markh@csd4.csd.uwm.edu (Mark William Hopkins) (05/22/91)
What is the velocity of the fuel propellant relative to the ship, in the case of (1) fuel from the booster during boost, and (2) fuel from the ship itself during orbital flight? How fast can NASA-generated technology expel propellant? What does the latest research on this issue have to say?
henry@zoo.toronto.edu (Henry Spencer) (05/22/91)
In article <12344@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: > What is the velocity of the fuel propellant relative to the ship, in the >case of (1) fuel from the booster during boost, and (2) fuel from the ship >itself during orbital flight? Boost vs. orbital flight makes no difference; engine technology, and to some extent ambient atmospheric pressure, do. Chemical-rocket exhaust velocities are typically a few thousands of meters per second. 3000-3500 is good for a non-hydrogen system, 4500 is good for hydrogen. You won't get large improvements on the latter with chemical fuels. > How fast can NASA-generated technology expel propellant? If you mean technology that is available for production use now, numbers on the order of the above are it. If you mean technology that NASA was investigating 20-30 years ago, an improvement of several orders of magnitude should be possible if you're willing to fund development. Thrusts will not be high enough, mostly, for use in launch to orbit, but 25-year-old technology would be ample for huge improvements in maneuvering once in orbit. > What does the latest research on this issue have to say? What latest research? There is only the barest dribble of research, and no development of flight-worthy hardware to speak of. -- And the bean-counter replied, | Henry Spencer @ U of Toronto Zoology "beans are more important". | henry@zoo.toronto.edu utzoo!henry
fcrary@earthquake.Berkeley.EDU (Frank Crary) (05/23/91)
In article <12344@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: > > What is the velocity of the fuel propellant relative to the ship, in the >case of (1) fuel from the booster during boost, and (2) fuel from the ship >itself during orbital flight? > > How fast can NASA-generated technology expel propellant? > > What does the latest research on this issue have to say? The exhaust velocity of the shuttle engines are as follows: (note convert to specific impulse by dividing by 9.8 m/s^2) Soild Rocket Boosters: 2700 m/s (approximate) Main Engines (used on take off only): 4400 m/s Orbital Manuvering System: 3070 m/s Reaction Control System: 2550 to 2750 m/s The highest exhaust velocity produced by a NASA rocket was 9600 m/s operational, upto about 10500 to 11000 m/s in very limited (and low thrust) conditions. This was the NERVA (Nuclear Energy for Rocket Vehicle Applications) static test reactor/engine. The program was terminated with NASA's post-Apollo Mars mission hopes. On paper studies say many things. Numbers as high as 50,000 m/s are projected but this involves very long term and risky research and development. I suspect that as far as the next 15 to 20 years go, a 14,000 m/s solid core nuclear thermal rocket is about the limit. Frank Crary UC Berkeley
markh@csd4.csd.uwm.edu (Mark William Hopkins) (05/25/91)
In article <1991May22.164754.22298@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes: >> What does the latest research on this issue have to say? > >What latest research? There is only the barest dribble of research, >and no development of flight-worthy hardware to speak of. Personally, I was expecting to hear an answer like: "the USAF is currently conducting research with fission-based propulsion with expellant velocities as high as 1000 KILOMETERS per second." No development? Say it ain't so.
henry@zoo.toronto.edu (Henry Spencer) (05/26/91)
In article <12430@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: >>What latest research? There is only the barest dribble of research, >>and no development of flight-worthy hardware to speak of. > >Personally, I was expecting to hear an answer like: "the USAF is currently >conducting research with fission-based propulsion with expellant velocities >as high as 1000 KILOMETERS per second." At any given time, there are a handful of very small research projects in advanced propulsion active. They come and go, and mostly don't lead anywhere because they never get funded to do anything more than research. We've had the research underpinnings for advanced-propulsion development since the 1960s. (We know more now, but we knew *enough* even then.) What's lacking is development money for real hardware. >No development? Say it ain't so. Wish I could. -- "We're thinking about upgrading from | Henry Spencer @ U of Toronto Zoology SunOS 4.1.1 to SunOS 3.5." | henry@zoo.toronto.edu utzoo!henry
markh@csd4.csd.uwm.edu (Mark William Hopkins) (05/27/91)
In article <1991May25.215849.15606@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes: >At any given time, there are a handful of very small research projects >in advanced propulsion active. They come and go, and mostly don't lead >anywhere because they never get funded to do anything more than research. It sounds like to me, that we private sector people have a wonderful opportunity to upstart the governmental space agencies of the world by applying our own personal funds towards such a project. The reality of the matter ... and irony ... is that relative speed of fuel expulsion is the determining factor for everything: how much fuel you need to bring on board, the size of the booster to accomodate it, how far you can get under sustained acceleration, (and thus) how fast you can get to the moon and Mars, (and thus) how much time you're forced to spend in weightlessness, as opposed to 1 G artificial gravity, and a whole lot more. The right fuel makes all the difference between having to take several whole days to float to the moon on inertia, and a few hours to ram it in full gear (and ram it to full stop when halfway there :)). It makes all the difference between a years' traversal to Mars on gravitation, and a week because of a few hours sustained thrust and deceleration. I am totally suprised that nobody seems to see how fundamental this one number (propellant velocity) is.
fcrary@lightning.Berkeley.EDU (Frank Crary) (05/27/91)
In article <12463@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: >It sounds like to me, that we private sector people have a wonderful opportunity >to upstart the governmental space agencies of the world by applying our own >personal funds towards such a project. > >The reality of the matter ... and irony ... is that relative speed of fuel >expulsion is the determining factor for everything: how much fuel you need to >bring on board, the size of the booster to accomodate it, how far you can get >under sustained acceleration, (and thus) how fast you can get to the moon and >Mars, (and thus) how much time you're forced to spend in weightlessness, as >opposed to 1 G artificial gravity, and a whole lot more. > Largely true, but it is very important to note that the non-chemical, e.g. all the high exhaust velocity, rockets tend to have low accelerations. There are many very good propulsion ideas that can offer order of magnitude improvements in this velocity, BUT they lack the acceleration to lift off from the Earth's surface under their own power, or even make a minimum energy transfer orbit injection to the Moon or Mars. Also, at least with chemical rockets, high exhaust velocity results in more expensive rockets and fuels which are awkard to handle. >The right fuel makes all the difference between having to take several whole >days to float to the moon on inertia, and a few hours to ram it in full gear >(and ram it to full stop when halfway there :)). It makes all the difference >between a years' traversal to Mars on gravitation, and a week because of a few >hours sustained thrust and deceleration. > No. Baring the use of nuclear explosives (e.g. like the Orion rocket) none of the current designs are capable of the high acceleration this requires. Even were the acceleration possible, the continued thrust would not be. To produce a constant 1-g (9,8 m/s^2) acceleration for 4 hours would require a 141.1 km/s change in velocity. Baring the nuclear explosives, the highest exhaust velocity I have seen in ANY published paper on advanced propulsion was about 50 km/s. With this exhaust velocity, the above delta-v would require a spacecraft that was 94% fuel. The remaining 6% would need to be fuel tanks, not payload. High, constant acceleration for long periods is just NOT possible with near term (e.g. the next 25 years) technology. By the way, the private sector is already working on this... OCS is looking at electric propulsion for use in upper stages (e.g. for transfers from low Earth orbit to Geostationary...) Frank Crary
henry@zoo.toronto.edu (Henry Spencer) (05/28/91)
In article <12463@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: >... we private sector people have a wonderful opportunity >to upstart the governmental space agencies of the world by applying our own >personal funds towards such a project. The problem with doing this in the private sector is, where's the market? Remember that both the comsat people and the science people put a much higher priority on avoiding risk than on better performance. There is a very high startup cost involved in building and test-flying the thing before people will accept it as a credible system. There is some private interest in the idea. For example, OSC has a project to build an ion-rocket orbital-maneuvering stage. Don't expect it to fly tomorrow, though. >The reality of the matter ... and irony ... is that relative speed of fuel >expulsion is the determining factor for everything... Well, no, not quite. While exhaust velocity is crucial, thrust is also significant. If engine thrust is not at least comparable to the local force of gravity, very inefficient trajectories result. The most notable example of this is what happens if you try to build an Earth-to-orbit launcher with inadequate thrust... :-) Sometimes it is better to trade off exhaust velocity for increased thrust. One can also benefit from reducing exhaust velocity if it reduces the mass of support systems enough. Electric rockets suffer from the mass of their power supply; high-energy nuclear rockets (e.g. fusion) suffer from the mass of the cooling systems needed to get rid of waste heat (at very high exhaust velocity, there is not enough mass flow through the rocket to use the fuel as coolant). Tradeoffs happen. -- "We're thinking about upgrading from | Henry Spencer @ U of Toronto Zoology SunOS 4.1.1 to SunOS 3.5." | henry@zoo.toronto.edu utzoo!henry
markh@csd4.csd.uwm.edu (Mark William Hopkins) (05/28/91)
(I wrote): >The right fuel makes all the difference between having to take several whole >days to float to the moon on inertia, and a few hours to ram it in full gear >(and ram it to full stop when halfway there :))... In article <1991May27.022456.2921@agate.berkeley.edu> fcrary@lightning.Berkeley.EDU (Frank Crary) writes: >To produce a constant 1-g (9,8 m/s^2) acceleration for 4 hours would require >a 141.1 km/s change in velocity. Baring the nuclear explosives, the highest >exhaust velocity I have seen in ANY published paper on advanced propulsion >was about 50 km/s. With this exhaust velocity, the above delta-v would >require a spacecraft that was 94% fuel. That's my point. The right fuel makes all the difference. The variation between required mass and exhaust velocity is exponential! So the difference between 50 km/s and 100 km/s exhaust velocity is correspondingly significant. With 50 km/s fuel there's no practical way to get much above a 50 km/s change of velocity. What's near term and what's long term is unsayable. Knowledge, creativity and inventiveness cannot be predicted, and breakthroughs may literally happen tomorrow. But it all needs support, and it makes sense to put top priority on it since it is so fundamental. Incidentally, 50 km/sec exhaust velocity would STILL enable you to reach the moon in 4 hours, ignoring the effects of having to pull against Earth's gravity, if you could accelerate at 1G to 50 km/sec and decelerate likewise. You'd get to Mars even with this fuel in a few weeks, and not a whole year.
henry@zoo.toronto.edu (Henry Spencer) (05/28/91)
In article <12468@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: >With 50 km/s fuel there's no practical way to get much above a 50 km/s change >of velocity. Not true; with 3-4 km/s fuels we routinely achieve orbit (8+ km/s) and escape (11+ km/s). A more precise statement is that achieving more than 2-3 times the exhaust velocity is difficult unless thrust can be very low, and going beyond 4-5x quickly becomes impossible. >Incidentally, 50 km/sec exhaust velocity would STILL enable you to reach the >moon in 4 hours, ignoring the effects of having to pull against Earth's >gravity, if you could accelerate at 1G to 50 km/sec and decelerate likewise. >You'd get to Mars even with this fuel in a few weeks, and not a whole year. You don't seem to understand that there are basic problems with this happy scenario *other than* the exhaust velocity involved. You do not accelerate at 1G with a 50 km/s engine! The high-exhaust-velocity engines are mostly eletrical rockets. To accelerate a ship of (say) 100 tons at 1G with a 50 km/s exhaust velocity requires 25 gigawatts of power. There is simply no foreseeable near-future power source that can pack that kind of electrical output into 100 tons. For the interested, the derivation of that number... by conservation of momentum, taking the time derivative we have mass_flow * exhaust_velocity = ship_mass * acceleration (This ignores the loss of ship mass in the exhaust; assume for the moment that that is negligible.) The power of the exhaust is the time derivative of its kinetic energy, which in the ship frame of reference -- the one that matters in this case -- is simply power = 0.5 * mass_flow * exhaust_velocity^2 Substituting the first into the second, we get power = 0.5 * ship_mass * acceleration * exhaust_velocity This assumes, of course, zero losses. Ha ha. Most power sources have more like 50-75% losses, never mind the efficiency of the engines themselves. Getting rid of 50+ GW of waste heat from a 100T ship is not going to be fun. Note, I'm not saying that respectable acceleration at an exhaust velocity of 50 km/s is impossible, but you *won't* do it with electrical rockets, and it won't be easy even with not-here-yet technology like fusion rockets. -- "We're thinking about upgrading from | Henry Spencer @ U of Toronto Zoology SunOS 4.1.1 to SunOS 3.5." | henry@zoo.toronto.edu utzoo!henry