My favorite "quasi-SSTO" approach would be:1- Airlaunched "Assisted SSTO"2- Use gamma maneuver (ie rocket assisted zoom-climb) with crossfeed from carrier aircraft to keep rocket topped until separation. This cuts the dV to orbit down into the ~8000m/s range.3- Use LOX/LH2 expander cycle engines with LOX-rich TAN to get both the thrust and the T/W ratio at ignition but high Isp in sustainer mode, with only two propellants and a good stage O/F ratio. The higher altitude start decreases the Isp penalty during TAN operations.Probably with helecopter landing and a retractable engine nozzle.I think that with LOX/LH2 + LOX-rich TAN, you could probably make a ground takeoff SSTO close.~Jon
If you build the Startram, and basically become THE go-to place for launch (including human missions), how much is inclination an issue? What percentage of current low to mid earth orbits require a flexible inclination?I'd imagine that almost all LEO and mid orbit industry and tourism would be captured at the megastructure's inclination, so flexibility wouldn't be much of an issue for that? Are missions to GEO, EM1 and lunar also affected substantially by inclination?Not crash hot on the orbital dynamics - apologies in advance.Edit: just finished reading the Startram book but inclination not really mentioned as a limiting factor in there except for their proposal for a polar orbit launcher in Antarctica (yikes!)
Nuclear rockets are much less good for SSTOs than you might think. Very low Thrust-to-weight basically kills a pure NTR SSTO, and the incredibly low density of hydrogen propellant (though only thing that's really worth putting in a NTR) drives a stake through its heart.
Quote from: ChrisWilson68 on 11/06/2014 01:32 amThat's not what McDonnell Douglas said. From a 1994 press release by McDonnell Douglas on NASA's web site:I hope you're not suggesting that the press release for a military program could ever be deliberately misleading.Quote from: ChrisWilson68They weren't practising to launch expendable missiles quickly.Where did I say they were? Read what I wrote or, ya know, go read one of the many great books which describe what Pete and the boys were doing.
That's not what McDonnell Douglas said. From a 1994 press release by McDonnell Douglas on NASA's web site:
They weren't practising to launch expendable missiles quickly.
There's other good reasons to aim for SSTO or even "assisted" SSTO. Especially as you start thinking about going beyond LEO.
My favorite "quasi-SSTO" approach would be:1- Airlaunched "Assisted SSTO"2- Use gamma maneuver (ie rocket assisted zoom-climb) with crossfeed from carrier aircraft to keep rocket topped until separation. This cuts the dV to orbit down into the ~8000m/s range.3- Use LOX/LH2 expander cycle engines with LOX-rich TAN to get both the thrust and the T/W ratio at ignition but high Isp in sustainer mode, with only two propellants and a good stage O/F ratio. The higher altitude start decreases the Isp penalty during TAN operations.
Probably with helicopter landing and a retractable engine nozzle.
I think that with LOX/LH2 + LOX-rich TAN, you could probably make a ground takeoff SSTO close.
...That's why people believe in SSTO - the numbers look to be just on the edge of the possible....It's not that SSTO is undoable. It's that it's SO marginal that we haven't got to where we've managed to make it work even as a demonstrator, never mind as an operational reusable vehicle.Being very light and being very structurally robust at the same time is hard.
Good post and points out SSTO is simply an engineering issue, not pie in the sky. Many concepts get lost in the numbers and forget the design must be a whole. Structurally, being a whole is called "fusion" where the tanks and TPS are partially load-bearing.
Numbers can suggest the high isp of hydrogen (Skylon), but as a whole, denser and non-cryogenic propellants win out. The simplicity and perceived weight savings of VTVL may stand out, but as was mentioned in the OP, HTHL wins out. And of course there's the pure engineering advances like one all-encompassing engine to keep mass low and aerodynamic tricks for low-speed through hypersonic efficiency that also provide structural fusion of the wings (as in the SR-71).
3- Use LOX/LH2 expander cycle engines with LOX-rich TAN...
Quote from: jongoff on 11/05/2014 06:17 am3- Use LOX/LH2 expander cycle engines with LOX-rich TAN...Expander? You'd need a lot of engines...
It seems to me that high chamber pressure leads to high thrust density which leads to low pressure losses for a conventional nozzle. If you can easily compensate for pressure losses by other means (TAN, aerospike, etc.), then chamber pressure probably isn't so important.
<long post full of speculation snipped.
While that's only up to about M5.5 the effect on the average Isp for the flight is substantial, as it would be for any air breathing system.
Quote from: john smith 19 on 11/08/2014 11:36 pmWhile that's only up to about M5.5 the effect on the average Isp for the flight is substantial, as it would be for any air breathing system.It's substantial but it tends to be overstated. For example, for a vehicle operating at 4000s over M=0-5 and 400s over M=5-25 the effective Isp over M=0-25 is 488s. A substantial improvement as you say but it is not obvious that the improvement in Isp makes up for the lower engine T/W, lower propellant density, and the rest of the air breathing requirements.
It's substantial but it tends to be overstated. For example, for a vehicle operating at 4000s over M=0-5 and 400s over M=5-25 the effective Isp over M=0-25 is 488s. A substantial improvement as you say but it is not obvious that the improvement in Isp makes up for the lower engine T/W, lower propellant density, and the rest of the air breathing requirements.
Quote from: jongoff on 11/05/2014 06:17 amMy favorite "quasi-SSTO" approach would be:1- Airlaunched "Assisted SSTO"2- Use gamma maneuver (ie rocket assisted zoom-climb) with crossfeed from carrier aircraft to keep rocket topped until separation. This cuts the dV to orbit down into the ~8000m/s range.3- Use LOX/LH2 expander cycle engines with LOX-rich TAN to get both the thrust and the T/W ratio at ignition but high Isp in sustainer mode, with only two propellants and a good stage O/F ratio. The higher altitude start decreases the Isp penalty during TAN operations.Probably with helecopter landing and a retractable engine nozzle.I think that with LOX/LH2 + LOX-rich TAN, you could probably make a ground takeoff SSTO close.~JonThere was some post on this board and a blog post which I can remember making a similar point. I did not understand the case at all.Particularly, why do you crossfeed the fuel? I mean, what is the design environment which leads to this making sense? As I understand it, this made sense for the shuttle. Why? Because you avoid putting any engines on the external tank. Makes sense. You're throwing it so obviously you don't want to use your mass putting engines on it.But if we're trying to make reusable rockets, the argument seems to fall apart. Isn't it much easier to transfer thrust than to transfer liquid? If you have 2 stages and you plan on reusing them both, why doesn't the first one just push the 2nd one until separation instead of dealing with the difficulties of crossfeed?
Quote from: jongoff on 11/05/2014 06:17 amMy favorite "quasi-SSTO" approach would be:1- Airlaunched "Assisted SSTO"2- Use gamma maneuver (ie rocket assisted zoom-climb) with crossfeed from carrier aircraft to keep rocket topped until separation. This cuts the dV to orbit down into the ~8000m/s range.3- Use LOX/LH2 expander cycle engines with LOX-rich TAN to get both the thrust and the T/W ratio at ignition but high Isp in sustainer mode, with only two propellants and a good stage O/F ratio. The higher altitude start decreases the Isp penalty during TAN operations.Pretty much the "Crossbow" concept IIRC?
QuoteProbably with helicopter landing and a retractable engine nozzle.Man after my own heart Maybe plug, or cluster-plug nozzle?
QuoteI think that with LOX/LH2 + LOX-rich TAN, you could probably make a ground takeoff SSTO close."Close" maybe but wouldn't an assist still make it more "economical" as in giving a higher payload? Your problem is still in your margins allowable versus your payload size to orbit.
and carrying the wings from M5.5 to M25, then the TPS for the wings and the jet engines coming back down, for the benefit of being able to fly around a bit at both ends. I am sorry unless we have high energy density FTR on the order of 500KW per kilo of engine, SSTO will not make sense when compared to the same technologies in TSTO.