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Everything posted by sevenperforce

  1. A solar-powered ion thruster is analogous....
  2. The reason you don't see stars during the day is because the Rayleigh scattering is brighter than the starlight behind it. Not quite. It's a function of particle size. Particles within the rings are mostly between 1 cm and 10 m with an exponential size-frequency relationship. The odds of a given photon intersecting a solid particle while traveling through a given path can be calculated if you estimate the size-frequency relationship, but it's not at all "X% of the path is blocked". Also, the ring particles are almost exclusively pure water ice...purer than found in most places in the solar system, actually. Though they are not transparent, so you're correct there. The rings are visible because they scatter sunlight, and so there is enough sunlight being scattered toward you to saturate. But you can definitely see through them. Consider this high-contrast Cassini image: From this vantage point, you can see how much light is blocked by the rings, both in their shadow projected on Saturn, and by the visibility of Saturn's disc through them at the top. The B ring obviously blocks more light than the C or A rings, but even it allows a little light through (the apparently-black portions of the stripes across Saturn are actually lighter than the black background.
  3. Air is dense enough to see when illuminated by sunlight but not so dense that it blocks starlight. Only 3% of the volume occupied by the rings is actually solid material. Starlight can shine through them easily, but stars are washed out by the ring brightness. Photos which show Saturn's rings typically do not show any stars because the dynamic range isn't high enough.
  4. Arguably, you would still see stars shining through. The rings aren't dense enough to actually attenuate starlight.
  5. It's easy to forget just how freaking unprecedented this is.
  6. Yes, they can, and easily. But that is, I believe, a variant of the Long March 2C with the addition of lower fins, a solid interstage, and grid fins. The Long March 2C is powered by four YF-20s in the YF-21C configuration, which has no centrally-mounted engine and cannot downthrottle low enough for even a hoverslam. I am unsure whether the YF-21C config has relight capability. The engine schematics don't show burst discs or anything but that isn't always reliable. The YF-20 also has a very rough fuel-lean start, as shown by the big clouds of N2O4 that puff up around the base of the rocket at ignition, so trying to control the landing in a preferred zone seems hazardous...then again, so is just dumping the whole rocket onto inhabited areas. I would also guess they are testing for another project.
  7. Late to this party, but given that this is clearly a hypergol booster, I don't foresee reignition and landing on it....
  8. Definitely looks like a full-size flaperon. I really wonder what kind of hydraulics they're going to be using.
  9. In theory, you could have an NTR which produces an ionized exhaust stream with an open-cycle coolant loop acting as the charge sink. The coolant loop would generate power, which could be used for an electromagnetic nozzle to accelerate the exhaust stream faster. The big problem with an NTR is temperature: you can only make the propellant move faster by making it hotter, and at some point you melt the reactor. A pure hydrolox rocket burned at stoichiometric ratio produces heat not significantly lower than the temperature of a solid-core NTR; the advantage with an NTR is that you're only pushing hydrogen, not that nasty and heavy oxygen, and so your specific impulse goes up. By accelerating the NTR exhaust products after they leave the chamber, you don't have to worry about overheating the reactor. If you could make it work.
  10. Odds of LOCV for STS-1 was determined retroactively to have been what...1:12? 1:9?
  11. LOCV failure modes for a commercial airliner include loss of control surface authority and catastrophic structural breaches. If the elevators fail, the pilot can't control pitch and the plane crashes. If the wings break up, the plane crashes. SS has the same failure modes; if it has a tank breach or a loss of control fin authority, it crashes.
  12. You can't have a chemical reaction that produces charged exhaust with only two reactants, because you need a charge sink. You can't just delete charge; it has to go somewhere. Any system that produces charged exhaust would accumulate electrostatic charge on itself. What you could do, conceivably, is have a multichamber tripropellant engine that produces two exhaust streams, one positively-charged and one negatively-charged. Maybe do it with an annular thrust chamber, like an aerospike engine inside a de laval. Then your net charge flux is zero and you can use magnetic fields to further accelerate one of the two charged exhaust streams..
  13. why? For an exhaust flow to be ionized in a way that would allow it to be manipulated my magnetic fields, you'd need to rip the electrons away in an organized fashion. You can't just annihilate the electrons, either. You'd almost need to have a tripropellant reaction, with one exhaust flow being net-positive and the other being net-negative. What I really want to see is a Hall Effect thruster (or other electrostatic thruster) which can have a working fluid injected into it to amp up the thrust at the expense of isp.
  14. Getting a reaction with a workably ionized exhaust stream is...tricky.
  15. At some point you create a kugelblitz, at which point, why not just go with a black hole starship? Collimation of the thrust beam is always going to be the challenge.
  16. Each leg has to be able to bear a certain minimum peak force onset regardless of the number of legs, since we must presume that the vehicle will not descend in a perfectly vertical state. One leg makes first contact with the ground. This places a minimum strength constraint on each leg and therefore requires a minimum weight for each leg. Therefore, increasing the number of legs for stability reasons will not reduce the mass of each individual leg and will increase the overall dry mass significantly. Three legs are best.
  17. I feel like your "fuel tank" would have rather poor mass ratio..........
  18. The time derivative of velocity is acceleration; the time derivative of acceleration is jerk. The time derivative of momentum is force; the time derivative of force is force onset rate or mass-jerk. The time derivative of mass flux is pressure; the time derivative of pressure would have units kg/m*s^3, which is power density...which I suppose represents the amount of energy being dissipated through a cross-sectional thickness of the fabric per second during the decompression. I love dimensional analysis. EDIT: Also the time-derivative of angular momentum is energy and the time-derivative of energy is power.
  19. If I was running the testing, I'd put the suit in a pressure chamber, pressurize the entire chamber to 3 atmospheres, seal the suit, and then gradually release pressure on the chamber. This tests pressure gradient resistance at 200% operational. Then, I'd repeat but only pressurize the chamber to 2.41 atmospheres, seal the suit, and then blow the chamber. Simulating explosive decompression at 200% of operational mass-jerk/area.
  20. Three atmospheres inside, one outside. The suit can't tell the difference between absolute pressure and gauge pressure; it is tested based on pressure differential.
  21. I'm sure they have done this. I know they've tested it up to double vacuum and I'm sure they did sudden/rapid depressurization in conjunction with that. In a lab, they can slam it with high doses of radiation, heat/cold cycles, whatever you need. I am pretty sure it has hose attachment.
  22. There is work with photons in quantum systems where the wavefunctions can be trapped in closed timelike curves, which should remove the capacity for quantum tunneling. Imagine if we had a quantum photon rocket powered by entangled particles in time-traveling curves! Or of course we could capture a black hole inside a resonating cavity and feed it mass whenever we wanted to fire our photon rocket.
  23. Ok thanks This also leads to an intuitive understanding of why engines with high exhaust velocity are more fuel-efficient than engines with low exhaust velocity. On Earth, we talk about fuel efficiency in the context of distance, because an automobile must continually expend energy in order to keep moving against the drag of the air and the ground. With a rocket, however, we talk about efficiency in the terms of fuel consumption rate. The technical term is "thrust-specific fuel consumption", i.e., the amount of fuel/propellant being expended every second in order to produce a specific amount of thrust. Since thrust is just force divided by time, you can cancel out the "per second" on both sides, which gives you the amount of fuel required to produce a specific amount of impulse, or change in momentum. Suppose you have a spaceship with a mass of 1 kg and a prop tank containing 10 kg of propellant. The initial mass of the system is 11 kg and the initial momentum of the system is zero. If you throw 1 kg of props out the back at -100 m/s, it has a momentum of -100 kg*m/s. By the conservation of momentum, the spaceship must have an equal and opposite momentum of +100 kg*m/s. The spaceship now masses 10 kg, so dividing its +100 kg*m/s by 10 kg gives you a positive velocity of 10 m/s. But if your propellant is more volatile (or, perhaps, is being accelerated by an extreme heat source like a nuclear reactor), it can come out the back end of your spaceship with much greater speed. For example, if you can throw propellant out the back of your spaceship at -1,090 m/s, then you only need to use 0.1 kg for the same effect. 0.1 kg * -1,090 m/s = -109 kg*m/s, and so the spaceship has a resulting momentum of +109 kg*m/s, and dividing by the remaining spaceship mass of 10.9 kg gives you a positive velocity of 10 m/s. You'd also need a way to collimate the photons so that their thrust pushed you in the correct direction rather than dispersing out at angles. And, as @kerbiloid's link indicates, photons do not actually remain trapped in resonant cavities indefinitely. Even with a perfectly-reflective mirror (which is itself impossible), there is a small but nonzero probability that the wavefunction of the photon will tunnel through, either being absorbed by the mirror substrate and turning into heat, or passing through entirely and escaping.
  24. Kinetic energy is half the mass times the square of velocity. As long as you "never let go" of the table, your velocity remains the same as its velocity, and thus the transfer of kinetic energy is zero. Why has this not been done yet? Low thrust and poor energy performance. You have to use massive amounts of energy to generate even a little thrust. We're talking about accelerating at 100% throttle for decades just to get 5 m/s of dV. Power the laser with a nuclear reactor A nuclear reactor uses fission to break apart large nuclei into smaller ones. Severing these nuclear bonds destroys nucleic mass and converts it into energy according to e=mc2. So you are converting mass into energy. A nuclear-powered photon rocket loses mass over time.
  25. The problem is inherent right there in your explanation. You "push" the thrust to the sides. Let's go back to our tennis ball analogy; our engine is a tennis-ball-throwing machine, and we want to recapture those lost tennis balls so we can "recycle" them. Follow along: In order to move the ship prograde, the tennis balls must be pushed retrograde. We have a magic magnetic mechanism to push the flying tennis balls in whatever direction we like. If we only push the tennis balls in the radial direction, they will head off at an angle, but their retrograde momentum will not be arrested. In order to deflect the tennis balls into a perpendicular direction, we must "push" them at an angle between radial and prograde. If our magic magnet mechanism pushes with any prograde component at all, then the tennis balls push back, by Newton's laws, in the retrograde direction. This produces a retrograde force on the mechanism. If the mechanism is attached to the ship, it transfers that "push" back to the ship, and the ship cannot move. If the mechanism is not attached to the ship, it rapidly recedes into the distance. It's not a free-energy machine, it's a reactionless thruster. Both are prohibited by physics. Confusing energy and momentum is one of the reasons this is often so hard to explain.... So have two springs. Or a system of springs. Remember, whatever a magnet can do, a frictionless spring can do. If you have a tube with springs at both ends, nothing will change the overall momentum of the system, no matter how much nuclear energy you add. Making the springs angled won't make a difference.