The KINETIC energy content of a gallon of gas in LEO is greater by a factor of about two than the CHEMICAL energy content, even if the oxygen was available for free.
Oh well!
But "would that it were true"!
Let's Capture Apophis: Not a Stargate Thread
Sorry Tom, but as I suspect you know full well, things don't add up that way. With velocity, 3*(0>1) does NOT equal 0>3.
The KINETIC energy content of a gallon of gas in LEO is greater by a factor of about two than the CHEMICAL energy content, even if the oxygen was available for free.
Oh well!
But "would that it were true"!
The KINETIC energy content of a gallon of gas in LEO is greater by a factor of about two than the CHEMICAL energy content, even if the oxygen was available for free.
Oh well!
But "would that it were true"!
Ah, the pitfalls of using kinetic energy as your standard in space. The problem is that kinetic energy must be relative to something, and it is easy to screw up.
The same problem crops up in collision of fuel species in a head-on collider (and the calculation applies to fusors and Polywells). If you have two nuclei approaching each other at V = 1 relative to the center of the machine (and center of mass of the system), either nucleus sees the other approaching it at V = 2. Thus, while in the COM analysis the sum of the kinetic energies of the two nuclei is mV^2/2 * 2, each nucleus thinks it is about to be swatted by m(2V)^2/2, twice the kinetic energy calculated in the COM case. Has conservation of energy been violated?
For a space probe with a small fuel mass relative to dry weight, you can approximate that a given amout of burn time gives a given amount of delta V. In flat space, if you are maneuvering to catch an unaccelerated target, this will work nicely, and your KE relative to the target has little to do with the overall KE of the probe.
An interesting consequence Dr. Bussard pointed out to me is that if you drop into a gravity well, and at perigee you do a burn to acquire a certain amount of delta V, you are doing so while the spacecraft is going faster than it is in flatter space. The result is that you pick up more KE than you would if you did the same burn in flat space!
My observation is that rocket types usually avoid using kinetic energy unless they are looking at the effects of one object hitting another. Otherwise you will give yourself a headache trying to make sense of the changing frames. Newton did not use kinetic energy ... the concept came along later.
For an object to get to 5 miles a second in LEO it must also, of course, climb about a hundred miles. And the energy expended to make thrust has everything to do with reaction mass flow. It should always take far more energy to increase velocity relative to a fixed starting point in flat space than the resulting kinetic energy, but the amount of delta V from a fixed amount of fuel burn may not relate to KE the way you expect. Rockets get far more energy efficient when going fast.
The same problem crops up in collision of fuel species in a head-on collider (and the calculation applies to fusors and Polywells). If you have two nuclei approaching each other at V = 1 relative to the center of the machine (and center of mass of the system), either nucleus sees the other approaching it at V = 2. Thus, while in the COM analysis the sum of the kinetic energies of the two nuclei is mV^2/2 * 2, each nucleus thinks it is about to be swatted by m(2V)^2/2, twice the kinetic energy calculated in the COM case. Has conservation of energy been violated?
For a space probe with a small fuel mass relative to dry weight, you can approximate that a given amout of burn time gives a given amount of delta V. In flat space, if you are maneuvering to catch an unaccelerated target, this will work nicely, and your KE relative to the target has little to do with the overall KE of the probe.
An interesting consequence Dr. Bussard pointed out to me is that if you drop into a gravity well, and at perigee you do a burn to acquire a certain amount of delta V, you are doing so while the spacecraft is going faster than it is in flatter space. The result is that you pick up more KE than you would if you did the same burn in flat space!
My observation is that rocket types usually avoid using kinetic energy unless they are looking at the effects of one object hitting another. Otherwise you will give yourself a headache trying to make sense of the changing frames. Newton did not use kinetic energy ... the concept came along later.
For an object to get to 5 miles a second in LEO it must also, of course, climb about a hundred miles. And the energy expended to make thrust has everything to do with reaction mass flow. It should always take far more energy to increase velocity relative to a fixed starting point in flat space than the resulting kinetic energy, but the amount of delta V from a fixed amount of fuel burn may not relate to KE the way you expect. Rockets get far more energy efficient when going fast.
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rjaypeters
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Yes, the Oberth maneuver: http://en.wikipedia.org/wiki/Oberth_effectTom Ligon wrote:An interesting consequence Dr. Bussard pointed out to me is that if you drop into a gravity well, and at perigee you do a burn to acquire a certain amount of delta V, you are doing so while the spacecraft is going faster than it is in flatter space. The result is that you pick up more KE than you would if you did the same burn in flat space!
Which we will do well to remember when we start moving large objects around the Solar System.
Yeah, I keep waiting for GWJohnson to start posting on this thread. He may be busy with his proposed Mars mission.Tom Ligon wrote:My observation is that rocket types usually avoid using kinetic energy unless they are looking at the effects of one object hitting another.
"Aqaba! By Land!" T. E. Lawrence
R. Peters
R. Peters