Physics, Astronomy & Cosmology / Re: Would a Hohmann Transfer be more fuel efficient to attain lunar orbit?« on: 20/02/2018 20:37:24 »
Quote from: petelamana
To achieve lunar orbit would it be more fuel efficient to utilize a Hohmann Transfer, rather than a "direct" approach, as the Apollo program used?Yes, the Hohmann transfer orbit is the most fuel-efficient, if all other things were unchanged - but they aren't.
The Hohmann transfer orbit uses just two (quite intense) engine burns.
But it takes about half an orbit between the two engine burns - in the case of an Earth to Moon transfer, that is 2 weeks, compared to about 3 days for the Apollo approach.
That means 4 times as much food, oxygen and muscle cramps, which means more mass, and more fuel.
In the case of Apollo 13, when their oxygen tank exploded, depriving them of electrical power and heating, they continued on their orbit around the Moon, and back to Earth after about 5 days. If they had been on a Hohmann transfer orbit, it would have taken about 4 weeks to return to Earth orbit - and they would have died.
And, for that matter, are HTs used to translate satellites into higher orbits?With a chemical rocket, you can achieve the very intense burn to kick a satellite into a Hohmann transfer orbit - and the second intense burn to keep it there once it reaches the farthest point in the orbit. However, these chemical rockets are not very efficient.
See, for example: https://en.wikipedia.org/wiki/Inertial_Upper_Stage
Many of today's geosynchronous satellites have ion thrusters, which are very efficient, but can't achieve a very intense burn. So these satellites reach geosynchronous orbit by doing a number of low-thrust burns on successive orbits. This takes longer to reach geosynchronous orbit, but by carrying less fuel, they can carry more payload (communications antennas, transmitter channels, etc).
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