RGClark
Mathematician
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- Jan 27, 2010
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This will be like the Delta Glider except the engines will not need a scramjet cycle.
Looking at the numbers I'm convinced now you can make a single stage to orbit vehicle with a combined ramjet/rocket engine, and without having to use scramjets.
The idea is to combine the turbo-ramjet/rocket into a single engine. This is what Skylon wants to do with their Sabre engine. But the Sabre will use hypersonic airbreathing propulsion up to Mach 5.5 before the rockets take over. This will require complicated air-cooling methods using heat exchangers with flowing liquid hydrogen for the Skylon.
However, just being able to get to say the Mach 3.2 reached by the SR-71 would take a significant amount off the delta-V required for orbit. Of course if the ramjet could get to Mach 5 that would be even better but key this would be doable with the existing engines of the SR-71. Note too the engines of the XB-70 Valkyrie bomber could operate at Mach 3 and as far as I know they didn't have ramjet operation mode. So it might not even be necessary for the engines to have a ramjet mode, turbojet might be sufficient.
The problem with using jets for the early part of the flight of an SSTO has been they are so heavy for the thrust they produce, generally in the T/W range of around 5 to 10, while rocket engines might have a T/W ratio in the range of 50 to 100. But a key point is the jet engine will be operating during the aerodynamic lift portion of the flight where the L/D ratio of perhaps 7. The XB-70 for instance had a L/D of about 7 during cruise at Mach 3. So if we take the T/W of the jet engine to be say 7 and the L/D to be 7, then the thrust to lift-off weight ratio might be about 50 to 1 comparable to that of rockets.
BTW, it is surprising there has been so little research on this type of combination with the jet and rocket combined into one. You hear alot about turbine-based-combined-cycle (TBCC) where it combines turbo- and scram-jets and rocket-based-combined-cycle (RBCC) , where the exhaust from a rocket is used to provide the compression for a ramjet. But not this type of combined turbojet/rocket engine. It doesn't seem to have an accepted name for example. It would not seem to be too complicated. You just use the same combustion chamber for rocket as for the jet. Probably also you would want to close off the inlets when you switch to rocket mode.
For the calculation the delta-V and propellant load would be feasible, note that for a dense propellant SSTO might require as much as 300 m/s lower delta-V than a hydrogen fueled SSTO, in the range of about 8,900 m/s, so I'll use kerosene as the fuel. Hydrogen might have an advantage though in being light-weight if what you wanted was horizontal launch. Say you were able to get to Mach 3+ with the jets, 1,000 m/s. The delta-V to be supplied by the rocket-mode is then 7,900 m/s. But note also you can get to high altitude say to 25,000 m. This might subtract another 300 m/s from the required rocket-mode delta-V, so now to 7,600 m/s.
A bigger advantage than this of the altitude is the fact that you get the full vacuum Isp during rocket-mode, call it an exhaust velocity of 3,600 m/s for kerosene rockets. Note this results in a mass-ratio for the rocket mode portion of e^(7,600/3,600) = 8.3, less than half that usually cited for a kerosene-fueled all rocket SSTO. Note the fuel required for the jet-powered portion would only be a fraction of the dry mass rather than multiples of it based on the fact the 1,000 m/s jet-powered speed is only a fraction of the 10,000 m/s or so effective exhaust speed of jet engines.
Note this brings the kerosene fuel load to be about that of hydrogen fueled SSTO's, except you still have the high density of kerosene. With modern lightweight materials this should be well doable.
Bob Clark
Looking at the numbers I'm convinced now you can make a single stage to orbit vehicle with a combined ramjet/rocket engine, and without having to use scramjets.
The idea is to combine the turbo-ramjet/rocket into a single engine. This is what Skylon wants to do with their Sabre engine. But the Sabre will use hypersonic airbreathing propulsion up to Mach 5.5 before the rockets take over. This will require complicated air-cooling methods using heat exchangers with flowing liquid hydrogen for the Skylon.
However, just being able to get to say the Mach 3.2 reached by the SR-71 would take a significant amount off the delta-V required for orbit. Of course if the ramjet could get to Mach 5 that would be even better but key this would be doable with the existing engines of the SR-71. Note too the engines of the XB-70 Valkyrie bomber could operate at Mach 3 and as far as I know they didn't have ramjet operation mode. So it might not even be necessary for the engines to have a ramjet mode, turbojet might be sufficient.
The problem with using jets for the early part of the flight of an SSTO has been they are so heavy for the thrust they produce, generally in the T/W range of around 5 to 10, while rocket engines might have a T/W ratio in the range of 50 to 100. But a key point is the jet engine will be operating during the aerodynamic lift portion of the flight where the L/D ratio of perhaps 7. The XB-70 for instance had a L/D of about 7 during cruise at Mach 3. So if we take the T/W of the jet engine to be say 7 and the L/D to be 7, then the thrust to lift-off weight ratio might be about 50 to 1 comparable to that of rockets.
BTW, it is surprising there has been so little research on this type of combination with the jet and rocket combined into one. You hear alot about turbine-based-combined-cycle (TBCC) where it combines turbo- and scram-jets and rocket-based-combined-cycle (RBCC) , where the exhaust from a rocket is used to provide the compression for a ramjet. But not this type of combined turbojet/rocket engine. It doesn't seem to have an accepted name for example. It would not seem to be too complicated. You just use the same combustion chamber for rocket as for the jet. Probably also you would want to close off the inlets when you switch to rocket mode.
For the calculation the delta-V and propellant load would be feasible, note that for a dense propellant SSTO might require as much as 300 m/s lower delta-V than a hydrogen fueled SSTO, in the range of about 8,900 m/s, so I'll use kerosene as the fuel. Hydrogen might have an advantage though in being light-weight if what you wanted was horizontal launch. Say you were able to get to Mach 3+ with the jets, 1,000 m/s. The delta-V to be supplied by the rocket-mode is then 7,900 m/s. But note also you can get to high altitude say to 25,000 m. This might subtract another 300 m/s from the required rocket-mode delta-V, so now to 7,600 m/s.
A bigger advantage than this of the altitude is the fact that you get the full vacuum Isp during rocket-mode, call it an exhaust velocity of 3,600 m/s for kerosene rockets. Note this results in a mass-ratio for the rocket mode portion of e^(7,600/3,600) = 8.3, less than half that usually cited for a kerosene-fueled all rocket SSTO. Note the fuel required for the jet-powered portion would only be a fraction of the dry mass rather than multiples of it based on the fact the 1,000 m/s jet-powered speed is only a fraction of the 10,000 m/s or so effective exhaust speed of jet engines.
Note this brings the kerosene fuel load to be about that of hydrogen fueled SSTO's, except you still have the high density of kerosene. With modern lightweight materials this should be well doable.
Bob Clark
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