So for a Mars mission high thrust is important! Let's say that we are using a methane engine with Isp of 380s. A low thrust electrical engine would require an Isp of at least 1520s to be as effective. Of course most electrical engines have an optimum Isp over 2000s, but that does not include the extra weight of the solar panels...
Quote from: Robotbeat on 02/08/2013 05:34 pmQuote from: R7 on 02/08/2013 05:32 pmQuote from: Robotbeat on 02/08/2013 05:19 pmQuote from: IRobot on 02/08/2013 04:59 pmI was under the impression that some orbital capture maneuvers required fast delta V change... No, nothing in-orbit actually /requires/ high thrust. There is a performance benefit (from the Oberth effect), but it's still dwarfed by the Isp advantage of electric propulsion.see emphasisI didn't miss it the first time. You just have to start thrusting earlier. You miss out on most of the Oberth effect (and trip time is slightly longer), but it's not a huge deal because of the huge advantage in Isp, which can also counter-act the delta-t penalty.For trans mars injection and mars capture, the oberth effect can make a very big difference. Electrical delta-v from earth C3=0 to mars C3=0 is roughly 6km/s, while chemical delta-v is roughly 1.5km/s. So a factor of 4. So I think discounting the oberth effect is not warranted unless you have a very good power source, or your destination does not have significant oberth effect (e.g. an asteroid).
Quote from: R7 on 02/08/2013 05:32 pmQuote from: Robotbeat on 02/08/2013 05:19 pmQuote from: IRobot on 02/08/2013 04:59 pmI was under the impression that some orbital capture maneuvers required fast delta V change... No, nothing in-orbit actually /requires/ high thrust. There is a performance benefit (from the Oberth effect), but it's still dwarfed by the Isp advantage of electric propulsion.see emphasisI didn't miss it the first time. You just have to start thrusting earlier. You miss out on most of the Oberth effect (and trip time is slightly longer), but it's not a huge deal because of the huge advantage in Isp, which can also counter-act the delta-t penalty.
Quote from: Robotbeat on 02/08/2013 05:19 pmQuote from: IRobot on 02/08/2013 04:59 pmI was under the impression that some orbital capture maneuvers required fast delta V change... No, nothing in-orbit actually /requires/ high thrust. There is a performance benefit (from the Oberth effect), but it's still dwarfed by the Isp advantage of electric propulsion.see emphasis
Quote from: IRobot on 02/08/2013 04:59 pmI was under the impression that some orbital capture maneuvers required fast delta V change... No, nothing in-orbit actually /requires/ high thrust. There is a performance benefit (from the Oberth effect), but it's still dwarfed by the Isp advantage of electric propulsion.
I was under the impression that some orbital capture maneuvers required fast delta V change...
Wasn't commenting the machine in original post, pointing out that high thrust/low Isp electric propulsion is not a break through, solutions been available. Then again, why use them because chemical solutions make more sense.
Quote from: rklaehn on 02/08/2013 06:57 pmFor trans mars injection and mars capture, the oberth effect can make a very big difference. Electrical delta-v from earth C3=0 to mars C3=0 is roughly 6km/s, while chemical delta-v is roughly 1.5km/s. So a factor of 4. So I think discounting the oberth effect is not warranted unless you have a very good power source, or your destination does not have significant oberth effect (e.g. an asteroid). Infinitely low-thrust assumption, yes, it would be almost 6km/s. that would assume spiraling around the sun for a very long time, years and years. But that assumption is clearly invalid here. The actual result would be closer to a Hohmann transfer, which still isn't great but is better than your factor of 4. And, of course, even the assumption of infinite thrust breaks down for chemical rockets as well.This paper shows the transfer between Earth and mars for far less than your 6km/s: http://goo.gl/2USvJ
For trans mars injection and mars capture, the oberth effect can make a very big difference. Electrical delta-v from earth C3=0 to mars C3=0 is roughly 6km/s, while chemical delta-v is roughly 1.5km/s. So a factor of 4. So I think discounting the oberth effect is not warranted unless you have a very good power source, or your destination does not have significant oberth effect (e.g. an asteroid).
And again, solar electric keeps improving.
Also, electric propulsion, especially solar-electric propulsion, benefits disproportionally when talking about short-stay Mars missions.
Yeah, I think Xenon is a great propellant for a lot of reasons. Agreed there.
There's an interesting fact about the Oberth Effect... The effect is greater the less delta-v you supply! So even a small chemical burn on an otherwise solar-electric propulsion system will be worth it. And in fact, the smaller the burn, the greater the multiplier. http://en.wikipedia.org/wiki/Oberth_effect#Detailed_proofCalculate the multiplier for a 10m/s burn at perigee, and you see the multiplier is almost 50, while if you do a 1km/s burn, the multiplier is just 5. So even using RCS jets at perigee would give you a quite significant boost, well worth the IMLEO.I guess this is why Boeing's Mars SEP architecture uses a chemical kick-stage. It really is an optimal arrangement to have a small chemical stage, more optimal than either a fully SEP or fully chemical architecture. (This also means it's really hard to decrease mission time by using a very large perigee burn in an all-chemical--or even all-NTR--architecture since the larger the burn, the less the Oberth multiplier is...).