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#1 2026-07-28 12:04:05
- tahanson43206
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- Registered: 2018-04-27
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Spiral Escape from Airless Bodies such as the Moon of Earth
This topic is inspired by a vision of Void, in mid-2026.
The vision (as I understand it) was to use chemical propulsion to reach a velocity sufficient to lift a vessel just above the surface of the Moon. The elevation would need to be great enough not to collide with the ridges of craters on the Moon.
The required elevation should be included in a detail post after this opening.
The intent of this topic is to collect knowledge sufficient to enable a reader to build and deploy a launch system for the Moon or any similar airless body, that would achieve the very best possible efficiency, by using chemical propulsion just long enough to achieve the momentum required to permit use of ion engines or similar highly efficient engines to spiral out from the Moon to achieve escape velocity.
It should be possible to assemble in this topic exact numbers that would govern this launch method.
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#2 2026-07-28 12:07:59
- tahanson43206
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Re: Spiral Escape from Airless Bodies such as the Moon of Earth
This post is reserved for an index to posts that may be contributed by NewMars members.
Needed posts include identification of the minimum elevation to be achieved by chemical propulsion or by a ground based system such as a maglev system to keep the vessel above the tallest obstructions on the object.
Needed posts include identification of the velocity needed for vessels needed at various stages of flight.
Related facts would include details of propellant needed for various masses of vehicle, including both chemical propulsion and ion propulsion.
Index:
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#3 2026-07-28 13:46:49
- GW Johnson
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Re: Spiral Escape from Airless Bodies such as the Moon of Earth
Electric propulsion typically has a very, very small device T/W. When you put it on a spacecraft, that acceleration capability is even smaller, because the W is larger still. We are talking milli-gee to micro-gee acceleration ranges. So it takes a very long time to add any noticeable speed. On the order of weeks to months, typically.
While the electric propulsion is trying to accelerate the craft, you do NOT want the craft to fall back due to gravity, a process measured in minutes. You MUST already be in circular orbit, in order not to fall back when you try to use your electric propulsion! Just getting above mountaintop altitude will NOT be adequate! It will NEVER be adequate! You MUST also have orbital speed, and in the horizontal direction! This is an inherent and absolute requirement of dynamics.
The formula for computing circular orbit speed is the same as the formula for computing escape speed, except that escape has a factor of 2 inside the square root radical that circular does not have. Therefore, for ANY body, at any given radius from is center, escape speed is square root of 2 larger than circular orbit speed.
Stated another way, circular orbit is just about 70.71% of escape speed. This is true at typical orbit altitudes, true on the surface, and true anywhere in between, including at a radius that just clears mountaintops on any airless world.
Whatever the escape speed is at your intended altitude, you are going to have to supply just about 70.71% of that with high-thrust propulsion, before you can even attempt to get the other 29.29% of it with your electric propulsion.
The formulas are: Vcirc = sqr rt(GM/R) and Vesc = sqr rt(2 GM/R) = sqr rt (2) * Vcirc, where R = Rbody + h, and h is your desired altitude. M is the body mass of the world, and G is the universal gravitation constant. As written, these formulas require consistent units of measure.
Vesc - Vcirc = 29.29% of Vesc, is the min ideal dV you need from your electric propulsion to reach escape from vircular orbit (design actual will be substantially higher due to gravity losses as you spiral out). Vcirc = 70.71% Vesc, is the min ideal dV you must get out of your chemical (or nuclear) high-thrust propulsion. If there are launch gravity losses (and there will be!), you must add them to that min dV = Vcirc, for the design dV of you launcher stage(s).
Sorry, them's just the ugly little facts of life. This can be done, but it will not be a "miracle of savings".
GW
Last edited by GW Johnson (2026-07-28 13:59:35)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#4 2026-07-29 10:19:46
- tahanson43206
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Re: Spiral Escape from Airless Bodies such as the Moon of Earth
GW Johnson contributed a new image to this topic, and text to go with it.
Meanwhile, Void has published a collection of data about various lunar orbits, with data about ones that are stable and the vast majority that are not due to mascons below the surface. (mass concentrations).
I ran the orbits spreadsheet for Earth's moon as the primary, at its
average radius to support any surface location, for an ellipse to ascend
from h = 0 km to only h = 10 km to clear any mountains. The results I got
are in the attached png sketch. Post it if you like, in that topic. Use
the text of this message with it, if you like.However one chooses to achieve it, the launch speed of just about 1.7 km/s
must be achieved, *and in the correct not-quite horizontal direction!* If
done with some sort of surface EM accelerator, presumably a straight track
device, there CANNOT be any mountains in the way of the ascent path, for
about a quarter of the way around the moon. The dV to circularize at
apoapsis is almost trivial at 2 to 3 m/s.The real problem here is mountains in the way, early in the ascent, when
your path is anywhere from 0 to at most 3-to-5 km above the surface. The
speed at release from an accelerator track MUST be in the correct pathwise
direction, otherwise you WILL NEVER enter the intended ascent ellipse!
And at speeds near 1.7 km/s, you CANNOT TOLERATE bends in the accelerator
track. It's an a = V^2/R thing for too many gees to tolerate. There will
be very few locations on the moon with a clear shot at reaching very low
orbit that way.The solution to this location dilemma is aiming for a higher circular
orbit. That is the only way to achieve higher path altitudes immediately
after release, that do not smack into mountains within a quarter of the
moon's circumference from the launch site.If you do an accelerator track launch, you must aim a few degrees up
(something like 1 to maybe 3 degrees), to miss the nearer mountains. That
INHERENTLY puts you onto a suborbital trajectory, with a non-trivial dV to
reach circular orbit speed at the apoapsis point. That's a study for which
I do not intend to put in the effort to do, right now.BTW, anyone can do this with the orbits spreadsheet I created for the
"orbits+" course materials. Even Void could do it, for himself.GW
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