Quote from: Hempsell on 06/17/2011 11:13 ambaldusi asks about the propellant ratio. For SABRE 3 in air breathing mode the equivalence ratio is 2.8 (at Mach 4). In rocket mode the mixture ratio is 6.Am I mistaken in thinking that the stoichiometric ration is 8:1? I've seen how the RL10 has improved efficiency going from 5, to 5.5 in the A4 and 5.88 in the B2. The RS-25D, RD-0120 and RD-0146 have a ratio of 6 to 1, even the proposed Raptor had a ratio of 6:1. When I use the mole solution I get a ratio of 8 to 1. Yet, all current and new future high efficiency LH/LOX seem to converge to 6:1. What am I missing? In particular, wouldn't it improve the mass fraction in the skylon?
baldusi asks about the propellant ratio. For SABRE 3 in air breathing mode the equivalence ratio is 2.8 (at Mach 4). In rocket mode the mixture ratio is 6.
Stoich is 8. Isp is primarily a function of exhaust molecular weight and chamber temp. Chamber temp maxes out at stoich, but the molecular weight is also higher. Running it at O/F~6 decreases the molecular weight because you have excess light hydrogen, and it doesn't impact temp that much (the curve is pretty flat right around stoich), so it's a net gain.Sorry to correct 93143 above, but the expansion ratio is actually independent of this consideration, and having stoich with an infinite expansion ratio would not be better than fuel rich.
Quote from: Hempsell on 06/17/2011 11:13 ambaldusi asks about the propellant ratio. For SABRE 3 in air breathing mode the equivalence ratio is 2.8 (at Mach 4). In rocket mode the mixture ratio is 6.Am I mistaken in thinking that the stoichiometric ration is 8:1? I've seen how the RL10 has improved efficiency going from 5, to 5.5 in the A4 and 5.88 in the B2. The RS-25D, RD-0120 and RD-0146 have a ratio of 6 to 1, even the proposed Raptor had a ratio of 6:1. When I use the mole solution I get a ratio of 8 to 1. Yet, all current and new future high efficiency LH/LOX seem to converge to 6:1. What am I missing?
Quote from: kraisee on 06/19/2011 08:12 pmIts not just engine efficiency that counts. The tanking mass for the vehicle increases a lot when using larger ratio's of LH2 to LOX, due to LH2's very low density.I might have understood it backwards, but the mixture is usually oxidized to fuel. So a mixture of 8:1 would be better than a 6:1. LH2 is about 16 lighter than LOX. So 8:1 would mean 15% less tank volume.
Its not just engine efficiency that counts. The tanking mass for the vehicle increases a lot when using larger ratio's of LH2 to LOX, due to LH2's very low density.
I'll take 93143's word for it that it's 4 or 5 to one.
Quote from: strangequark on 06/20/2011 06:06 amStoich is 8. Isp is primarily a function of exhaust molecular weight and chamber temp. Chamber temp maxes out at stoich, but the molecular weight is also higher. Running it at O/F~6 decreases the molecular weight because you have excess light hydrogen, and it doesn't impact temp that much (the curve is pretty flat right around stoich), so it's a net gain.Sorry to correct 93143 above, but the expansion ratio is actually independent of this consideration, and having stoich with an infinite expansion ratio would not be better than fuel rich.I hadn't thought about this before, but I think 93143 is correct: if the expansion ratio were infinite, stoichiometric would be the way to go. Here's why. The advantage of burning a hydrogen-rich mixture is not that the exhaust has lower molecular weight, it's that the exhaust contains a higher fraction of relatively simple molecules (hydrogen), which have fewer ways (principally rotational modes) of soaking up energy. That means that for a given amount of expansion, more energy goes into bulk gas motion and less into internal modes.At infinite expansion, however, the exhaust temperature falls to absolute zero and all energy is converted to bulk motion, regardless of the complexity of the molecules (in classical thermodynamics, of course; quantum reality would be a little different). In that case, it's best simply to maximize the chemical energy per unit mass of the propellants, and that means a stoichiometric mixture ratio.
I hadn't thought about this before, but I think 93143 is correct: if the expansion ratio were infinite, stoichiometric would be the way to go. Here's why. The advantage of burning a hydrogen-rich mixture is not that the exhaust has lower molecular weight, it's that the exhaust contains a higher fraction of relatively simple molecules (hydrogen), which have fewer ways (principally rotational modes) of soaking up energy. That means that for a given amount of expansion, more energy goes into bulk gas motion and less into internal modes.At infinite expansion, however, the exhaust temperature falls to absolute zero and all energy is converted to bulk motion, regardless of the complexity of the molecules (in classical thermodynamics, of course; quantum reality would be a little different). In that case, it's best simply to maximize the chemical energy per unit mass of the propellants, and that means a stoichiometric mixture ratio.
I'd add that you also run less chance of bathing the engine in superhot oxygen if something goes wrong. The Russians do have some lox-rich engines though IIRC, fancy alloys and all that.
Is that actually true that you would recover the vibratory energy in bulk motion, even moving down to 0K? I thought there were internal damping mechanisms for molecular vibration. In that case, and given the infinite amount of time to reach infinite expansion, there should be losses that scale as a function of molecular complexity.
Though, what I said should be true for any finite nozzle of arbitrary size, correct?
Well, may be the solution would be to run the chamber at 8:1 and use TAN with pure H2?
Have you considered advanced carbon carbon for the aeroshell? It flew on the x37 and is meant to be only a quarter the density of RCC. It is also stronger and more oxidant resistance than RCC.
Quote from: Hempsell on 06/17/2011 11:13 amI would rather like that, but as the passenger module is not part of the SKYLON development plan it won’t be up to us.Interesting. So REL wouldn't design a baseline passenger module, but would leave that up to the customers (unless, presumably, they hire you do it)? Sounds like a reasonable conservative approach.How involved will REL be in the design of the ground facilities?
I would rather like that, but as the passenger module is not part of the SKYLON development plan it won’t be up to us.
Re passenger module: Annex D of the Skylon User's Manual describes a conceptual Personnel/Logistics Module that would accommodate 24 people.http://www.reactionengines.co.uk/downloads/SKYLON_User_Manual_rev1-1.pdfIt sounds as though the current 'D' Skylon designs will have more internal space than when this was written, so perhaps more capacity for passengers.
Hemsell... this question is not exactly is not exactly about Skylon, but about the somewhat related A2 Lapcat.many people doubt hipersonic (or even supersonic) aircraft will exist in the next decades because of the economics of fuel... Boeing and Airbus will spend billions to make airplanes that are more economical rather than being faster or anything.how much $$ of fuel would an A2 Lapcat spend per km compared with a common 747?