tag:blogger.com,1999:blog-8628325.post113745159260032524..comments2024-03-15T05:59:53.929-07:00Comments on Ambivalent Engineer: Slow LaunchAmbivalent Engineerhttp://www.blogger.com/profile/16491915174390340818noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-8628325.post-54570736680157153042012-08-03T20:32:50.533-07:002012-08-03T20:32:50.533-07:00Higher Isp engines generally have less thrust/weig...Higher Isp engines generally have less thrust/weight. If that's not true, then higher Isp is always higher performance, just as you expect.<br /><br />To figure out these tradeoffs, I built a simulator which flies the rocket into orbit. Quite a bit of the computation goes into finding, for any given rocket, the optimal trajectory to get into orbit.<br /><br />Unfortunately, that simulator sits on disks in a computer by my right knee which has not been turned on in many years, so I can't just give it to you because I don't know how to recover the RAID5. Phhbbbt.Ambivalent Engineerhttps://www.blogger.com/profile/16491915174390340818noreply@blogger.comtag:blogger.com,1999:blog-8628325.post-3722407920356748892012-06-14T18:05:00.451-07:002012-06-14T18:05:00.451-07:00I can see how a higher Isp stage might have a grea...I can see how a higher Isp stage might have a greater total gravity loss, but why should it necessarily be greater than the increase in delta-V resulting from the higher Isp? It's the net (post-gravity) delta-V that counts. Gross (pre-gravity) delta-V goes up linearly with Isp, and that's pretty desirable considering that it goes up only logarithmically with mass fraction.<br /><br />I think of gravity loss as driven by two things: burn time and pitch angle. The faster you burn, the less time for gravity to accelerate you downward. And the more horizontal your attitude, the lower the fraction of your thrust subject to gravity loss; it's equal to the sine of the pitch angle.<br /><br />If we could do orbital launches the way Newton originally showed us (taking off horizontally from the ground) then we'd have very small gravity losses. But we have an atmosphere that exerts a lot of drag, so you have to sacrifice some effiency to get above it. Without an atmosphere to worry about, LM ascents from the moon flew vertically for just a few seconds to clear terrain, then pitched down sharply to drive into orbit.<br /><br />I think achieving horizontal flight more quickly is one of the main ideas behind the Pegasus. Although the altitude and velocity of the aircraft is only a tiny fraction of orbital energy, that initial altitude allows the launcher to fly nearly horizontally immediately after release. Its first stage has wings to compensate for some of its gravity loss with aerodynamic lift. And finally, by starting above much of the atmosphere the peak aerodynamic forces are reduced, lightening the structure.<br /><br />When you're horizontal, as near the end of upper stage powered flight, your gravity loss is zero and you can afford to use high Isp (e.g., LH2/LOX) engines with relatively low thrust, like the J-2. You can even afford to shift the mixture and gain a few seconds on Isp at the expense of thrust. But at liftoff, you're vertical and will be nearly so for a while to get above the atmosphere, so you're subject to very high gravity losses. Then you want as much thrust as you can reasonably get, which means either a solid or a big but inefficient engine like the F-1 (Isp only 263 sec even with RP1/LOX).Phil Karnhttp://www.ka9q.netnoreply@blogger.comtag:blogger.com,1999:blog-8628325.post-37919253318973898812007-11-27T00:49:00.000-08:002007-11-27T00:49:00.000-08:00Gravity Loss has a retrospective look at this year...Gravity Loss has a retrospective look at this year's Lunar Lander Challenge. There's also analysis of where things will go from here in the competition, as well as comparisons with DARPA's challenges and the Ansari X PRIZE.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8628325.post-1143661639217192422006-03-29T11:47:00.000-08:002006-03-29T11:47:00.000-08:00One thing to watch for with high-Isp propellants i...One thing to watch for with high-Isp propellants is that they also increase your gravity losses, so the performance boost isn't as good as it looks in the rocket equation. <BR/><BR/>(in case you find this counterintuitive: the better your mass fraction, the more mass you have gravity pulling on all the way to orbit)Anonymousnoreply@blogger.com