> 4. Drag losses vs. gravity losses (was: [ERPS] New lube, VTVL in > Israel and TRIZ) (Randall Clague) > --__--__-- > > Message: 4 > From: Randall Clague <[EMAIL PROTECTED]> > To: [EMAIL PROTECTED] > Subject: Drag losses vs. gravity losses (was: [ERPS] New lube, VTVL in Israel and >TRIZ) > Date: Mon, 01 Jul 2002 00:13:34 -0700 > > On Sat, 15 Jun 2002 12:40:28 -0700, Randall Clague > <[EMAIL PROTECTED]> wrote: > > >As you've discovered, SSTO with a small vehicle requires trajectory > >optimization, which in turn is a balancing act as you trade off drag > losses against gravity losses. <snip>
> So I did a structural model for this really cool pressure fed TSTO. > Nothing fancy, 6061 T6, about 1000 kg GLOW, 10 kg payload, delta-v > 9000 m/sec. You could take the thing out to the launch site in a > small U-Haul truck, and put 22 lbs in orbit for about $1000/lb. > Definitely within the reach of motivated amateurs, and probably > marketable as well. Way cool. > > Next step, trajectory modeling. > > Drag *killed* me. It didn't even make a decent sounding rocket. > > I'm currently doing a parametric analysis, varying thrust, propellant > load, aspect ratio, and payload. What I've seen just from the first > few runs is that crawling along through the troposphere at M 0.5 or so > gets me the best final altitude. I throw away a lot of fuel "standing > on a pillar of fire," as Mitch puts it, but the alternative is burning > harder to get through the atmosphere quicker. This decreases my > gravity losses at the price of increasing my drag losses even more. > (Gravity losses are linear, but drag is proportional to the square of > the velocity.) > > This sure is frustrating. > > -R Randall & the group: I came to the same conclusion modelling an X-Prize vehicle. My conclusion was that you don't even *think* of going supersonic below FL300 (30K feet), at which the density is about 1/4 of sea level. From there on up, it's a race between 'rho' and 'vee-squared.' Once you are above 50-100K feet, you do want to burn as hard as possible to minimize the gravity losses, only throttling back to avoid crushing the pilot/payload/airframe. A secondary conclusion based on this model is that a turbojet engine makes an excelent first stage. In fact one of the most cost-effective first stages is a mass-produced jet transport aircraft. Pegasus has been doing good work in this area, and Pioneer uses this principle both for the spaceplane, and the tanker. Of course the jet engines on the spaceplane make early flight test and ferry operations a no-brainer. (((>>> Doug Drummond, Kittyhawk Technologies. _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
