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#1
March 1st 04, 08:37 PM
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Bear Cub to tether
I've been playing with Len Cormier's Bear Cub. As a tether
enthusiast, I'm interested in the impact of using a tether with the Bear Cub. A hanging tether can give as much as 800 m/s sub orbital grapple or trapeze target. The payload of the Bear Cub grows from 455 kg to about 1200 kg for such a tether. I use the rocket equation to get a table of the net velocity available to this configuration with increased payloads. This gave me the following table. Payload Ideal DV $/kg 455 9.472 549 620 9.233 403 900 8.865 278 1000 8.744 250 1200 8.516 208 Len estimated a per flight cost of about $250,000. I couldn't resist adding the cost per kilogram to this table. I'm a bit tentative about the conversion from "Ideal DV" to orbital altitude. If I use the Buzz Aldrin 15% loss to gravity, air resistance, etc. Then 9.474 km/s should produce an effective 8.23 km/s. If I spend 8.07 to get into a transfer orbit, and 0.16 to circularize the orbit, that looks like 550 km altitude. Does that work? Henry For sanity; these are the figures I used in my spreadsheet. Len gave an expected ideal DV for each of the pieces; the Tupelov carrier, the booster stage, and the orbiter. He quoted the ISP for the RL-10. The booster stage uses Kerosene, Len does not provide an ISP for the booster stage. I infer an exhaust of about 3200 m/s from the mass and DV. Payload Ideal DV ISP m/s Empty Fuel GLOW 455 5.553 4422 Orbiter 1945 6025 8425 3.249 3200 Booster 2340 19000 29765 0.670 Tupelov95 94400 Total 9.472 
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#3
March 3rd 04, 03:10 AM
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Bear Cub to tether
(Len) wrote in message . com...
(Henry Cate, Jr) wrote in message I use the rocket equation to get a table of the net velocity available to this configuration with increased payloads. This gave me the following table. Payload Ideal DV 455 9.472 620 9.233 900 8.865 1000 8.744 1200 8.516 Henry Len gave an expected ideal DV for each of the pieces; the Tupelov carrier, the booster stage, and the orbiter. He quoted the ISP for the RL-10. The booster stage uses Kerosene, Len does not provide an ISP for the booster stage. I infer an exhaust of about 3200 m/s from the mass and DV. Payload Ideal DV ISP m/s Empty Fuel GLOW 455 5.553 4422 Orbiter 1945 6025 8425 3.249 3200 Booster 2340 19000 29765 0.670 Tupelov95 94400 Total 9.472 This last table is essentially correct. The Isp assumed for the booster is 3195 m/s. However, I calculate that reduction of the orbiter delta v requirement by 800 m/s to 4753 m/s would reduce the required mass ratio to 2.93, which would give a payload of about 930 kg, rather than 1200 kg-- which is still a very nice improvement. I assumed the Tupelov could carry and extra mass and not notice the difference. I assumed the fuel load stayed constant, and only the final payload changed, though that changes the effective mass ratio of the booster stage. That's how the 1200 worked out. Is the Bear Cub payload mass limited, or volume limited? How hard would it be to stuff an extra 1000 kg into the orbiter? Of course, the tether isn't free, so this would also add to total costs. The $250,000 per flight recurring cost is a goal, not an estimate--although I think it is a reasonable goal. I agree that this is a reasonable goal. It seems high if we could get a high flight rate (I.e. if we fly often, the cost per flight should be lower.) The tether isn't free, but the operating costs are low, so the major cost would be paying for the R&D. (I'm interested in tourists, where there is round trip traffic, so there is little or no net energy cost to the tether.) I believe the investment dominates the Bear Cub flight costs also. The gross orbiter mass should remain the same, in order to maintain initial thrust-to-weight ratio. Higher payload means higher burnout and reentry mass; so this would have an impact on TPS, reducing the payload somewhat. I don't follow this. The initial mass has changed by 465 (or 750) kg, out of 29.7 tons. That doesn't seem to impact the thrust to weight very much? Higher payload means lower burnout velocity for the booster. I don't see what impact on reentry mass. Why would the reentry mass change by adding 465 kg to the payload (the booster delivers 8880 instead of 8425)? My spread sheet assumed both the booster and the orbiter deliver less DV due to the increase in payload. I assumed I could carry the same fuel load, since more payload doesn't impact the fuel tank? The Tu-95 would launch the same mass. The booster would also remain about the same (although I might opt for a different recovery concept). I'd assumed we could add 465 or 750 kg to payload in the Bear Cub orbiter without changing either the Tu-95 or the booster. As I said above, the Tu-95 wouldn't notice the difference between 29.7 tons and 30.1, would it? I'm a bit tentative about the conversion from "Ideal DV" to orbital altitude. If I use the Buzz Aldrin 15% loss to gravity, air resistance, etc. Then 9.474 km/s should produce an effective 8.23 km/s. If I spend 8.07 to get into a transfer orbit, and 0.16 to circularize the orbit, that looks like 550 km altitude. Does that work? And the total delta v requirement would be the same for the same basic ISS orbit. So the Bear Cub is aimed at the ISS orbit, of 400 km altitude at 57 degree inclination? BTW most of the delta v losses are early in the game; this--along with the near-elimination of the altitude compensation requirement--are the main benfits of subsonic launch at altitude. I thought the Aldrin 15% would be generous. It appeared to my logic that just getting to 15 km altitude could easily cost 350-400 m/s in gravity loss, for example (with vertical launch rockets). I don't know how to calculate gravity and other Delta V losses with confidence.
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#4
March 4th 04, 04:16 AM
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Bear Cub to tether
(Henry Cate, Jr) wrote in message . com...
(Len) wrote in message . com... .....snip... I assumed the Tupelov could carry and extra mass and not notice the difference. I assumed the fuel load stayed constant, and only the final payload changed, though that changes the effective mass ratio of the booster stage. That's how the 1200 worked out. Is the Bear Cub payload mass limited, or volume limited? How hard would it be to stuff an extra 1000 kg into the orbiter? A ton or so more on the Tupolev is probably noise level. However, 30 tonnes is lot of concentrated load for any existing airplane--I'm not sure that one could push that amount much higher without asking for trouble. Of course, the tether isn't free, so this would also add to total costs. The $250,000 per flight recurring cost is a goal, not an estimate--although I think it is a reasonable goal. I agree that this is a reasonable goal. It seems high if we could get a high flight rate (I.e. if we fly often, the cost per flight should be lower.) Our stated goal for development costs for the Bear Cub is $100,000,000. IMO, this adds about $30,000,000 per year to total costs. At 100 flights per year, this amounts to $300,000 per flight for return on development investment. At 200 flights per year, this reduces to $150,000 per flight. If your tether costs only $10,000,000, then it makes economic sense. If it costs $100,000,000, then it would probably make more sense to fly more unassisted Bear Cub flights. Of course, there could be a decent argument for a larger payload per flight--but even than argument can go the other direction. The tether isn't free, but the operating costs are low, so the major cost would be paying for the R&D. (I'm interested in tourists, where there is round trip traffic, so there is little or no net energy cost to the tether.) I believe the investment dominates the Bear Cub flight costs also. The gross orbiter mass should remain the same, in order to maintain initial thrust-to-weight ratio. Higher payload means higher burnout and reentry mass; so this would have an impact on TPS, reducing the payload somewhat. I don't follow this. The initial mass has changed by 465 (or 750) kg, out of 29.7 tons. That doesn't seem to impact the thrust to weight very much? The orbiter gross mass is currently 8425 kg. Adding 465 kg or 750 kg could start adding to losses, since the thrust of the RL10 would not change. Adding 465 kg or 750 kg to the booster would not improve orbital payload much. Higher payload means lower burnout velocity for the booster. I don't see what impact on reentry mass. Why would the reentry mass change by adding 465 kg to the payload (the booster delivers 8880 instead of 8425)? We would be adding 465 kg to the reentry mass. These details have a way of adding up to a serious problem. Higher planform loading means higher temperature, which adds to TPS mass, which adds to planform loading, etc. My spread sheet assumed both the booster and the orbiter deliver less DV due to the increase in payload. I assumed I could carry the same fuel load, since more payload doesn't impact the fuel tank? The Tu-95 would launch the same mass. The booster would also remain about the same (although I might opt for a different recovery concept). I'd assumed we could add 465 or 750 kg to payload in the Bear Cub orbiter without changing either the Tu-95 or the booster. As I said above, the Tu-95 wouldn't notice the difference between 29.7 tons and 30.1, would it? No, but the orbiter would notice the burnout mass difference. The orbiter would also notice a correspondingly larger difference in gross mass, if you try to apply the tether benefit to other than the orbiter. BTW, I think you will find that applying the delta vee benefit to the orbiter would maximize the potential benefit of the tether. ....snip... And the total delta v requirement would be the same for the same basic ISS orbit. So the Bear Cub is aimed at the ISS orbit, of 400 km altitude at 57 degree inclination? Well, that's one obvious potential mission. BTW most of the delta v losses are early in the game; this--along with the near-elimination of the altitude compensation requirement--are the main benfits of subsonic launch at altitude. I thought the Aldrin 15% would be generous. It depends upon the vehicle concept, but 15% is not generous--especially if one includes back-pressure nozzle losses as part of total losses. A high acceleration rocket concept can have somewhat lower losses--perhaps in the 15% range. Airbreathing concepts are likely to have 40% or 50% losses. It appeared to my logic that just getting to 15 km altitude could easily cost 350-400 m/s in gravity loss, for example (with vertical launch rockets). I don't know how to calculate gravity and other Delta V losses with confidence. 15 km is rather difficult to achieve. I usually count subsonic (mach 0.8) at 12 km as being worth about 670 m/s delta vee ideal, including equivalent losses. At some point, of course, one should run a actual trajectory for a specific concept. But I generally assume about 9510 m/s for a typical, well-designed rocket vehicle to something like the ISS orbit. Best regards, Len (Cormier) PanAero, Inc. (change x to len) http://www.tour2space.com
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#5
March 4th 04, 08:04 PM
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Bear Cub to tether
....snip... frequent trims ;-)
A ton or so more on the Tupolev is probably noise level. However, 30 tonnes is lot of concentrated load for any existing airplane--I'm not sure that one could push that amount much higher without asking for trouble. With a gross mass of 185 tonnes, empty mass of 94.4 tonnes, I'd assumed the TU-95 could carry close to 90 tonnes. It the problem putting mass outside? I assume some mass is fuel load? Of course, there could be a decent argument for a larger payload per flight--but even than argument can go the other direction. I buy the argument that small payload and fly often beats large payload and fly rarely. The gross orbiter mass should remain the same, in order to maintain initial thrust-to-weight ratio. Higher payload means higher burnout and reentry mass; so this would have an impact on TPS, reducing the payload somewhat. I don't follow this. The initial mass has changed by 465 (or 750) kg, out of 29.7 tons. That doesn't seem to impact the thrust to weight very much? The orbiter gross mass is currently 8425 kg. Adding 465 kg or 750 kg could start adding to losses, since the thrust of the RL10 would not change. The hope was that the tether could provide a slower target for the orbiter, and allow the larger payload. Adding 465 kg or 750 kg to the booster would not improve orbital payload much. I don't know any way for a tether to interact with the booster. Higher payload means lower burnout velocity for the booster. I don't see what impact on reentry mass. Why would the reentry mass change by adding 465 kg to the payload (the booster delivers 8880 instead of 8425)? We would be adding 465 kg to the reentry mass. These details have a way of adding up to a serious problem. Higher planform loading means higher temperature, which adds to TPS mass, which adds to planform loading, etc. If I add 465 kg to the payload mass, I was planning to leave it in orbit, not bring it back. I'm thinking of building a station (and a larger tether) in LEO, so one way traffic has been my main focus. The orbiter would also notice a correspondingly larger difference in gross mass, if you try to apply the tether benefit to other than the orbiter. BTW, I think you will find that applying the delta vee benefit to the orbiter would maximize the potential benefit of the tether. My model is that the orbiter grabs the tether, saving some delta V. This borrows some momentum from the tether and ballast. An elecrodynamic reboost pumps the momentum back up. The payload would be unloaded to the tether, then the orbiter returns for another trip. I thought the Aldrin 15% would be generous. It depends upon the vehicle concept, but 15% is not generous--especially if one includes back-pressure nozzle losses as part of total losses. A high acceleration rocket concept can have somewhat lower losses--perhaps in the 15% range. Airbreathing concepts are likely to have 40% or 50% losses. I was too terse. I thought the Aldrin 15% was generous for an air launch such as the Bear Cub. 15 km is rather difficult to achieve. I usually count subsonic (mach 0.8) at 12 km as being worth about 670 m/s delta vee ideal, including equivalent losses. At some point, of course, one should run a actual trajectory for a specific concept. But I generally assume about 9510 m/s for a typical, well-designed rocket vehicle to something like the ISS orbit. Thanks for the sanity check. I recognize that I am long on theory and short on practical application. Henry
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#6
March 5th 04, 04:25 AM
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Bear Cub to tether
(Henry Cate, Jr) wrote in message . com...
....snip... frequent trims ;-) A ton or so more on the Tupolev is probably noise level. However, 30 tonnes is lot of concentrated load for any existing airplane--I'm not sure that one could push that amount much higher without asking for trouble. With a gross mass of 185 tonnes, empty mass of 94.4 tonnes, I'd assumed the TU-95 could carry close to 90 tonnes. It the problem putting mass outside? I assume some mass is fuel load? Fuel load in the wings is relieving. A concentrated load on the centerline is usually harder to handle. Also length is a problem, since the nose gear folds aft. But, additional mass on the centerline might be possible; I don't have enough information at this point. And I don't have a Technical Assistance Agreement from our State Dept. to discuss the problem with Tupolev. Of course, there could be a decent argument for a larger payload per flight--but even than argument can go the other direction. I buy the argument that small payload and fly often beats large payload and fly rarely. ....snip... The hope was that the tether could provide a slower target for the orbiter, and allow the larger payload. Yes, and since the orbiter would have less work to do, some of the mass of the propellant (and tankage) could be allotted to payload. Adding 465 kg or 750 kg to the booster would not improve orbital payload much. I don't know any way for a tether to interact with the booster. The interaction would be indirect; the delta vee split could be different, but this would change initial orbiter thrust-to-weight, which might not be good. ....snip... If I add 465 kg to the payload mass, I was planning to leave it in orbit, not bring it back. I'm thinking of building a station (and a larger tether) in LEO, so one way traffic has been my main focus. If, for some reason, you can't get rid of the payload, then you have to allow for possible reentry at essentially burnout mass. Abort jettisoning would be an added complication. The orbiter would also notice a correspondingly larger difference in gross mass, if you try to apply the tether benefit to other than the orbiter. BTW, I think you will find that applying the delta vee benefit to the orbiter would maximize the potential benefit of the tether. My model is that the orbiter grabs the tether, saving some delta V. This borrows some momentum from the tether and ballast. An elecrodynamic reboost pumps the momentum back up. The payload would be unloaded to the tether, then the orbiter returns for another trip. I thought the Aldrin 15% would be generous. It depends upon the vehicle concept, but 15% is not generous--especially if one includes back-pressure nozzle losses as part of total losses. A high acceleration rocket concept can have somewhat lower losses--perhaps in the 15% range. Airbreathing concepts are likely to have 40% or 50% losses. I was too terse. I thought the Aldrin 15% was generous for an air launch such as the Bear Cub. I tend to look at the total delta vee requirement. The allowance for subsonic launch is basically equivalent to what would have happened with a rocket launch from the surface. 15 km is rather difficult to achieve. I usually count subsonic (mach 0.8) at 12 km as being worth about 670 m/s delta vee ideal, including equivalent losses. At some point, of course, one should run a actual trajectory for a specific concept. But I generally assume about 9510 m/s for a typical, well-designed rocket vehicle to something like the ISS orbit. Thanks for the sanity check. I recognize that I am long on theory and short on practical application. Henry You never know where good ideas may come from, and tethers do have significant potential, IMO. I find a lot of my own ideas that I initially get quite excited about get less and less attractive with analysis. Different concepts can emphasize different potential benefits. Howevever, the total system concept has to be consistent with respect to how all the different components fit together. Inconsistency with respect to components and analysis, is the biggest fault I find with all airbreathing acceleration concepts--the components do not all fit together in one consistent design and, most important, airbreathing acceleration fails when it comes to realistic, objective trajectory and cost analyses. Airbreathing cruise is another matter. Best regards, Len (Cormier) PanAero, Inc. (change x to len) http://www.tour2space.com
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