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.
Quote from: QuantumG on 02/14/2016 10:25 pmIf 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.
...The very low T/W means that systems like SEP CAN NOT (rather than "do not" as you suggest) use the Oberth Effect. ...
Quote from: RanulfC on 02/18/2016 04:32 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.
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.
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.
Quote from: Robotbeat on 02/23/2016 02:50 amQuote from: RanulfC on 02/18/2016 04:32 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'.
QuoteI 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:Quote from: Nilof on 02/23/2016 05:56 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?
Quote from: guckyfan on 02/25/2016 08:01 amI 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
Quote from: STS-200 on 02/25/2016 09:13 amQuote from: guckyfan on 02/25/2016 08:01 amI 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.
No gravity = no interchange of energy = no Oberth effect.
Quote from: STS-200 on 02/25/2016 12:28 pmNo 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.
Quote from: guckyfan on 02/25/2016 02:25 pmQuote from: STS-200 on 02/25/2016 12:28 pmNo 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
Quote from: RanulfC on 02/24/2016 03:23 pmQuote from: Robotbeat on 02/23/2016 02:50 amQuote from: RanulfC on 02/18/2016 04:32 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...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.Quoteand 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?QuoteQuoteI 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:Quote from: Nilof on 02/23/2016 05:56 pm...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.