more-so. I think there's another killer: cost.
SEP basically stops trying to get benefit from the Oberth Effect, which is about a factor of 2, and it can afford to do so because it has Isp in the range of 2000-5000+. Nuclear thermal can't afford to do so since half of 900s is 450s, the same as chemical propulsion.

But the ISP isn't so much of a help when the T/W is as low as a nominal SEP system since it requires a great deal of time to move out of a gravity well. The very low T/W means that systems like SEP CAN NOT (rather than "do not" as you suggest) use the Oberth Effect. Not sure why you seem to assume that somehow the standard ISP of a propulsion system is halved, if a propulsion system is pushing "straight out" from a gravity well it's ISP is still it's standard ISP so an NTR would push along at 900sec while having a T/W of around 1 (assuming non-LOX augmented, standard NERVA design) to as high as 4 in the case of more modern NTR designs. This is the standard Nuclear Lunar Taxi computed trajectory. (Though that tends to use LOx augmentation which decreases average ISP by around 100 which would drop the NTR to around 800 seconds but increase the T/W to around 10)

If starting from LEO the NTR (or chemical for that matter) vehicle can push to escape velocity in a few minutes and pass through the Van Allan's in under an hour. Meanwhile the low T/W SEP would take several months to reach escape velocity and spend a great majority of that time within the belts during that period. Starting from much higher in the Cis-Lunar gravity field, (L2 for example) the SEP greatly benefits as it can simply increase speed to inject into an interplanetary trajectory with little additional velocity and steadily increases as time goes on due to the high ISP, (but low T/W) SEP system. Due to the high ISP overall propellant usage (and therefore requirement) is very low but trip times are normally higher than higher thrust/lower ISP systems. (I've seen quotes as low as 6 months but the average is between two years and a year and a half with near term SEP systems)

With the same parameters an NTR or abundant chemical mission launching from L2 and using an Earth passing Oberth Effect can average 6 months or less. The trade off is much higher initial mass due to propellant carried versus much higher transit time. Direct transfer from LEO using NTR or Chemical averages about the same transfer time with increased propellant mass.

It comes down to various trade offs which in most cases ends up being chemical or NTR for crew (which is time sensitive due to radiation and other factors) and SEP for high-mass but rather insensitive cargos for which higher overall efficiency is more of a requirement.

Randy
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Offline Robotbeat

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Re: NASA's new nuclear engine
« Reply #42 on: 02/23/2016 02:50 am »
...The very low T/W means that systems like SEP CAN NOT (rather than "do not" as you suggest) use the Oberth Effect. ...
That is factually incorrect. Oberth effect doesn't happen due to how high your thrust is, it happens due to where that thrust occurs. If you had an SEP craft with very low T/W ratio but only turned on your thruster when your orbit was near perigee, then you would still get a very strong Oberth effect, the same as if you were using a chemical thruster at that altitude. But this would take a LOT longer and you'd require dozens or hundreds of passes to get enough impulse to get where you're going, and you'd usually be better off just thrusting nearly constantly but perhaps operating at a higher Isp to compensate.

...but it isn't just academic that very low thrust/weight ratio SEP can take advantage of the Oberth Effect. Sometimes a strategy of avoiding thrusting at apogee pops up when you're constrained by Isp range if you're using a low-thrust trajectory optimizer program.

With SEP, you could easily harness the Oberth effect by just thrusting in the lowest 5% of your orbit, but it'd take you a lot longer and would be suboptimal since you'd basically be wasting the energy you collected in 95% of your orbit (energy which could've been used to operate at a higher Isp). Additionally, you don't get much hyperbolic excess with this method, but there's no reason the Oberth effect couldn't fully be applied to achieve escape velocity with SEP, it's just not as optimal.


I don't really feel like responding to the rest of your comment until this is acknowledged, though it definitely isn't the only misconception in there.
« Last Edit: 02/23/2016 03:13 am by Robotbeat »
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Offline Nilof

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Re: NASA's new nuclear engine
« Reply #43 on: 02/23/2016 05:56 pm »
Also, unlike NTR, SEP has very good (better than chemical) propellant tank mass ratios due to the high density of common propellants, which means there is essentially no penalty for not staging away empty propellant tanks. That tends to encourage single stage, fully reusable designs.

It also means that if you do compromise on total thrust, the mass ratio isn't bottlenecked by the propellant tank mass as it is with NTR.
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 RanulfC

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Re: NASA's new nuclear engine
« Reply #44 on: 02/24/2016 03:23 pm »
...The very low T/W means that systems like SEP CAN NOT (rather than "do not" as you suggest) use the Oberth Effect. ...
That is factually incorrect. Oberth effect doesn't happen due to how high your thrust is, it happens due to where that thrust occurs. If you had an SEP craft with very low T/W ratio but only turned on your thruster when your orbit was near perigee, then you would still get a very strong Oberth effect, the same as if you were using a chemical thruster at that altitude. But this would take a LOT longer and you'd require dozens or hundreds of passes to get enough impulse to get where you're going, and you'd usually be better off just thrusting nearly constantly but perhaps operating at a higher Isp to compensate.

...but it isn't just academic that very low thrust/weight ratio SEP can take advantage of the Oberth Effect. Sometimes a strategy of avoiding thrusting at apogee pops up when you're constrained by Isp range if you're using a low-thrust trajectory optimizer program.

With SEP, you could easily harness the Oberth effect by just thrusting in the lowest 5% of your orbit, but it'd take you a lot longer and would be suboptimal since you'd basically be wasting the energy you collected in 95% of your orbit (energy which could've been used to operate at a higher Isp). Additionally, you don't get much hyperbolic excess with this method, but there's no reason the Oberth effect couldn't fully be applied to achieve escape velocity with SEP, it's just not as optimal.

"Can Not" was a direct quote from several sites and papers on SEP propulsion just so you're aware and then you point out the very correct reasons why this is so. "Not/Sub optimal" is an understatement by a large degree and it's not been suggested for a very good reason, it is totally unworthy even considering for anything other than a very few scenarios'.

Quote
I don't really feel like responding to the rest of your comment until this is acknowledged, though it definitely isn't the only misconception in there.

Up to you but I'm not making an argument that one is inherently better for every mission than the other which is what you appear to keep trying to say and what I'm objecting to.

For example:
Also, unlike NTR, SEP has very good (better than chemical) propellant tank mass ratios due to the high density of common propellants, which means there is essentially no penalty for not staging away empty propellant tanks. That tends to encourage single stage, fully reusable designs.

It also means that if you do compromise on total thrust, the mass ratio isn't bottlenecked by the propellant tank mass as it is with NTR.

You forgot "or chemical" at the end there since you've already established that both NTR and chemical have the bottleneck you're pointing out :)

SEP has a wonderful propellant tank mass ratio, and if that were of course the ONLY metric of importance for any mission that might actually be a point. It isn't though and while SEP is ONE of a number of propulsion systems, including chemical, NTR and even NEP that can and should be used to open the Solar System.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Robotbeat

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Re: NASA's new nuclear engine
« Reply #45 on: 02/25/2016 03:40 am »
...The very low T/W means that systems like SEP CAN NOT (rather than "do not" as you suggest) use the Oberth Effect. ...
That is factually incorrect. Oberth effect doesn't happen due to how high your thrust is, it happens due to where that thrust occurs. If you had an SEP craft with very low T/W ratio but only turned on your thruster when your orbit was near perigee, then you would still get a very strong Oberth effect, the same as if you were using a chemical thruster at that altitude. But this would take a LOT longer and you'd require dozens or hundreds of passes to get enough impulse to get where you're going, and you'd usually be better off just thrusting nearly constantly but perhaps operating at a higher Isp to compensate.

...but it isn't just academic that very low thrust/weight ratio SEP can take advantage of the Oberth Effect. Sometimes a strategy of avoiding thrusting at apogee pops up when you're constrained by Isp range if you're using a low-thrust trajectory optimizer program.

With SEP, you could easily harness the Oberth effect by just thrusting in the lowest 5% of your orbit, but it'd take you a lot longer and would be suboptimal since you'd basically be wasting the energy you collected in 95% of your orbit (energy which could've been used to operate at a higher Isp). Additionally, you don't get much hyperbolic excess with this method, but there's no reason the Oberth effect couldn't fully be applied to achieve escape velocity with SEP, it's just not as optimal.

"Can Not" was a direct quote from several sites and papers on SEP propulsion just so you're aware...
Well too bad, they're wrong. It's clearly not impossible, which is what "can not" means, and even sometimes makes sense (if you're Isp limited but not severely time-constrained). Next time, do your own analysis (or at least quote a source) before trying to pull a gotcha on someone. I worded the original statement carefully to be fully correct. If I had said it was impossible, I would've been wrong.

Of course, remember Akin's Laws: 17. The fact that an analysis appears in print has no relationship to the likelihood of its being correct.

Quote
and then you point out the very correct reasons why this is so.
No, I said why it's not usually optimal. Impossible is an entirely different standard, and it's what we reserve the New Physics forum section for.

Quote
"Not/Sub optimal" is an understatement by a large degree and it's not been suggested for a very good reason, it is totally unworthy even considering for anything other than a very few scenarios'.
...and?

Quote
Quote
I don't really feel like responding to the rest of your comment until this is acknowledged, though it definitely isn't the only misconception in there.

Up to you but I'm not making an argument that one is inherently better for every mission than the other which is what you appear to keep trying to say and what I'm objecting to.

For example:
...
I'm not Nilof.


...and this has turned into one of those long refuting things. I hate those, and it's what I was trying to avoid. Just admit you were wrong and let's get back to the discussion.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline guckyfan

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Re: NASA's new nuclear engine
« Reply #46 on: 02/25/2016 08:01 am »
I got to admit that I am still not completely familiar with the Oberth effect. Is its efficiency not dependent on ISP or more precisely speed of exhaust vs. speed of the accelerating vehicle. Which means the Oberth effect is less relevant for high ISP engines even when only applied during perigee?

Offline STS-200

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Re: NASA's new nuclear engine
« Reply #47 on: 02/25/2016 09:13 am »
I got to admit that I am still not completely familiar with the Oberth effect. Is its efficiency not dependent on ISP or more precisely speed of exhaust vs. speed of the accelerating vehicle. Which means the Oberth effect is less relevant for high ISP engines even when only applied during perigee?

The Oberth Effect is ultimately due to gravitation and has no direct link to any specific type of engine. The Wikipedia article on the effect is quite good. The usual rocket equation applies regardless of where the burn is performed.

However, a consequence of maximising the benefit of the Oberth effect is to want very short burn times (i.e. high thrust), which is more difficult to achieve with high Isp engines.

SEP/STR/NEP engines tend to be low thrust, so it is more difficult (but not impossible) to benefit from the effect when using them. SEP/NEP designs often suppose such high Isp that Oberth becomes irrelevant (i.e. the engine is so efficient and thrust is so low that the gain offered by Oberth doesn't matter).

NTRs and chemical engines can have higher thrusts and therefore designs using these engines can be optimised to benefit from the effect more than the others. As NTR/chemical tends to have lower Isp, use of the "gain" provided by Oberth is more important, but the Isp and Oberth gain are not directly related.
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Offline guckyfan

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Re: NASA's new nuclear engine
« Reply #48 on: 02/25/2016 09:42 am »
I got to admit that I am still not completely familiar with the Oberth effect. Is its efficiency not dependent on ISP or more precisely speed of exhaust vs. speed of the accelerating vehicle. Which means the Oberth effect is less relevant for high ISP engines even when only applied during perigee?

The Oberth Effect is ultimately due to gravitation

I know one thing for sure. It has got nothing to do with gravitation. About all the rest of your argument, that is where I am not sure.

Offline Nilof

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Re: NASA's new nuclear engine
« Reply #49 on: 02/25/2016 09:52 am »
I got to admit that I am still not completely familiar with the Oberth effect. Is its efficiency not dependent on ISP or more precisely speed of exhaust vs. speed of the accelerating vehicle. Which means the Oberth effect is less relevant for high ISP engines even when only applied during perigee?

No, these things neatly cancel out so that for an instantaneous burn it only cares about the delta-v vector, the velocity vector at the point where the delta-v change happens, and the value of the gravitational potential at that point(to get V_infinity or the apoapsis height). Engine specific impulse is irrelevant, the Oberth changes delta-v independently of what your spacecraft looks like, and only depends on Isp in the sense of optimizing the spacecraft for a given delta-v requirement.

However, fully using the Oberth effect requires thrusting near periapsis, and Kepler's second law implies that this is precisely the portion of the orbit where the spacecraft spends the least time. So high thrust (isp irrelevant) allows you to use the Oberth effect more in a single burn and optimal use of the Oberth effect really amounts to (burn time/time near periapsis).

Essentially, this means that high thrust engines get a free bonus for raising apoapsis or increasing v_infinity, while high and low thrust engines are equally efficient for raising/lowering periapsis. This is why SEP systems on current state of the art satellites are aimed towards raising periapse to GEO/MEO's which have traditionally been challenging orbits to reach with chemical systems.

However, SEP still does get a noticable bonus from the Oberth effect, and if it has extra time it can make a spiral-out slightly elliptical and not thrust at the points closest to apoapse. You can continuously deform a SEP spiral out from the case of a pure circular spiral to the one where it only thrusts at periapse to get equal-to-chemical oberth effect, and essentially get any bonus inbetween the two at a medium-range time cost.

For interplanetary transfers where SEP has reasonably high thrust relative to the orbital period, SEP spacecraft can get the full solar Oberth effect, and can also get noticeable boosts from a planet's gravity which are not due to the Oberth effect, by using the long transfer burn to also stay behind/ahead of a planet in its solar orbit longer to get a "free" boost which requires the full three-body description to analyze.

Basically, low-thrust trajectories are much harder to optimize than those with instantaneous burns, because the former is a rather difficult functional optimization problem while the latter just involves optimizing a small number of variables that can be done with a patched conic approximation. Every number you will see for SEP delta-v to a given orbit in a given number of days is going to be an overestimate because finding optimal trajectories is a very hard problem, though good-enough trajectories are reasonably easy to find.
« Last Edit: 02/25/2016 09:58 am by Nilof »
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 STS-200

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Re: NASA's new nuclear engine
« Reply #50 on: 02/25/2016 12:28 pm »
I got to admit that I am still not completely familiar with the Oberth effect. Is its efficiency not dependent on ISP or more precisely speed of exhaust vs. speed of the accelerating vehicle. Which means the Oberth effect is less relevant for high ISP engines even when only applied during perigee?

The Oberth Effect is ultimately due to gravitation

I know one thing for sure. It has got nothing to do with gravitation.

This is probably why you are struggling to understand the Oberth effect.
Ask yourself:
Does gravity have any effect on a vehicle in free space as its distance to a mass changes?
Is it the same when seen from the frame of reference of the vehicle and from outside the system?
Which frame of reference is relevant to the operation of a rocket engine on the vehicle?
Which frame is relevent when considering what happens to the vehicle as it flies away from the mass?

All that can be mathematically boiled down to a nice, simple conservation of energy equation, but the effects of gravity are still relevant there - so final question:
What fundamental property of the universe underlies the concept of "escape velocity" ?

No gravity = no interchange of energy =  no Oberth effect.

Once you are there, you can start to have fun by considering:
Is the effect of gravity only one-way?
and therefore what happens if both the vehicle and the mass are moving relative to the outside observer.

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Offline Stormbringer

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Re: NASA's new nuclear engine
« Reply #51 on: 02/25/2016 02:02 pm »
That question reminds me of the Felber effect: For relativistic approaching masses there is a velocity window where gravity is repulsive if the initial path of the lesser mass is chosen carefully.
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Offline guckyfan

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Re: NASA's new nuclear engine
« Reply #52 on: 02/25/2016 02:25 pm »
No gravity = no interchange of energy =  no Oberth effect.

I suggest you think again. The Oberth effect is not a swing by acceleration utilizing a nearby celestial object.

Offline Robotbeat

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Re: NASA's new nuclear engine
« Reply #53 on: 02/25/2016 06:50 pm »
No gravity = no interchange of energy =  no Oberth effect.

I suggest you think again. The Oberth effect is not a swing by acceleration utilizing a nearby celestial object.
The Oberth effect has everything to do with gravity. You're expelling your propellant deep inside a gravity well. That's how it works and why it's not some sort of violation of conservation of energy.

Doesn't anyone even bother to read even the Wikipedia article at least??

http://en.wikipedia.org/wiki/Oberth_effect
« Last Edit: 02/25/2016 06:54 pm by Robotbeat »
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Offline Kansan52

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Re: NASA's new nuclear engine
« Reply #54 on: 02/25/2016 07:06 pm »
No gravity = no interchange of energy =  no Oberth effect.

I suggest you think again. The Oberth effect is not a swing by acceleration utilizing a nearby celestial object.
The Oberth effect has everything to do with gravity. You're expelling your propellant deep inside a gravity well. That's how it works and why it's not some sort of violation of conservation of energy.

Doesn't anyone even bother to read even the Wikipedia article at least??

http://en.wikipedia.org/wiki/Oberth_effect

Good article. Can't do the math but the rest seems clear.

Offline JasonAW3

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Re: NASA's new nuclear engine
« Reply #55 on: 02/29/2016 12:03 am »
Getting back to the core topic here,

      I know many of you are kind of sick hearing about Thorium, but I was wondering if pelletizing Thorium with a uranium core to the pellets, to act as a neutron source, could be stacked as rods and utilized as a heat source in a NERV type nuclear engine.

      The only types of Thorium based reactors that I hear about are typically gaseous core.  It seems like it would be possible to use a solid type core, which should have broader tolerances with a system like this.

     Does anyone know if a reactor like this was ever set up?
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Offline Hanelyp

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Re: NASA's new nuclear engine
« Reply #56 on: 02/29/2016 04:25 pm »
Thorium is not a fissile fuel.  Rather, like U238 is converted to Pu239, Th232 can be converted to U233.  I see no reason U233, provided you had a sufficient stock, couldn't be used to different kinds of fission reactor.

The =liquid= core salt reactor so commonly associated with thorium is well suited to the breeder reactor needs of that fuel.  A thorium salt blanket is exposed to neutron flux from a U233 salt core, then fairly quickly removed for processing.  A second neutron hitting the thorium before the Th233 can beta decay messes up the conversion.  I'm unaware of any gaseous core proposals for thorium.

Offline QuantumG

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Re: NASA's new nuclear engine
« Reply #57 on: 02/29/2016 10:34 pm »
More importantly, it takes 23 days or more to make U-233 from naturally occurring Thorium.
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Offline Alf Fass

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Re: NASA's new nuclear engine
« Reply #58 on: 03/01/2016 03:39 am »
...The very low T/W means that systems like SEP CAN NOT (rather than "do not" as you suggest) use the Oberth Effect. ...
That is factually incorrect. Oberth effect doesn't happen due to how high your thrust is, it happens due to where that thrust occurs. If you had an SEP craft with very low T/W ratio but only turned on your thruster when your orbit was near perigee, then you would still get a very strong Oberth effect, the same as if you were using a chemical thruster at that altitude. But this would take a LOT longer and you'd require dozens or hundreds of passes to get enough impulse to get where you're going, and you'd usually be better off just thrusting nearly constantly but perhaps operating at a higher Isp to compensate.

...but it isn't just academic that very low thrust/weight ratio SEP can take advantage of the Oberth Effect. Sometimes a strategy of avoiding thrusting at apogee pops up when you're constrained by Isp range if you're using a low-thrust trajectory optimizer program.

With SEP, you could easily harness the Oberth effect by just thrusting in the lowest 5% of your orbit, but it'd take you a lot longer and would be suboptimal since you'd basically be wasting the energy you collected in 95% of your orbit (energy which could've been used to operate at a higher Isp). Additionally, you don't get much hyperbolic excess with this method, but there's no reason the Oberth effect couldn't fully be applied to achieve escape velocity with SEP, it's just not as optimal.

"Can Not" was a direct quote from several sites and papers on SEP propulsion just so you're aware...
Well too bad, they're wrong. It's clearly not impossible, which is what "can not" means, and even sometimes makes sense (if you're Isp limited but not severely time-constrained). Next time, do your own analysis (or at least quote a source) before trying to pull a gotcha on someone. I worded the original statement carefully to be fully correct. If I had said it was impossible, I would've been wrong.

Of course, remember Akin's Laws: 17. The fact that an analysis appears in print has no relationship to the likelihood of its being correct.

Quote
and then you point out the very correct reasons why this is so.
No, I said why it's not usually optimal. Impossible is an entirely different standard, and it's what we reserve the New Physics forum section for.

Quote
"Not/Sub optimal" is an understatement by a large degree and it's not been suggested for a very good reason, it is totally unworthy even considering for anything other than a very few scenarios'.
...and?

Quote
Quote
I don't really feel like responding to the rest of your comment until this is acknowledged, though it definitely isn't the only misconception in there.

Up to you but I'm not making an argument that one is inherently better for every mission than the other which is what you appear to keep trying to say and what I'm objecting to.

For example:
...
I'm not Nilof.


...and this has turned into one of those long refuting things. I hate those, and it's what I was trying to avoid. Just admit you were wrong and let's get back to the discussion.

I probably shouldn't stir the pot, but. .

1. While high thrust systems using a single pass can get a very high Oberth effect by accelerating to a delta V way above escape velocity, aren't systems using repeated perigee burns limited to the Oberth effect to escape velocity, after that final low pass that pushes you to escape velocity the opportunity's gone?

2. With a high Isp system the benefits from the Oberth Effect are going to be far less useful simply because the benefit from the effect would be a smaller fraction of a high Isp vehicles delta V compared to a low Isp vehicle (assuming the reasaon for choosing high Isp is to get higher delta v)?
When my information changes, I alter my conclusions. What do you do, sir?
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Offline Robotbeat

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Re: NASA's new nuclear engine
« Reply #59 on: 03/01/2016 04:30 am »
Both true.
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