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).
Quote from: Robotbeat on 05/26/2010 06:26 pmSEP 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?
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).
Quote from: Robotbeat on 05/28/2010 02:58 amFrom 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.
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.
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.
Quote from: mlorrey on 05/28/2010 07:24 amA 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.
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?
Quote from: daveklingler on 05/28/2010 02:44 amThe 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).
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?
Quote from: kfsorensen on 05/28/2010 12:35 pmQuote from: mlorrey on 05/28/2010 07:24 amA 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.pdfhttp://www.physorg.com/news186397741.htmlhttp://dma.ing.uniroma1.it/users/bruno/Petro.prn.pdfhttp://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/f_wsc02.pdfNow, care to rescind your accusations?
Quote from: mlorrey on 05/29/2010 06:56 pmIt 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.
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 .
Quote from: kfsorensen on 05/29/2010 08:06 pmQuote from: mlorrey on 05/29/2010 06:56 pmIt 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.