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This comment seems to come from ignorance of basic physics and chemistry rather than an acute awareness of it. The specific energy of the best batteries we hav

This comment seems to come from ignorance of basic physics and chemistry rather than an acute awareness of it. The specific energy of the best batteries we have at the moment is only about 2% of that of gasoline. And gasoline isn't that great for a rocket. http://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials http://en.wikipedia.org/wiki/Energy_density#Energy_densities... If you want to talk about taking a jet to mach4, well, now things are starting to get really nuts. Airplanes only work because they can produce lift in excess of their weight. Putting enough batteries on an airplane to make it fly is completely doable; just look at RC airplanes and quadcopters. But getting to mach4 is bonkers at the moment. Drag goes up by speed squared. Doubling your speed quadruples your drag. Of course, air breathing craft to get above most of the atmosphere are totally viable and so are space elevators (maybe not today but in the future) and the launch loop might be feasible too from a basic physics and material science standpoint. Once you're in orbit ion drives still aren't awesome at least for human-scale space exploration, but they become viable.

maxerickson 12y ago
There are manned electric planes quite far into development. For example: http://www.gizmag.com/e-fan-airbus-electric-plane/31823/ http://www.gizmag.com/e-fan-airbus-electric-plane/31823/ (I'm just pointing out that they are further along than just RC stuff, not trying to argue that it is significant to space flight)
- msandford 12y ago
  Yeah, sure it's further along. But these kinds of planes fly at 100-200mph. But mach4 is at least 1000mph or more (depending on altitude). So while the envelope of feasibility might be doable at 100mph or 200mph to go from 100mph->1000mph requires 100x the amount of power. Going from 200mph to 1000mph requires 25x the amount of power. So here's the rub: not only are your power requirements going up but also your total energy requirements are going up. This is why the Concorde stopped flying. We're too cheap to go fast. Using energy at 100x the rate to go 10x as fast means that your total energy budget has to somehow miraculously go up 10x and 10x more batteries could easily keep a plane from flying. As battery tech gets better we might find it's possible to do really useful stuff with electric planes. But until really major breakthroughs happen we won't see electric cargo jets or electric-to-launch-orbit. It's not nay-saying that will prevent this but basic physics.
  - deleted 12y ago
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  - maxerickson 12y ago
    How could I have better put not trying to argue that it is significant to space flight? The basic thought behind my comment was that people reading your comment might find electric airplanes interesting but not be aware of the progress being made.
deleted 12y ago
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Retric 12y ago
You misunderstood. Some aircraft carriers have swapped to electrical propusion systesm vs steam to launch craft. While a rocket sled is no net gain you could launch a rocket to Mach 4+ using rail / coil gun etc which is fairly significant fuel savings. Pushing past that has major issues, but a potential advantage is you can build a air breathing scram jet without needing design your engine for lower speeds. I agree that: http://eandt.theiet.org/magazine/2014/10/rise-of-electric-aircraft.cfm http://eandt.theiet.org/magazine/2014/10/rise-of-electric-ai... is just aboit pointless for rockets. Building the 32km of track to hit mach 4 at 3g's of acceleration would one of many issues, but my point is simply building another rocket does not really change much. There actually fairly close to the limits of what you can do with a chemical roket.
- msandford 12y ago
  I think another real problem is that at the end of the 32km of track you need to stop going sideways and start going up. At mach4 (at sea level this is really, really fast) you're going to need either a huge radius or you're going to have incredible g forces. Either way, big engineering problems.
  - Retric 12y ago
    Not 100% clear what you mean by huge g forces. You would not make a round track to do this. You want something close to a straight line with some adjustment for the curved surface of the earth and local gravity. After launch you’re going to want to rapidly gain altitude as fast as the passengers can tolerate (so ~3g), but even if you lose 30-50% of the input energy your still dramatically reducing your on board fuel needs as in ~30+ seconds of main engine burn. More importantly you can have an air breathing sage without including a sub sonic air breathing engine which vastly simplifies your design. Further, one proposal was instead of trying to accelerate using an air breathing engine you simply try and maintain velocity as you gain altitude which further expands your useful design space. Net result, reusable single stage to orbit, with a useful mass fraction, might be possible. Though, I agree trying to hit mach 4 near sea level is going to be a major design issue. But, Mach 2 is defiantly possible and probably a better first goal.

maxerickson 12y ago

There are manned electric planes quite far into development. For example: http://www.gizmag.com/e-fan-airbus-electric-plane/31823/ http://www.gizmag.com/e-fan-airbus-electric-plane/31823/ (I'm just pointing out that they are further along than just RC stuff, not trying to argue that it is significant to space flight)

msandford 12y ago
Yeah, sure it's further along. But these kinds of planes fly at 100-200mph. But mach4 is at least 1000mph or more (depending on altitude). So while the envelope of feasibility might be doable at 100mph or 200mph to go from 100mph->1000mph requires 100x the amount of power. Going from 200mph to 1000mph requires 25x the amount of power. So here's the rub: not only are your power requirements going up but also your total energy requirements are going up. This is why the Concorde stopped flying. We're too cheap to go fast. Using energy at 100x the rate to go 10x as fast means that your total energy budget has to somehow miraculously go up 10x and 10x more batteries could easily keep a plane from flying. As battery tech gets better we might find it's possible to do really useful stuff with electric planes. But until really major breakthroughs happen we won't see electric cargo jets or electric-to-launch-orbit. It's not nay-saying that will prevent this but basic physics.
- deleted 12y ago
  [deleted]
- maxerickson 12y ago
  How could I have better put not trying to argue that it is significant to space flight? The basic thought behind my comment was that people reading your comment might find electric airplanes interesting but not be aware of the progress being made.

deleted 12y ago

[deleted]

Retric 12y ago

You misunderstood. Some aircraft carriers have swapped to electrical propusion systesm vs steam to launch craft. While a rocket sled is no net gain you could launch a rocket to Mach 4+ using rail / coil gun etc which is fairly significant fuel savings. Pushing past that has major issues, but a potential advantage is you can build a air breathing scram jet without needing design your engine for lower speeds. I agree that: http://eandt.theiet.org/magazine/2014/10/rise-of-electric-aircraft.cfm http://eandt.theiet.org/magazine/2014/10/rise-of-electric-ai... is just aboit pointless for rockets. Building the 32km of track to hit mach 4 at 3g's of acceleration would one of many issues, but my point is simply building another rocket does not really change much. There actually fairly close to the limits of what you can do with a chemical roket.

msandford 12y ago
I think another real problem is that at the end of the 32km of track you need to stop going sideways and start going up. At mach4 (at sea level this is really, really fast) you're going to need either a huge radius or you're going to have incredible g forces. Either way, big engineering problems.
- Retric 12y ago
  Not 100% clear what you mean by huge g forces. You would not make a round track to do this. You want something close to a straight line with some adjustment for the curved surface of the earth and local gravity. After launch you’re going to want to rapidly gain altitude as fast as the passengers can tolerate (so ~3g), but even if you lose 30-50% of the input energy your still dramatically reducing your on board fuel needs as in ~30+ seconds of main engine burn. More importantly you can have an air breathing sage without including a sub sonic air breathing engine which vastly simplifies your design. Further, one proposal was instead of trying to accelerate using an air breathing engine you simply try and maintain velocity as you gain altitude which further expands your useful design space. Net result, reusable single stage to orbit, with a useful mass fraction, might be possible. Though, I agree trying to hit mach 4 near sea level is going to be a major design issue. But, Mach 2 is defiantly possible and probably a better first goal.