#1
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booster propeller
One of the limiting things in Earth to orbit rockets
is first stage initial thrust. The rocket engine of the first stage has to carry all the weight of the stages and fuel in the rocket and thus be big and often expensive. If you increase rocket fuel mass (since some people advice "fuel is cheap"), but don't increase first stage thrust, you get lower acceleration and thus bigger gravity losses. It can be thought that if the rocket is only powerful enough to just hover, the gravity losses approach deltav = useful deltav becomes zero. So designers have thought of ways around this, like putting small solid rocket boosters around the base of the rocket. Then you can put on a big upper stage, and the thing still takes off. The small solids are shed pretty soon, but they have done their important job of giving some initial velocity, thus reducing gravity losses. Upper stages can have lower, below 1 T/W ratios, since the stack is by then already going very fast and their slower acceleration doesn't thus contribute that much to the total flight time / increase gravity loss. They will eventually go faster again after they burn some propellants away. You also probably know that jet engines have often been proposed for start thrust augmentation on rockets. But they are actually awful engines in this job- their thrust to weight ratio is only close to 10. They are also quite expensive. Jet engines are only good at medium speeds - not at standstill or at multiple mach numbers. Only the latest high-tech fighter planes have T/W ratios over 1. But helicopters have been around for over 50 years. They certainly have thrust to weight over 1. How is this possible, since their power density is actually worse than with jets? It is physically very simple: they have big rotors. Thrust is relative to reaction mass times the velocity change of said mass. Power on the other hand is half the times of reaction mass times the velocity change *squared*. So, the higher your velocity change, the more *power* you need for the same *thrust*. Mathematically, it's said as F=2*P/deltav, where F is thrust, P is power and deltav is the reaction mass' exhaust velocity minus input velocity. That's why helicopters can hover with big payloads and jets can't. Rockets are the worst in this degree, that their reaction mass is accelerated to many km/s compared to a few tens m/s in helicopters - rockets waste a huge amount of energy to generate the same thrust as compared to a helicopter. So, in theory, if we tap some of the energy use of a rocket engine, but use external reaction mass (a propeller in ambient air), we could accelerate with much less energy and thus use much less fuel. The drawback is that propellers become less efficient at high speeds while rockets don't care about speed at all. (jets are somewhere between.) But we could use this only for the first 100-150 m/s. That's Mach 0.36 to 0.54. I'll come to that later. For example the RD-171 engine has a 170 MW turbine/ turbopump assembly.* It only burns a fraction of the propellants going to the main thrust chamber. Now, we change the pump so it doesn't put anything to the thrust chamber. The nozzle is silent. Then we attach a gearbox, a shaft and a big propeller to the turbine. The propeller gives a 50 m/s velocity change to the outside air - the resulting thrust would be 2 * 1.7e8 W / 50 m/s = 6.8e6 N = 6.8 MN. This would be able to hover about 700 metric tons. The total mass of a Zenit rocket is about 470 tons. The RD-171 in normal operation has 7.3 MN of thrust. What use would this be if we assume we get 100 m/s "for free"? Well, we could simply put in a bigger upper stage or bigger payload or both, just like with the help of solid booster rockets. Without gravity loss considerations, the effect is approximated by the rocket equation: If we assume for first stage deltav = v_ex log(m_o/m_dry) = 5 km/s or m_1o/m_1dry = exp(deltav/v_ex) Now if we reduce the required deltav by 100 m/s, we get deltav' = 0.98deltav m_1o/m_1dry = exp(deltav'/v_ex) = exp(0.98deltav/v_ex) = ( exp(deltav/v_ex) )^0.98 If original mass ratio was 3 (whole rocket / [whole rocket - first stage propellant]), now it's 3^0.98, which is 2.93. About a 2.3% decrease, or in other words, the dead weight of the whole thing can be increased by 10 tons and speed after first stage burnout is similar to normal situation. That might just be the mass of the propeller, gearbox and the propellant burned during 100 m/s acceleration. If we assume 3 m/s^2 acceleration (below T/W constraints), the vertical traveled distance to 100 m/s velocity is 5 km. I haven't calculated the benefit from possible lessened gravity losses or starting the rocket motor at higher altitude. The world's biggest helicopter, Mil mi-26 has the max liftoff mass at 56 tons. It has a 5-bladed 32 meter diameter rotor and 15 MW of engine power. So scale is about one tenth of the system designed above. Ceiling 4.6 km. One interesting question is the staging operation where the propeller is dropped and the thrust chamber started. Also, the cost of the gearbox and propeller would probably be prohibitive for the whole system unless they could be reused (and would probably still be). I haven't found any references to this idea. No wonder, it's quite bad. Discussion? *)It's a bad example since it's too complicated of a cycle and uses nozzle cooling energy for first stage pumps etc.. A gas generator engine would be best for this, but I didn't find quick good data on their gas generators' powers. -- -meiza |
#2
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booster propeller
I haven't found any references to this idea.
----- You would want to search rotary rocket Here's one reference: http://www.spaceandtech.com/spacedat...tary_sum.shtml Scaling that up you may want to research Hiller's work on flying cranes: http://avia.russian.ee/helicopters_e...r_crane-r.html There were problems with using gas turbines at the tip, so research was going towards a resonant pulse-jet (valveless) design that would allow even bigger flying crane designs. |
#3
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booster propeller
Anvil* wrote:
I haven't found any references to this idea. ----- You would want to search rotary rocket Here's one reference: http://www.spaceandtech.com/spacedat...tary_sum.shtml Scaling that up you may want to research Hiller's work on flying cranes: http://avia.russian.ee/helicopters_e...r_crane-r.html There were problems with using gas turbines at the tip, so research was going towards a resonant pulse-jet (valveless) design that would allow even bigger flying crane designs. These ideas are opposite to my proposal of using the torque from the rocket turbopump and a "dry" rotor. Also, Armadillo aerospace tried rotors with tip jets and found it's hard to control the fluid column in the blade. -- -meiza |
#4
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booster propeller
Forget trying to get deltaV --Just some high into thinner air *mite* be
worth it. That the problem with all these different schemes. When it come to the question of which is better (ie better than a plain rocket) the answer it: it depends on the details. Not just big crude detials, but real fine detials that are real hard to put numbers on without bent metal. Take this idea as a example, with a adjustment or two. So we use a "zero" stage that is really just a big chopper. Tip jets and all. It get us to where the air pressure is 1/3 at (~30KPa) then we fire the engines on our almost SSTO RLV. Now we can get much better T/W ratio on modest chamber pressure engines and we can use a more aggresive trajectory (max Q is now reduced for example) . But we need to make the zero stage. Will it help? well that depends on just were you want to spend your money. It seems likley that just reducing the payload a bit and launching from sea level would be much simpler with less things to go wrong. But then again it depends on how hard you think it is to make a good SSTO engines that work well at sea level (probably high chamber pressure + extendable nozzel say). Personaly I don't *belive* devlopment costs favour these kinds of complicated small payback schemes. Operationaly almost certianly a true SSTO will be better. I see this kinda scheme and its kin (air launch, airship launch) biggest advantage when we are lauching SSTO at rates of many a week. Then it seems to be one of the only ways to get rid of the noise problem. |
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