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Air Breathing for VTVL
Until recently I have considered air breathing engines on VTVL to
not be worth consideration. Now I believe they *might* be worth consideration if they can reach certain performance goals. The performance goals I have in mind are, Dry mass/payload ratio to remain similar if, and only if, some substantial performance or operational gain is made. or Cost of construction, testing, and operations drop a considerable amount for similar mission capability. I would like to quantify the performance requirement for the curve in which the addition of a subsonic air breathing engine reaches break even vs an all rocket VTVL. There are 3 places I am aware of to get the mass for the air breathing engine I am interested in. Replacement of some rocket engine mass from launch for a minute or so. Minor fuel savings during launch. Elimination of some landing fuel required by all rocket landing. For figuring purposes, a 160,000 lb GLOW vehicle with 10,000 lb dry mass including payload and landing fuel. This is a dense fuel SSTO as I am not sold on hydrogen for various reasons. If terminal velocity is 100 m/s and 3 gee deceleration is used with no margin, then a 30k rocket engine will burn 500 lbs of fuel in 5 seconds before landing. To match this a 15k air breathing engine will have a 20 second burn using 150 to 300 lbs of fuel at Isp=2,000 and Isp=1,000 respectively. Those two points on the curve suggest that the engines could mass between 350 and 200 lbs to match the pure rocket performance. This requires a T/W of 43 and 75 for the 2 cases. Replacement of some rocket engine mass on launch can only be a contribution to the available air breathing engine mass. If the orriginal rocket is 200k at sea level, then the thrust replaced can only be about half of the 15k as the air breathing engine loses thrust during the climb and craps out entirely at 30,000 to 50,000 feet. The gravity losses after that cut the gains some. Say 7.5k of rocket thrust traded at T/W 125 for a 60 lb savings applied to the air breathing engine. Fuel savings on launch are argueable. Say 60 seconds at 10k average thrust. This is 300 to 600 lbs of fuel vs 2,000 lbs for the rocket. This minor savings on fuel is during the low slow part of the trajectory. The best I can guess is it translates to 100-150 lbs in orbit. This suggests that there would be 550 lbs available to the 2,000 second air breather and 360 lbs available for the 1,000 second air breather. T/W ratios required would be 28 and 42 respectively for these systems to match pure rocket performance on a VTVL. Leaving aside whether it is worthwhile to use air breathing at all, are these numbers accurate enough to start an honest decision curve? |
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Air Breathing for VTVL
johnhare wrote:
This suggests that there would be 550 lbs available to the 2,000 second air breather and 360 lbs available for the 1,000 second air breather. T/W ratios required would be 28 and 42 respectively for these systems to match pure rocket performance on a VTVL. What kind of airbreathing engines do you have in mind here? If turbomachinery based your Isps are too low and your T/Ws are too high. More importantly, I think if you want your airbreathers to do double duty during ascent and recovery you're going to have to address the issue of increased installation weight. An installation optimized for deceleration/hover may not work well while accelerating and vice versa. Jim Davis |
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Air Breathing for VTVL
==
I would like to quantify the performance requirement for the curve in which the addition of a subsonic air breathing engine reaches break even vs an all rocket VTVL. There are 3 places I am aware of to get the mass for the air breathing engine I am interested in. Replacement of some rocket engine mass from launch for a minute or so. Minor fuel savings during launch. Elimination of some landing fuel required by all rocket landing. For figuring purposes, a 160,000 lb GLOW vehicle with 10,000 lb dry mass including payload and landing fuel. This is a dense fuel SSTO as I am not sold on hydrogen for various reasons. Agreed! Coincidentaly I just finished up a article for the http://www.rocketmanblog.com blog about a notianal HTHL SSTO using jets up to about 100,000 - 150,000 feet using a jet engine modification Darpa is pushing for their TSTO Rascel program. (You might find the spreadsheet usefull? E-mail me if you want a copy.) The Rascal paperâÿóÿý™s a http://cism.jpl.nasa.gov/events/work...ton_Carter.pdf http://www.darpa.mil/TTO/rascal/RASCAL_PS_Final.pdf http://hypersonic2002.aaaf.asso.fr/papers/17_5148.PDF Darpa has been doing tests on the engines the craft would need, and is designing the system around off the shelf parts. Their 0 to maybe up to Mach 6 jet engines are modified versions of the engines that have been flying in the F-15's for 30 years. No need for new rocket based combined cycle engines (though they might be nice). They just spray water into the engine intake to cool the incoming air to save the turbojets from overheating, and spray some liquid oxygen ahead of the burners to make up for the thin air. Its like hot roding up a normal car engine for occasional drag racing. Its been increasing the thrust, without hurting the engines, for short bursts up to Mach 6. It will get the Rascel mother ship up to 200,000 feet or so (in a glide after flame out) where you drop of the upper stages out off a bomb bay like chamber. Obviously after reentering and restarting the engines, the winnged Rascel simply flies back to base. T/W on the best fighter turbojets on the market are about 10-1, but the papers list "significant" increases in the thrust with the mass and oxegen augmenntation, but I couldn't find more details. T/W on airliner like turbofans is much higher, and I've head of designs using them with ramjets in the ducts to get higher T/W and speed, though possibly more speed limitations then the Rascel design. Given vertical launch SSTOs seem to burn about half their fuel/LOx to get to Mach 6 and out of the air, this seems very usable. I found with a LOX Kerosene fueled DC-X style SSTO. the fuel/LOx weighs 13 times as much as the rest of the ship combined. If you use jets like Darpa is working on to Mach 6, then boost to orbit with rockets. Instead of needing 13 times the unfueled weight of the vehicle to get into orbit, you only need about 6 times the dry weight. For a Winged craft like I was connsidering you'ld need to mid air "refuel" with LOx like the Pioneer / Blackhorse concept, or fill the LOx tanks from the air in midflight like http://www.andrews-space.com/en/corporate/Alchemist(200311).html. Liquid hydrogen fueld concept is proposeing. So adding it up and skipping over the spreadsheet and equations. Assuming a craft with cargo and everything it needs in space, weighs about 50 ton's. My concept would: - Takes off with 90 tons of Kerosene. (A smaller fraction then the 1950's SR-71'a could. If you want you can take off with less kerosene and add more in flight.) - Its carrying 10-16 tones of modified F-15 jet engines (Depending on the acceleration rate you want) which is about the weigh fraction of the engines in a F-15. - Under 2 tons of empty LOx tanks and less then a ton of kerosene tank. Note the Kerosene "tank" is likely mostly the wing, so it will likely weigh less. - Under 5 tons of rocket engines like P&W's RD-180s. (Depending on your assent trajectory. Shuttles engine thrust is less then its weight after SSRB sep, but gets to orbit either way.) In flight you add 190 tons of LOx (liquefied oxygen). Maybe with mid air refueling. Maybe with mid-air oxygen mining and liquefaction. The plane can fly with this much weight as long as its going several hundred miles per hour. Though its probably handling like a pig. You bring all your jet engines to full power and boost out for speed and altitude. When the engines finally flame out your at Mach 6 and leaving all but wisps of the atmosphere. You start the RD-180 ish rocket engines. They consume the rest of that huge fuel reserve, bringing the 50 tons of craft and cargo into orbit. Assuming heaviest assumptions for jet and rocket engine weights, everything else now weighs 27 tons. You drop your cargo (likely 5 tons or less) or dock with a station. When you want to come down you use a small burn to decelerate you, and you renter. One of the oldest serious proposals to build a HTHL SSTO (though on a far larger scales) was Star-Raker by Rockwell in the '70's. http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld047.htm It took off and landed from a runway with its fuel and oxygen load. Used rocket engines and ramjets for flight and boost to orbit. Expected little issue with reentry heating due to its large size and proportionally low reentry weight. Hope this helps. Kelly Kelly Starks "Humans are a race of compassionate predators." |
#4
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Air Breathing for VTVL
"Jim Davis" wrote in message . 1.4... johnhare wrote: This suggests that there would be 550 lbs available to the 2,000 second air breather and 360 lbs available for the 1,000 second air breather. T/W ratios required would be 28 and 42 respectively for these systems to match pure rocket performance on a VTVL. What kind of airbreathing engines do you have in mind here? If turbomachinery based your Isps are too low and your T/Ws are too high. I believe I have invented a new arraingement for the turbomachinery that is much lighter than that in current equipment. The aero/thermodynamic cycle is a hybrid turbojet and air-turborocket. I finally figured a way to build test models on my budget. The key point was finding a way to build a blade row that operates efficiently as compressor over part of its cycle, and partial admission turbine over the other part. The blade row is regeneratively cooled during the compressor portion. This allows much higher turbine admission temperatures than normal and eliminates the afterburner. I am looking for the target performance curve where the airbreather does not have a performance penalty vs a pure rocket system. More importantly, I think if you want your airbreathers to do double duty during ascent and recovery you're going to have to address the issue of increased installation weight. An installation optimized for deceleration/hover may not work well while accelerating and vice versa. Any increased installation weight will have to be charged off to the airbreather, which makes the true curve possibly more difficult to reach. I believe I have found an installation scheme for this particular engine type that will operate somewhat effectively in both orientations. There are three performance curves on the graph. The low one is what many people shoot for. Airbreathing is so desirable that substantial performance penalties are going to be overlooked in order to incorporate them. The middle curve is where performances just match. The most difficult one is where not including airbreathers must be justified in terms of simplicity or cost. I am interested in the middle curve. Jim Davis John Hare |
#5
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Air Breathing for VTVL
"Kelly St" wrote in message ... == I would like to quantify the performance requirement for the curve in which the addition of a subsonic air breathing engine reaches break even vs an all rocket VTVL. There are 3 places I am aware of to get the mass for the air breathing engine I am interested in. Replacement of some rocket engine mass from launch for a minute or so. Minor fuel savings during launch. Elimination of some landing fuel required by all rocket landing. For figuring purposes, a 160,000 lb GLOW vehicle with 10,000 lb dry mass including payload and landing fuel. This is a dense fuel SSTO as I am not sold on hydrogen for various reasons. Agreed! Coincidentaly I just finished up a article for the http://www.rocketmanblog.com blog about a notianal HTHL SSTO using jets up to about 100,000 - 150,000 feet using a jet engine modification Darpa is pushing for their TSTO Rascel program. (You might find the spreadsheet usefull? E-mail me if you want a copy.) I couldn't find your article at that site. I have a bit of a problem buying HTHL SSTO as being able to close the technical case. The landing gear and aero surfaces are unavoidable mass that has to be hauled in both directions. For suborbital or first stage work HTHL is more competative. The Rascal paper s a http://cism.jpl.nasa.gov/events/work...ton_Carter.pdf http://www.darpa.mil/TTO/rascal/RASCAL_PS_Final.pdf http://hypersonic2002.aaaf.asso.fr/papers/17_5148.PDF I have some familiarity with this. Darpa has been doing tests on the engines the craft would need, and is designing the system around off the shelf parts. Their 0 to maybe up to Mach 6 jet engines are modified versions of the engines that have been flying in the F-15's for 30 years. No need for new rocket based combined cycle engines (though they might be nice). They just spray water into the engine intake to cool the incoming air to save the turbojets from overheating, and spray some liquid oxygen ahead of the burners to make up for the thin air. Its like hot roding up a normal car engine for occasional drag racing. Its been increasing the thrust, without hurting the engines, for short bursts up to Mach 6. It will get the Rascel mother ship up to 200,000 feet or so (in a glide after flame out) where you drop of the upper stages out off a bomb bay like chamber. Obviously after reentering and restarting the engines, the winnged Rascel simply flies back to base. For my purposes, modifications of existing hardware are somewhat interesting. I have been unable to locate a single paper, book, or proposal that makes a convincing case for high mach modifications of existing equipment. Everything I have seen so far gets too complex too fast for the purported advantage over mach 3 or so. T/W on the best fighter turbojets on the market are about 10-1, but the papers list "significant" increases in the thrust with the mass and oxegen augmenntation, but I couldn't find more details. My interest is in finding the T/WxIsp curve that makes a compelling arguement for a new type engine. IMO T/W of 25+ is possible with 4 digit Isp. T/W on airliner like turbofans is much higher, and I've head of designs using them with ramjets in the ducts to get higher T/W and speed, though possibly more speed limitations then the Rascel design. T/W on turbofans is typically lower in exchange for much better fuel economy. The curve is very different for cruise. I have not seen a convincing paper for using ramjets. By the time ramjets start working well, it's past time to get out of the atmosphere. Given vertical launch SSTOs seem to burn about half their fuel/LOx to get to Mach 6 and out of the air, this seems very usable. Verticle launch runs out of useful propulsion air long before mach 6. LOX is cheap, I am comparing against the dry mass. I found with a LOX Kerosene fueled DC-X style SSTO. the fuel/LOx weighs 13 times as much as the rest of the ship combined. If you use jets like Darpa is working on to Mach 6, then boost to orbit with rockets. Instead of needing 13 times the unfueled weight of the vehicle to get into orbit, you only need about 6 times the dry weight. The ratio is more like 16. The problem is that those engines to mach 6 have to be carried to orbit and back in addition to the rockets required anyway. The mass breakdown has not been shown to work that I am aware of. You might get GLOW down some, usually at the expense of more hardware, complexity, and flight path difficulties. For a Winged craft like I was connsidering you'ld need to mid air "refuel" with LOx like the Pioneer / Blackhorse concept, or fill the LOx tanks from the air in midflight like http://www.andrews-space.com/en/corporate/Alchemist(200311).html. Liquid hydrogen fueld concept is proposeing. Familiar with them. Not the question I'm working on. So adding it up and skipping over the spreadsheet and equations. Assuming a craft with cargo and everything it needs in space, weighs about 50 ton's. My concept would: - Takes off with 90 tons of Kerosene. (A smaller fraction then the 1950's SR-71'a could. If you want you can take off with less kerosene and add more in flight.) - Its carrying 10-16 tones of modified F-15 jet engines (Depending on the acceleration rate you want) which is about the weigh fraction of the engines in a F-15. - Under 2 tons of empty LOx tanks and less then a ton of kerosene tank. Note the Kerosene "tank" is likely mostly the wing, so it will likely weigh less. - Under 5 tons of rocket engines like P&W's RD-180s. (Depending on your assent trajectory. Shuttles engine thrust is less then its weight after SSRB sep, but gets to orbit either way.) In flight you add 190 tons of LOx (liquefied oxygen). Maybe with mid air refueling. Maybe with mid-air oxygen mining and liquefaction. The plane can fly with this much weight as long as its going several hundred miles per hour. Though its probably handling like a pig. You bring all your jet engines to full power and boost out for speed and altitude. When the engines finally flame out your at Mach 6 and leaving all but wisps of the atmosphere. You start the RD-180 ish rocket engines. They consume the rest of that huge fuel reserve, bringing the 50 tons of craft and cargo into orbit. Assuming heaviest assumptions for jet and rocket engine weights, everything else now weighs 27 tons. You drop your cargo (likely 5 tons or less) or dock with a station. When you want to come down you use a small burn to decelerate you, and you renter. One of the oldest serious proposals to build a HTHL SSTO (though on a far larger scales) was Star-Raker by Rockwell in the '70's. http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld047.htm It took off and landed from a runway with its fuel and oxygen load. Used rocket engines and ramjets for flight and boost to orbit. Expected little issue with reentry heating due to its large size and proportionally low reentry weight. Hope this helps. My orriginal question is simply stated as, "what performance is required of a new style air breathing engine that would win an ICH tee shirt?" Kelly Kelly Starks "Humans are a race of compassionate predators." |
#6
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Air Breathing for VTVL
"johnhare" writes:
Airbreathing is so desirable that substantial performance penalties are going to be overlooked in order to incorporate them. I very much question this claim --- especially for VTVL. Air-breathing T/W ratios are so wimpy that they almost always force wings and horizontal lift-off in the final analysis, since the engine cannot lift the weight of the fully loaded vehicle. You can dream all you want about air-breathing engines with a T/W ratio of "43 to 75," but I very much doubt that you or _anyone_ will be shipping one any time soon !!! Furthermore, you appear to have made the common false assumption that air-breathing performance is independent of airspeed. In point of fact, the effective I_sp of an air-breathing engine is roughly inversely proportional to airspeed above roughly Mach 1, so that at Mach 6, the effective I_sp of an air-breather is only a few times better than a rocket burning the same fuel. And since you are also assuming water _AND_ LOX injection, you must include these in your propellant input, so that your effective I_sp is even further degraded. At this point, your engine is starting to look more like a bad rocket than a air-breather. And as Henry Spencer has pointed out many times in this newsgroup, when a careful performance analysis is done, one usually finds in the end that it is better to build a good rocket that can double as a bad air-breather than an air-breather that can double as a bad rocket. Finally, since most of the propellant will still be consumed after air-breathing has become useless, unless you go to two stages or otherwise drop off your fancy air-breathing engines when you reach Mach 6 or so, all that heavy turbomachinery becomes so much useless dead mass for most of the trajectory to orbit. In summary, I continue to remain unconvinced that air-breathing is even the _least_ bit desirable for anything except possibly the first stage of TSTO. Furthermore, the claim that air-breathers can achieve a T/W exceeding 40, and will be useful for VTVL makes me fall down and roll on the floor, laughing my head off... -- Gordon D. Pusch perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;' |
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Air Breathing for VTVL
"Gordon D. Pusch" wrote in message ... "johnhare" writes: Airbreathing is so desirable that substantial performance penalties are going to be overlooked in order to incorporate them. I very much question this claim --- especially for VTVL. It is somewhat irritating that you snipped a single sentence from the paragraph to make it seem that my position is far from where it really is. I pasted the paragraph in below to show the difference. There are three performance curves on the graph. The low one is what many people shoot for. Airbreathing is so desirable that substantial performance penalties are going to be overlooked in order to incorporate them. The middle curve is where performances just match. The most difficult one is where not including airbreathers must be justified in terms of simplicity or cost. I am interested in the middle curve. This is my position. I am looking for useful answers to what airbreathers have to achieve to match rockets. The many people that write papers and advocate airbreathing at any cost should be keelhauled. Air-breathing T/W ratios are so wimpy that they almost always force wings and horizontal lift-off in the final analysis, since the engine cannot lift the weight of the fully loaded vehicle. You can dream all you want about air-breathing engines with a T/W ratio of "43 to 75," but I very much doubt that you or _anyone_ will be shipping one any time soon !!! You should have noted that my post suggested that this would be the required performance to match rockets, not that the 43-75 was feasable in the forseeable planning horizon. Furthermore, you appear to have made the common false assumption that air-breathing performance is independent of airspeed. In point of fact, the effective I_sp of an air-breathing engine is roughly inversely proportional to airspeed above roughly Mach 1, so that at Mach 6, the effective I_sp of an air-breather is only a few times better than a rocket burning the same fuel. And since you are also assuming water _AND_ LOX injection, you must include these in your propellant input, so that your effective I_sp is even further degraded. At this point, your engine is starting to look more like a bad rocket than a air-breather. And as Henry Spencer has pointed out many times in this newsgroup, when a careful performance analysis is done, one usually finds in the end that it is better to build a good rocket that can double as a bad air-breather than an air-breather that can double as a bad rocket. Are you responding to my post or someone elses? I am comparing performance usefullness of a subsonic airbreathing system. Supersonic requires fairly heavy (by comparison) intakes. Finally, since most of the propellant will still be consumed after air-breathing has become useless, unless you go to two stages or otherwise drop off your fancy air-breathing engines when you reach Mach 6 or so, all that heavy turbomachinery becomes so much useless dead mass for most of the trajectory to orbit. This is definately in response to someone else. In summary, I continue to remain unconvinced that air-breathing is even the _least_ bit desirable for anything except possibly the first stage of TSTO. Furthermore, the claim that air-breathers can achieve a T/W exceeding 40, and will be useful for VTVL makes me fall down and roll on the floor, laughing my head off... My orriginal question stands unaddressed, What performance is required of an air breathing engine in order to match an all rocket LV performance. I questioned whether the numbers I have derived for a requirement are accurate enough for reasonable decisions to be made. You should note that I also said that T/W of 25 was in reach maybe. While you are rolling on the floor laughing, care to make a small wager on the capabilities of a concept demonstrater? Since you are so certain, you should be willing to offer really good odds. -- Gordon D. Pusch perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;' |
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Air Breathing for VTVL
Opps,
Sorry I didn't respond to your post earlier. I've been busy and hadn't checked in. My article isn't on the site yet. The site editor is busy and getting behind. You said, "I have a bit of a problem buying HTHL SSTO as being able to close the technical case.". I'm not clear what your questions were. For a Mach 3 to 6 airbreathing bird I was getting take off weights ( LOx Added later) of 170-200 tons. And crafte weight on orbit of 50. Weight on orbit minus all tanks and engines of 23-29 tons. Even cuting out 5 tons for reentry systems, and 4-5 tons for landing gear, that still leaves 13-20 tons for frame crew, crago, and such. Which seemed doable from what I could see. I think the main difference between us is goals: My interest is in finding the T/WxIsp curve that makes a compelling arguement for a new type engine. IMO T/W of 25+ is possible with 4 digit Isp. === My orriginal question is simply stated as, "what performance is required of a new style air breathing engine that would win an ICH tee shirt?" I was interested in seeing if you could do it with current equipment, not searching for a justification for a new one. I'm not saying a new engine with better perfomance wouldn't make things, but I'm not at all clear Its needed to get the job done. A first generation craft that could drop cost to orbit by a order of magnitude or two, and greatly increase flexibility, would get the ball runing and later bankroll the dev of better engines. Kelly Starks "Humans are a race of compassionate predators." |
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Air Breathing for VTVL
"Kelly St" wrote in message ... Opps, Sorry I didn't respond to your post earlier. I've been busy and hadn't checked in. My article isn't on the site yet. The site editor is busy and getting behind. You said, "I have a bit of a problem buying HTHL SSTO as being able to close the technical case.". I'm not clear what your questions were. For a Mach 3 to 6 airbreathing bird I was getting take off weights ( LOx Added later) of 170-200 tons. And crafte weight on orbit of 50. Weight on orbit minus all tanks and engines of 23-29 tons. Even cuting out 5 tons for reentry systems, and 4-5 tons for landing gear, that still leaves 13-20 tons for frame crew, crago, and such. Which seemed doable from what I could see. I have read a lot of information on using existing airbreathing engines, on paper, disk, and web. None of them have been able to make a good case. Most of the ones I have looked at use GLOW as the standard to judge by. I'm looking for dry mass as the standard. A turbojet costs orders of magnitude more per launch pound than LOX, or even RP. So far, I have not seen one source that has dropped dry mass with the use of any current airbreathing engines. I think the main difference between us is goals: My interest is in finding the T/WxIsp curve that makes a compelling arguement for a new type engine. IMO T/W of 25+ is possible with 4 digit Isp. === My orriginal question is simply stated as, "what performance is required of a new style air breathing engine that would win an ICH tee shirt?" I was interested in seeing if you could do it with current equipment, not searching for a justification for a new one. I have concluded no on this question. I am trying to find the break point where that answer becomes yes. Airbreathing engines offer certain operational advantages. The target is to find the point where these operational advantages can be had without a performance penalty compared to pure rocket operation. I'm not saying a new engine with better perfomance wouldn't make things, but I'm not at all clear Its needed to get the job done. A first generation craft that could drop cost to orbit by a order of magnitude or two, and greatly increase flexibility, would get the ball runing and later bankroll the dev of better engines. Kelly Starks "Humans are a race of compassionate predators." |
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Air Breathing for VTVL
"Kelly St" wrote in message ... I was interested in seeing if you could do it with current equipment, not searching for a justification for a new one. I'm not saying a new engine with better perfomance wouldn't make things, but I'm not at all clear Its needed to get the job done. A first generation craft that could drop cost to orbit by a order of magnitude or two, and greatly increase flexibility, would get the ball runing and later bankroll the dev of better engines. I should have put some numbers on my other reply to clarify my points a bit better. An all rocket VTVL SSTO is probably feasible at this time. The margins will be tight with no slack for extra systems that don't pull their own weight. Adding any current airbreathing engine to the projected VTVL SSTO will cut into the payload for a given dry mass. Take a theoretical VTVL SSTO at 100,000 lbs GLOW. At mass ratio of 16, there will be 6,250 lbs mass in orbit. If you add some hypothetical jet which cuts the GLOW to 80,000 lbs and increases the mass in orbit to 7,250 lbs for the same payload, you have lost. The 1,000 lb jet you added could cost something on the order of $1M. That $1M will buy 20,000,000 lbs of the LOX you replaced. That is 1,000 launches to reach break even all else being equal. All else is seldom equal. That 1,000 lb jet must be accelerated all the way to orbit, reentered, and landed. Most studies would seem to indicate that adding the jet will prevent SSTO operation without seriously advanced tech across the board. Increasing the reentry mass and landing mass also increases the cost of the vehicle. 1 lb of hardware at $1,000 lb will buy 20,000 lbs of LOX. For HTHL, you are lifting wings and landing gear sized for GLOW all the way to orbit and back. Add all the weights of HTHL and you don't reach SSTO mass ratio requirements. Add a second type all up propulsion system to the required rockets and it becomes more difficult yet. You are forced into some form of air tow or refueling to even attempt making it to orbit. Most of the papers I have read assume one or more advanced technologies to make this happen. If you go TSTO, then many possibilities open up. Air breathing becomes more feasible in several of them. I am basing on SSTO because it is a useful math model. If pure rocket SSTO is feasible, and adding jets requires an additional stage, then the jets hamper performance. You shouldn't add systems that hamper performance without a very good reason. I would like to find the point where the operational flexibilities possible with an airbreathing engine do not cost performance in real terms. Kelly Starks "Humans are a race of compassionate predators." |
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