A kerosene-fueled X-33 as a single stage to orbit vehicle.
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From: J. Clarke on 22 Mar 2010 14:34 On 3/22/2010 1:56 PM, Jeff Findley wrote: > "J. Clarke"<jclarke.usenet(a)cox.net> wrote in message > news:ho7rdm02ink(a)news7.newsguy.com... >> On 3/22/2010 8:32 AM, Jeff Findley wrote: >>> "Greg D. Moore (Strider)"<mooregr_delet3th1s(a)greenms.com> wrote in >>> message >>> news:X6-dnTN9SfCfDzvWnZ2dnUVZ_rWdnZ2d(a)earthlink.com... >>>> J. Clarke wrote: >>>>> Why do people think that launching from 50,000 feet will help somehow? >>>>> Going into orbit is not a matter of going high, it's a matter of going >>>>> _fast_. Launching from 50,000 feet or from sea level you still need >>>>> to impart 18,000 miles an hour of delta-v. That's the hard part. >>>> >>>> Because 50,000 feet gets you above the bulk of the atmosphere which >>>> provides a decent bonus. >>> >>> Specifically, you can optimize your engines for the much lower pressure >>> of >>> 50,000 feet (to vacuum), as opposed to the compromises necessary to make >>> them run at sea level. >> >> So how much do you think this gains you? > > For a conventional bell engine design, quite a bit of ISP as you can > optimize the engine bell shape for vacuum. How much Isp? And how much of the time during boost is it running in vacuum? > Jeff
From: J. Clarke on 22 Mar 2010 14:33 On 3/22/2010 1:55 PM, Jeff Findley wrote: > "J. Clarke"<jclarke.usenet(a)cox.net> wrote in message > news:ho7rdn12inj(a)news7.newsguy.com... >> SSTO, if it can be done at all with chemical fuels, is _barely_ doable. > > There are several expendable stages which could theoretically do SSTO, with > a usable payload, if launched by themselves. Which would those be? > Note that Atlas was able to > put Mercury into orbit, but it did cheat a bit by dropping the two outer > engines on the way up, partly to reduce thrust and partly to reduce the dry > mass of the booster. Yep, it's called a half-stage". > That said, a resuable SSTO is a matter of debate. Some say it's possible, > others say it's too hard or impossible. In any case, do you think that it's going to be achieved by replacing the hydrogen aerospike engines in the X-33 with something burning kerosene and sticking it on top of an airliner?
From: J. Clarke on 22 Mar 2010 15:10 On 3/22/2010 2:37 PM, Jeff Findley wrote: > "J. Clarke"<jclarke.usenet(a)cox.net> wrote in message > news:ho8cvh2rfj(a)news6.newsguy.com... >> While I'm not going to give a cite for it, it is generally accepted that >> all else being equal a kerosene rocket will have lower specific impulse >> than a hydrogen rocket, so whatever performance the X-33 achieves with a >> kerosene rocket will be less than for a hydrogen rocket. > > ISP is one measure of engine performance. Vehicle performance is much more > complicated and depends on many more variables besides engine ISP. In > particular, LH2 isn't very dense. Kerosene is far more dense than LH2 plus > it doesn't need cryogenic storage. In a vehicle design, kerosene has some > distinct advantages which may make up for its lower ISP. And those are going to put an X-33 in orbit?
From: Peter Stickney on 22 Mar 2010 23:07 On Mon, 22 Mar 2010 14:34:15 -0400, J. Clarke wrote: > On 3/22/2010 1:56 PM, Jeff Findley wrote: >> "J. Clarke"<jclarke.usenet(a)cox.net> wrote in message >> news:ho7rdm02ink(a)news7.newsguy.com... >>> On 3/22/2010 8:32 AM, Jeff Findley wrote: >>>> "Greg D. Moore (Strider)"<mooregr_delet3th1s(a)greenms.com> wrote in >>>> message >>>> news:X6-dnTN9SfCfDzvWnZ2dnUVZ_rWdnZ2d(a)earthlink.com... >>>>> J. Clarke wrote: >>>>>> Why do people think that launching from 50,000 feet will help >>>>>> somehow? Going into orbit is not a matter of going high, it's a >>>>>> matter of going _fast_. Launching from 50,000 feet or from sea >>>>>> level you still need to impart 18,000 miles an hour of delta-v. >>>>>> That's the hard part. >>>>> >>>>> Because 50,000 feet gets you above the bulk of the atmosphere which >>>>> provides a decent bonus. >>>> >>>> Specifically, you can optimize your engines for the much lower >>>> pressure of >>>> 50,000 feet (to vacuum), as opposed to the compromises necessary to >>>> make them run at sea level. >>> >>> So how much do you think this gains you? >> >> For a conventional bell engine design, quite a bit of ISP as you can >> optimize the engine bell shape for vacuum. > > How much Isp? And how much of the time during boost is it running in > vacuum? It depends on the chamber pressure of the engine - but a fair bit - the J2 engine optimized for Sea Level has a vacuum Isp of 390, and the vacuum optimized J2 has an Isp of 421. = a gain of 8% over Sea Level. A launch vehicle engine spends most of its time in vacuum. The initial trajectory is as much vertical as possible to get it out of the thick air. When a reasonably high altitude is reached, you pitch over to accelerate. -- Pete Stickney Failure is not an option It comes bundled with the system
From: Peter Stickney on 22 Mar 2010 23:11
On Mon, 22 Mar 2010 09:12:41 -0400, J. Clarke wrote: > On 3/22/2010 4:07 AM, Pat Flannery wrote: >> On 3/21/2010 4:53 PM, Marvin the Martian wrote: >> >> >>> It is apparent you're not acquainted with rocket science. Getting >>> through the "dense lower atmosphere" is no big deal. Von Braun did >>> that with an single stage alcohol fueled rocket 65 years ago. >>> >>> The problem is getting up to orbital velocity. >> >> If you can put the LOX aboard the rocket at altitude, where the >> humidity is very low, you can eliminate the weight and complexity of >> having to put insulation on the outside of the oxidizer tank section, >> as ice won't form on it like it would if it were fueled and launched >> from the surface. Not only does the booster then end up carrying the >> weight of ice still sticking to it during ascent, but the ice that >> sheds can damage the booster due to its mass and impact speed. > > So how much "weight and complexity" is involved with a little bit of > spray-on foam? And in practical terms how much difference is this going > to make? I'm sorry, but you're trying to reduce launch costs by > tackling an at best second order effect without dealing with the major > cost drivers. In any case the tankage on the X-33 is does not have > surfaces exposed to the airflow so this becomes a non-issue. > > And if you're talking an X-33 it has to have a thermal protection system > for reentry anyway. > > And the X-33 could not achieve more than half of orbital velocity on > HYDROGEN so how in the Hell do you expect it to do that with kerosene? > > SSTO, if it can be done at all with chemical fuels, is _barely_ doable. While LH2 can provide high Isp, its Energy Density (Cubic Ergs, if you will) is quite poor. Since an SSTO is fairly limited in volume, you need a high energy density fuel. Kerosene has about 6 times the energy density of LH2. The drawback is it weighs more, and thus incurs structural weight penalties. -- Pete Stickney Failure is not an option It comes bundled with the system |