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Why is a LOX/Kero SSTO not rather easy?
I am not a particular fan of SSTO, but it appears to me that SSTO has long been within our grasp. The best figures that I could get for the Titan II 1st stage (based on Rusty Barton's reply to my earlier post) are a GLOW 0f 258000 lbs, and a dry weight of 10900, giving it a mass ratio of 23.7. Now comparing it to a LOX/kero vehicle powered by something like the Russian NK-33, I see the following plus and minus factors affecting its mass ratio (I am not including a payload in these calculations): On the minus side, the Titan II 1st stage is not a complete vehicle. It: (a) lacks a nose cone (b) lacks most avionics (c) lacks a cargo bay (d) has 6% denser fuel so a slightly smaller fuel tank On the plus side: (a) the NK-33 is 420 lbs lighter than the Titan II engine yet it is sufficient for a vehicle nearly 10% heavier (b) the structure does not have to support the 32 ton 2nd stage and so can be significantly lighter. My guess is that those factors mostly cancel out. I also assume that making a vehicle reusable adds about 40% to its dry weight: Wings add 7% Landing gear add 3% TPS add 15% Other add 15% So if we crank those factors in: Dry weight = 10900*1.4 = 15300 lbs Glow = 258000 + (15300 - 10900) = 262400 MR = GLOW/(GLOW-Dry weight) = 262400/15300 = 17.15 Now for the NK-33 we have an average Isp of 331, and given a required dV of 9200 m/s (300 m/s less than a LH2/LOX rocket due to less air resistance, lower back pressure losses, and earlier peak acceleration) we get a required MR of 17.01, which is slightly less than what we can achieve. So we can make orbit with a single stage using very old technology. Of course, this is without payload, but given the fact that the Titan II 1st stage was not optimized for weight (you would not normally optimize a 1st stage) and we have lighter materials today, such as aluminum-lithium and carbon fiber, I would think we would have the necessary margin for a significant payload. We also might fly with a wet wing and eliminate the kerosene tank altogether. And of course if we scaled it up by a factor of 3 we would gain a substantial economy of scale. So it appears to me that we have had reusable SSTO capability for dense fuel vehicles for a long time. -- Larry |
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Why is a LOX/Kero SSTO not rather easy?
Hi Larry!
Well, I'd like to add a few nitpicks. I removed a lot of the text where I do not disagree. Larry Gales wrote: [...] My guess is that those factors mostly cancel out. I also assume that making a vehicle reusable adds about 40% to its dry weight: Wings add 7% Landing gear add 3% TPS add 15% Other add 15% I do not think wings could be done for only 7% of the weight, especially if one takes into account the added structure needed for a winged vehicle. However I am partial to wingless vehicle and I did the following calculations: The spaceship has in the earth a terminal velocity of 50...100 m/s (example: cw=1, 10 m diameter, 50t weight = 60m/s) Say you want 200m/s fuel including reserves, so you would need 6-7% mass depending on Isp - and the engines and structure needed are already there. Question: which items do you include among "other"? I see only restfuel and a bit more avionics, but this should not amount to 15%. However, I might easily miss something... So if we crank those factors in: Dry weight = 10900*1.4 = 15300 lbs Glow = 258000 + (15300 - 10900) = 262400 MR = GLOW/(GLOW-Dry weight) = 262400/15300 = 17.15 Now for the NK-33 we have an average Isp of 331, and given a required dV of 9200 m/s (300 m/s less than a LH2/LOX rocket due to less air resistance, lower back pressure losses, and earlier peak acceleration) we get a required MR of 17.01, which is slightly less than what we can achieve. So we can make orbit with a single stage using very old technology. I notice most modern rockets are more "fat" than Titan. However, I am not sure how much weight could be saved if the tanks have more diameter and less length, and there is some delta-V loss due to additional air resistance. Additional advantage to the fat version: I want a lot of drag for breaking during descend. Of course, this is without payload, but given the fact that the Titan II 1st stage was not optimized for weight (you would not normally optimize a 1st stage) and we have lighter materials today, such as aluminum-lithium and carbon fiber, I would think we would have the necessary margin for a significant payload. You can optimize a RLV a bit more than a ELV: increased construction costs for the airframe would be spread over a high number of flights. However, initially you would *have* to keep down development costs, so this might not be possible for the first generation. We also might fly with a wet wing and eliminate the kerosene tank altogether. And of course if we scaled it up by a factor of 3 we would gain a substantial economy of scale. As befo I think you underestimate the weight of a wing. So it appears to me that we have had reusable SSTO capability for dense fuel vehicles for a long time. Well, I agree. I just do not think a winged vehicle would be the way to go. Robert Kitzmueller |
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