Zero energy trajectories - a thread to discuss this topic

John Creighton · 2004-03-29 07:42:07

Slightly off topic. Well, I guess that space tug ends the hubble debate. Hubble will be saved.

Mark Friedenbach · 2004-03-29 07:43:42

It's roughly analagous to a chemical reaction where you have two similar energy states seperated by a high energy activation barrier. This system seems to act as a catalyst that lowers that energy barrier, making it eaiser to get between the 2 equivalent energy states but I'm a bit unclear on HOW. I can easily understand how lunar L1 can easily feed to Earth L1 but Lunar L1 to MArs L1 just seems almost too good to be true.

It is. The analogy you use (lowering the activation barrier) is a little flawed. Think of getting from Earth to Mars as getting from one side of a hill to the other. Rather than go over it the "zero energy" trajectory will have you go around the hill. But if there's a fundamental difference in elevation with the hill (i.e. we're at 2,000km and we need to get to a place at 4,000km on the other side) you still have to move up or down by that amount to get to your destination (+2,000km in our case). Moving from L1 to L2 in the Earth-Moon system is "free" because the L points lie more or less at the same "elevation" (they have very close gravitational potential energies). On the other hand, there's still a fundamental delta-v involved if you want to go from the Earth to Mars.

The standard Hohmann trajectory would have you waste precious energy to get to the top of the sun-earth-mars gravity well and then give you back that gravitational potential energy as kinetic energy--speed--- as you approach Mars. Thus when you get to mars you'll be going something like twice the orbital speed you want and you'll need to slow down (hence the aerobrake). All that extra energy you have to get rid of is unnecessary.

The "zero energy" trajectories are not really zero energy in this case, but "zero extra energy". They'd go around any "gravitational hills" to conserve energy, but there's still a fundamental delta-v involved (which is very large in comparison, you'd probibly lower the delta-v requirement by less than 1km/s in the best realistic circumstances). In addition, these trajectories are very hard to calculate, and like gravity assists, they might be available very infrequently and are likely to add years or even decades to travel time.

Hope that helps.

Bill White · 2004-03-29 08:24:41

The standard Hoffman trajectory would have you waste precious energy to get to the top of the sun-earth-mars gravity well and then give you back that gravitational potential energy as kinetic energy--speed--- as you approach Mars. Thus when you get to mars you'll be going something like twice the orbital speed you want and you'll need to slow down (hence the aerobrake). All that extra energy you have to get rid of is unnecessary.

This makes sense to me. Zero extra energy. Thank You.

So, my original point stands. :;): Spending computational resources to better understand these routes around the hills is a worthwhile endeavor.

Mark Friedenbach · 2004-03-30 01:43:12

This makes sense to me. Zero extra energy. Thank You.
So, my original point stands. :;): Spending computational resources to better understand these routes around the hills is a worthwhile endeavor.

Glad I could help!

Bill White · 2004-04-02 15:51:09

Another link to explain the usefulness of L point trajectories:

[http://www.marsinstitute.info/rd/facult … /in14.html]http://www.marsinstitute.info/rd/facult … /in14.html

In the present paper, Mendell calls L-1 "the gateway to the solar system" because it is always accessible from Earth and provides access to the Weak Stability Boundary (WSB), a system of dynamic pathways linking the L points. "Objects moving within the WSB," he explains, "do not have the kinetic energy necessary to fully escape the Earth-Moon system, but neither are they captured by the Earth itself nor by the Moon itself." Once within the WSB, objects can move between the L points using minimal energy (thus propellant). Mendell notes that the WSB allowed Japan's Hiten to maneuver through multiple lunar encounters in 1990-93, and let AsiaSat reach a useful orbit around Earth after it became stranded by upper stage failure in 1999. He adds that "escape to interplanetary travel is energetically cheap" once a spacecraft is within the WSB. In 1984-85, controllers took advantage of this aspect of the WSB to launch the ISEE/ICE spacecraft past Comet Giacobinni-Zinner. For Mendell, the WSB also serves a political purpose - that of ending the "'Moon vs. Mars' religious wars" which, he writes, have split the community of people interested in piloted solar system exploration. These squabbles have, for example, caused Mars advocates to oppose using the moon as an experience-building stepping stone to Mars. Mendell points out that few people know about the WSB, let alone have an "emotional attachment" to it. "The development of tools, infrastructure, and capability to exploit the WSB," he writes, "can reduce the cost and risk of future exploration programs while avoiding the negative reactions to a decision that could be interpreted as favoring a final destination for planetary development."

SpaceNut · 2015-03-18 18:13:54

RobertDyck wrote:

Terraformer wrote:
Why do you think launching from L1 is a stupid idea?
It takes fuel to get there, and has no advantage. When you measure total propellant starting from the launch site on Earth, it's far less to launch directly from Earth to trans-Mars trajectory. Earth-Moon L1 or L2 are just mathematically calculated positions in space, with nothing there. It takes propellant to get there, so there is a cost with no benefit.
Low Earth Orbit has some benefits. Although stopping in LEO takes more propellant than direct launch, the benefits are radiation protection, and ISS. That is LEO is inside the Earth's magnetosphere, so therefor protected from Radiation. So astronauts working there for any length of time have less radiation exposure. And ISS provides living quarters for astronauts, tools, the large robot arm, and all sorts of support for any on-orbit assembly. If you want any sort of orbital assembly before proceeding to Mars, the only sensible place to do it is ISS.

Tom Kalbfus wrote:

What about using high efficiency thrusters which provide sub-g acceleration? You see one of the big losses in rocket launches from the Earth is that 1st g of acceleration is entirely negated by Earth's gravity at the launch pad. If you accelerate straight upward at 9.81 meters per second all you do is hold your position against Earth's gravity. If you accelerate upwards at 2 g you are only actually accelerating upwards at 9.81 meters per second, though you feel the full 2 g of acceleration as you sit in your acceleration couch. If you launch from Low Earth orbit, you are already traveling at about 8 km/second around the Earth, adding to that would put you in a higher orbit, add 3 more km/sec to your velocity and you will escape the Earth's gravity entirely. Orbit is a good place to assemble large spaceships that can't be launched directly in one piece from the Earth's surface.

SpaceNut · 2015-03-18 18:15:43

RobertDyck wrote:

But what you said is true of "Earth orbit". Why L1? That's close to the gravitational half-way point between two bodies. For the Earth-Moon system, it's much closer to the Moon. Why would you assemble anything at L1? LEO would be far better.

Decimator · 2015-03-19 08:40:26

Spacenut, what's up with all these block quotes with no comments you keep posting?

SpaceNut · 2015-03-19 18:15:31

Its the only method I currently have as a mod to copy posts and to redirect topics of discusion to keep original topics on track....

The L1 location means that a craft is stationary with respect to gravitational pull but its not zero as it still does orbit the sun at that constant distance, so while it makes it still accessible from Earth or the moon, it still does not change the other planets alignment of windows to make use of it as a mission launch location than starting from earth or moon. You might as well have lots of stations just orbiting the sun all at the same distance to each other.

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#26 2004-03-29 07:42:07

Re: Zero energy trajectories - a thread to discuss this topic

#27 2004-03-29 07:43:42

Re: Zero energy trajectories - a thread to discuss this topic

#28 2004-03-29 08:24:41

Re: Zero energy trajectories - a thread to discuss this topic

#29 2004-03-30 01:43:12

Re: Zero energy trajectories - a thread to discuss this topic

#30 2004-04-02 15:51:09

Re: Zero energy trajectories - a thread to discuss this topic

#31 2015-03-18 18:13:54

Re: Zero energy trajectories - a thread to discuss this topic

#32 2015-03-18 18:15:43

Re: Zero energy trajectories - a thread to discuss this topic

#33 2015-03-19 08:40:26

Re: Zero energy trajectories - a thread to discuss this topic

#34 2015-03-19 18:15:31

Re: Zero energy trajectories - a thread to discuss this topic

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