Quote from: Jim on 12/14/2008 05:01 pmCurrent flight rates don't support the use of RLV'sWell, more correctly current flight rates combined with current ways of doing things don't support the use of RLVs. There are actually ways that you could get enough flight rate to support an RLV even with only existing demand. But it would take some change in the way some things are done. ~Jon
Current flight rates don't support the use of RLV's
What are those methods? PDs or putting everything on one launcher?
If you have to throttle down the engine in the atmosphere (for powered landing for instance), FSC techniques allow you to do that without sacrificing expansion ratio for your in-space operations, or requiring separate landing engines.
With the ejectable ring doesn't that require inserting a new ring in the engine before coming back for a landing? ...
{snip}Now, there are better methods (small depot tanks up the kick stage instead of doing tinkertoys modules, eventually having a reusable kick stage, etc) but that's the basic concept. The big challenge is that GEO sat customers are really conservative. While they definitely will go with lower launch cost options once they are proven (people still launch on Protons and Zenits, both of which have worse records than some other more expensive but less used launchers), they're sticklers about not being the first or second customer for something. Now, if NASA did the smart thing, and acted as an anchor tenant for such services by using them to send a probe or two to the Moon or Mars, that might pave the way...But there's a lot of work between here and there--both propellant transfer and RLVs. Personally, I find depots to be the less daunting of the two (though my day job is working on the other problem).~Jon
What about borane fuels? They were supposed to offer the performance of hydrogen at the density of kerosene, but I believe that their was a problem.
Quote from: tnphysics on 01/03/2009 04:11 pmWhat about borane fuels? They were supposed to offer the performance of hydrogen at the density of kerosene, but I believe that their was a problem.The rockets proposed at the time used Fluorine as oxidant and Borane as fuel. The problems were toxicity, corrosion and clumping IIRC. There are ways of improving the ISP of kerosene rockets slightly. e.g. the Russians pre-chilled kerosene to make it denser, and used sintin (synthetic kerosene) which allegedly is only made of some specific hydrocarbon compound instead of the mix-mash that is regular kerosene. Then there are the middle ground hydrocarbons between kerosene and hydrogen. Proposals to use liquid propane or methane are often bandied around. As for LOX/LH2 you can increase thrust at the cost of ISP by, varying the mixture ratio, adding more LOX. LOX has a much higher density than LH2 so you get higher overall propellant density. Having a variable mixture ratio LOX/LH2 engine reduces a lot of the advantage of going for a LOX/LH2/LCH4 tri-propellant engine for example.
It wouldn't cost that much to put a sea delivery terminal/pipe offshore from Canaveral or Wallops.
I think it is more an issue of no one has actually flow a Methane rocket (yet).
Actually, the very first liquid-fuel rocket in Europe (about 75 years ago) burned methane and lox! But certainly no current system uses methane.What I wonder, though, is why everybody is so excited about methane and not propane. According to Bruce Dunn and others, propane generally outperforms methane, and it's less cryogenic to boot.The one place where I can see methane being preferable is for in situ fuel production, because it's probably easier to manufacture methane than propane. This consideration, however, is irrelevant for earth launch and is far in the future for any other system, except maybe on the moon.
Quote from: Proponent on 01/26/2009 09:51 amActually, the very first liquid-fuel rocket in Europe (about 75 years ago) burned methane and lox! But certainly no current system uses methane.What I wonder, though, is why everybody is so excited about methane and not propane. According to Bruce Dunn and others, propane generally outperforms methane, and it's less cryogenic to boot.The one place where I can see methane being preferable is for in situ fuel production, because it's probably easier to manufacture methane than propane. This consideration, however, is irrelevant for earth launch and is far in the future for any other system, except maybe on the moon.Though people are playing with methane and propane as we type...I would argue if you are going to use LOX (I can not see a chemical SSTO without it) you have to handle Cryo fuels anyway. Methane does have a slightly higher ISP but lower density than propane so we it is as always a tradeoff.Not to pull us off topic, but the only "in situ" methane production I can think of is mars. Both Carbon and Hydrogen are scare on the moon, so it makes sense to use the H2 directly. At this point we have detected methane releases on mars. Meaning we (big unknown what form it really is in and how it is locked up) might be better off drilling for natural gas on mars. Big drilling rig vs. big plumbers wet dream
... has almost as good of Isp as methane ...
Quote from: jongoff on 01/26/2009 02:50 pm... has almost as good of Isp as methane ...I understand everything except this statement. The C/H mass ratio of propane (~36:4) is higher than methane (~12:4) and that of the exhaust products correspondingly similar. With consistent mixture ratios across a range of inlet fuel temperatures, I'd expect Isp to stay relatively constant. Is the mixture ratio in "sub-cooled" propane engines altered to make use of the different thermal properties of the fuel (cooling) or is there something else I'm missing?
If you burn stoichiometrically, the average molecular weight of the exhaust is 29.1 for propane (3 CO2 @ 44, 4 H2O @ 18) and 26.7 for methane (1 CO2 @ 44, 4 H2O @ 18), and 31 for RP-1 (1 CO2, 1 H2O). For LH2/LOX it's 18.If you run fuel-rich to leave some unburned hydrogen the numbers change a bit, but not a huge amount. Propane is roughly halfway between methane and kerosene, and all are a lot higher than liquid hydrogen.
Quote from: yinzer on 01/26/2009 09:40 pmIf you burn stoichiometrically, the average molecular weight of the exhaust is 29.1 for propane (3 CO2 @ 44, 4 H2O @ 18) and 26.7 for methane (1 CO2 @ 44, 4 H2O @ 18), and 31 for RP-1 (1 CO2, 1 H2O). For LH2/LOX it's 18.If you run fuel-rich to leave some unburned hydrogen the numbers change a bit, but not a huge amount. Propane is roughly halfway between methane and kerosene, and all are a lot higher than liquid hydrogen.Precisely what I was thinking. Higher exhaust velocities could potentially offset the [average] molecular mass of the combustion products, but I don't see that being possible given the energy potentials of the fuel/oxidizer combinations being discussed (see http://www.engineeringtoolbox.com/adiabatic-flame-temperature-d_996.html) for some comparative adiabatic flame temperatures, albeit at room conditions).
[T]he only "in situ" methane production I can think of is mars. Both Carbon and Hydrogen are scare on the moon, so it makes sense to use the H2 directly