Yup , another thing to think about is if you have a lot of nozzles in your 100 ratio plug , after 50,000 ft you shut off 1/2 of them , Instant 200 ratio ! ( Isp 300 to 340 ) Not sure I like the idea of blowdown, by the time you add the extra volume of tank , it weighs the same as the pressurizing system .
Kevin Bollinger ----- Original Message ----- From: "John Carmack" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Friday, February 28, 2003 1:37 PM Subject: [ERPS] blowdown SSTO > If plug nozzles actually work to allow automatic altitude compensation, it > seems like it should be possible to do an SSTO with blowdown pressurization > and light tanks. > > My reasoning is that the bulk of the delta-V that an SSTO needs is > generated outside the atmosphere, and with no back pressure, a very large > expansion ratio nozzle can extract essentially the same Isp regardless of > chamber pressure. If you had a huge plug nozzle over the entire base of a > vehicle, you could start out with a 150 psi tank pressure and get a crappy > 200 or so Isp at sea level (98% peroxide / kerosene), but it would rise to > 300 when out of the atmosphere. The time spent at the lower Isp is not > actually all that great -- under 20% of the time in the sensible > atmosphere, with the full Isp at the end where you really need it. > > If you have an engine capable of deep throttling, say, with a moveable > pintle injector, starting with some ullage and letting the pressure decay > to 20% of its starting pressure probably isn't a bad thing at all, since > you are going to throw away 95% of your mass by that point. Making a > pressurization subsystem just go away is a big deal optimization. > > Extremely low engine pressures also make the cooling task a lot easier -- > the plug nozzle could almost certainly live with a pretty lightweight heat > barrier / high temp metal fabrication for a six minute burn to orbit. > > Composite tanks with a 200 psi burst pressure can be fabricated with SSTO > mas fractions fairly easily. > > John Carmack > > _______________________________________________ > ERPS-list mailing list > [EMAIL PROTECTED] > http://lists.erps.org/mailman/listinfo/erps-list > _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
