cjameshuff

Shift my orbit by a hundred kilometers, or by a thousand. Do a large Hohmann transfer. Do a plane change. Do a precision reentry. Eventually, transfer into orbit around another body. So what? The SAS module makes it easier to avoid introducing the error in the first place.

It's relevant for tens of seconds if trying to do a precision burn, and multiple orbits if you're using the navball as a time-of-orbit indicator. Except it's not dead weight, while winglets would be. Your 'test' is completely irrelevant. SAS is useful in orbit, winglets are not.

The capsule SAS damps out rotation, it doesn't maintain orientation. I keep a single SAS module on my uppermost stages just for this purpose. If you define sloppy design as anything that's particularly assisted by use of SAS...and they're an assist to piloting, allowing you to spend more time monitoring altitude and velocity instead of constantly tweaking your orientation. As for winglets, I've never seen the need for them. I'm out of the atmosphere almost immediately anyway. Real orbital launchers rarely use them, for this reason...they mostly just add drag.

Real capsules carrying humans have better instruments and actually fly aerodynamically on reentry, adjusting angle of attack to control their trajectory. Or at least, are intended to...some Soyuz capsules have come in purely ballistic due to loss of active control, resulting in rougher reentry forces and landing off target. (Still, preferable to getting scattered across the countryside...) Anyway, it's not actually that inaccurate with good instrumentation and control. The later Apollo missions came down within 1.85 km of their targets, and SpaceX's Dragon splashed down within 800 meters of its target on its first flight. They also plan to come down on land, using rocket propulsion for final braking and perhaps maneuvering before touchdown...it's possible they'll be able to reliably hit a specific landing field.

That's a graph of velocity versus altitude...for a given altitude, there's nothing to graph. If you just want to calculate the needed velocity to just intersect atmosphere, my calculator will do that easily: http://files.arklyffe.com/orbcalc.html?a=1234500&p=634500&s=1e6 I had considered adding this (well, a transfer to 50 km, allowing you to go to a low orbit to better pick your landing point, or just burn a little bit longer to directly reenter) to the table of circular/escape velocities and orbit periods. If there's interest, I can generate an updated version. inzmru, you might want to add your plot to this thread: http://kerbalspaceprogram.com/forum/index.php?topic=647.0

Just released a new version. Any particular remarks on usability? Unexpected behavior, unclear functionality? Especially at periapsis. The new version lets you adjust velocity with the mouse by shift-dragging, which illustrates the sensitivity to velocity pretty well.

Use fewer solids and make sure you're turning your SAS on. Fewer solids, and solids only on the bottom stage...they can get you moving early on, but they burn out before giving you any real speed, so you don't want to carry them around. Stack fuel tanks...you've barely given your rocket any fuel...and put a liquid stage above the tricoupler.

See this thread: http://kerbalspaceprogram.com/forum/index.php?topic=650.0

Well, I was talking interstellar missions on human timescales in general, and...

All of the above assumes a lightsail craft of the same mass as the VASIMR craft. What you miss is that the thrust can be applied to a much lighter payload. The laser craft doesn't have to carry any reactors, and requires a tiny fraction of the thrust to achieve the same acceleration. Rather than massing a few hundred tons, it can be a few kilograms. It might take more power to exert a newton of thrust, but the same result will be achieved with on the order of a hundred thousand times less thrust. The laser craft has no power density requirements...you could run it off hamster wheels if you stacked enough together, because the power source stays stationary. VASIMR carries its power source, and current power sources are really just not good enough to let it reach its potential (though they're plenty good enough to put it to work as an orbital tug or bulk transport craft). No, VASIMR can't (usefully) run on such a small solar array. We're putting a small, low power one (200 kW) on the ISS soon, complete with battery pack to charge it up so we can actually run it, in pulses for a few minutes at a time. VASIMR scales up well to high powers, but realistically requires nuclear reactors (or beamed power) as a power source. If you're just throwing a few kilowatts at it, you may as well just use some form of ion drive, it'll be lighter and perform better. A 10 MW beam could push a 1 kg sailprobe at 0.067 m/s^2. After a month, that adds up to 175 km/s, while still within 1.5 AU distance, without playing any fancy tricks like doing the boost during a solar flyby (like a beam-assisted H-reversal trajectory). All you need is a bigger aperture to increase effective range so you can keep accelerating. All you need to increase acceleration is total power...power density and energy density are irrelevant. A 10 MW 50 metric ton VASIMR craft with 50 kW/N will get 10.5 km/s in that same time period. Improving overall delta-v significantly without making it far slower means squeezing more power and more energy out of a given mass of reactors, radiators, and other power systems. It'll take a lot more where it's going, but it isn't winning a speed race.

In terms of setting velocity records, I suspect we could do better right now with laser propulsion, actually. You can scale up lasers and optical apertures more easily than you can scale up reactor power and energy density. Double the reactors for a beam launch means double the acceleration, double the aperture means double the effective range of the beam. Double the reactors for a VASIMR launch means a considerably larger dry mass and a relatively small boost in acceleration and delta-v. It's a bit early for missions to other stars, though. I'm thinking more along the lines of heliosphere probes consisting of a bare minimum of instrumentation, a few kilograms of sail and electronics, powered by the same beam used for propulsion.

First post in the thread, second paragraph of the documentation bit.

Delta-v is change in velocity. If you're going at 2371.3 m/s and need to be going at 2419.1 m/s, it's easier to burn until your velocity is 2419.1 m/s than to burn enough to change your velocity by 47.8 m/s. The Wikipedia articles on orbits are reasonably complete. Googling for 'orbital mechanics' gave several good results.

Some sanity checks: the kinetic energy of a 1 kg at 0.9c is would take annihilating 1.3 kg of matter and antimatter to release. Relativistic delta-v: dv = c*tanh(Isp/c*ln(m0/m1)) Assuming proton-antiproton annihilation for the bulk of the energy, Isp/c is 0.6: 0.9 = tanh(0.6*ln(m0/m1)) e^(atanh(0.9)/0.6) = m0/m1 e^(atanh(0.9)/0.6)*1 kg = 11.6 kg initial spacecraft mass. 5.3 kg of antimatter for each kg of spacecraft accelerated to 0.9c. Then you have to stop, delivering 1 kg to the destination for every 11.6 kg that you accelerate to 0.9c. This clearly requires unreasonable amounts of antimatter. Some form of matter annihilation that doesn't require antimatter would work...if we ever discover it. Fusion has similar problems, it's only going to get you a few percent of c at best. Bussard ramjets do solve the problem of carrying mass+energy source, but in our region of space, they produce more drag than thrust once they get above a few percent of c. They might be useful for decelerating if you use something else to accelerate, though. Given how difficult antimatter is to make, beam propulsion really seems like the only thing that is likely to get a payload to another system in a human lifetime. It completely sidesteps the big issue of having to carry a power plant and reaction mass on the accelerating craft. It's also not limited to lasers, though the particle beam options are probably better suited to lower power, higher thrust, shorter range applications within the solar system.

Yes, that's it. To transfer to another orbit, burn to match velocities at a point where positions and directions coincide. A Hohmann transfer is just a transfer between circular orbits via an intermediate elliptical orbit. The calculator reports the overall delta-v required at each end of an elliptical orbit for transfer to/from a circular orbit, but this is mostly useful for planning purposes, to determine if you've got enough propellant to do the job...it's easier to burn to match a target velocity than to try to achieve a specific delta-v. I might add another input mode to echo your entries to both apsides to let you directly set a circular orbit, but I'm not sure how much use it'd be. Maybe a keyboard shortcut to cycle parameter entry modes.