## Orbits Q&A

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IsaacKuo
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 « Reply #105 on: 07/05/2012 09:24 PM »

The requirements for the plane change are rather complex, and depend on the exact timing and details of the transfer between Earth and Mars.  In the ideal situation, no plane change is required at all--you simply insert directly into the desired orbit plane.  This desirable situation is possible when the asymptotic approach path of your transfer orbit is parallel to any line which lies on the desired orbit plane.
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 « Reply #106 on: 07/05/2012 09:48 PM »

So is there an easy way (excel spreadsheet like the one above?) to calculate the approximate dv for just a plane change?

-MG.

If you're changing direction but not speed, the before and after velocity vectors are sides of an isosceles triangle with the plane change vector as base.

With each isosceles side having the magnitude v, magnitude of plane change vector is 2 * v(sin(a/2)) where a is angle of plane change.

mgfitter
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 « Reply #107 on: 07/05/2012 09:56 PM »

Thanks. Can I check my math quick?

a = 15 deg.
v = 7,800 m/s (roughly 200km circular).

So 2 * v(sin(a/2)) = 2,036 m/s, right?

-MG.
Hop_David
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 « Reply #108 on: 07/05/2012 10:18 PM »

Thanks. Can I check my math quick?

a = 15 deg.
v = 7,800 m/s (roughly 200km circular).

So 2 * v(sin(a/2)) = 2,036 m/s, right?

-MG.

You got it.

If you wanted a plane change and a speed change, the triangle would no longer be isosceles. Then the Law of Cosines could do the job.
IsaacKuo
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 « Reply #109 on: 07/05/2012 10:25 PM »

Sorry I misunderstood the question.  I was assuming the question was about the plane change requirements to get from a Earth-to-Mars transfer orbit to an equatorial synchronous orbit.  In other words, you don't know the angle of the plane change beforehand.

If you do know the angle of the desired plane change, then yes it's a simple calculation.

But the required angle for getting to an equatorial Mars orbit could be anything from 0 degrees to 25 degrees, depending on the details of the transfer orbit (including the date of arrival).
mgfitter
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 « Reply #110 on: 07/06/2012 04:42 AM »

You got it.

If you wanted a plane change and a speed change, the triangle would no longer be isosceles. Then the Law of Cosines could do the job.

Thanks. Circular PC's are good enough for me for now, but I now know who to ask if I have more complex questions in the future. Thanks for the education!

-MG.
baldusi
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 « Reply #111 on: 07/06/2012 02:36 PM »

Thank you for all this replies, you are being very instructive and exteremely helpful!
kevin-rf
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 « Reply #112 on: 07/31/2012 02:43 PM »

Does anyone know the DeltaV and time required to reach the Sun/Earth L3 Lagrangian point?

Yes I know communications would be "non-trivial" (Jim would say non starter), but just curious about it as an alternative for Space Weather Stereo type missions.
baldusi
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 « Reply #113 on: 07/31/2012 02:59 PM »

Does anyone know the DeltaV and time required to reach the Sun/Earth L3 Lagrangian point?

Yes I know communications would be "non-trivial" (Jim would say non starter), but just curious about it as an alternative for Space Weather Stereo type missions.
It's a trade off of delta-v and delta-t. Once you reach C3=0, you could have an arbitrary bigger orbit, and drift one way or the other, but it would take longer. Also, you have to consider in which direction you leave on C3=0, because if you leave sunwards or anti sunward, you'd already be on a slightly different orbit. once you reach L3, you have to calculate the delta-v to reach L3. Technically, I think that if you use a very slightly bigger orbit, you'd be pull in. But it would be really slow.
mgfitter
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 « Reply #114 on: 08/03/2012 03:18 PM »

Is there actually a stable orbit that would take a craft in a permanent figure-of-8 around both the Earth and the Moon?

If there is, can someone shed some light on what sort of parameters it would have?   Perigee and Perilune, orbital duration etc.

I assume such an orbit wouldn't be completely stable and would require some RCS activity on each orbit, but I'm curious whether some form of "transport hub" in such an orbit might not be a workable idea.

Thanks!

-MG.
QuantumG
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 « Reply #115 on: 08/03/2012 03:22 PM »

Is there actually a stable orbit that would take a craft in a permanent figure-of-8 around both the Earth and the Moon?

If there is, can someone shed some light on what sort of parameters it would have?   Perigee and Perilune, orbital duration etc.

I assume such an orbit wouldn't be completely stable and would require some RCS activity on each orbit, but I'm curious whether some form of "transport hub" in such an orbit might not be a workable idea.

See http://cbboff.org/UCBoulderCourse/documents/LunarCyclerPaper.pdf
sdsds
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 « Reply #116 on: 08/03/2012 09:26 PM »

Is there actually a stable orbit that would take a craft in a permanent figure-of-8 around both the Earth and the Moon?

See http://cbboff.org/UCBoulderCourse/documents/LunarCyclerPaper.pdf

Wow, from the "AAS 91-105" is it correct to conclude this paper was published in 1991? That's awhile ago! These trajectories would seem to provide strong competition to those that orbit near the Lagrange points. Has there been any comparison of those two classes of trajectories as possible locations for a prototype deep space habitat?
truth is life
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 « Reply #117 on: 10/31/2012 10:21 PM »

Short question here: Supposing I have a known C3 and have a known delta-V capability, how can I calculate my final C3 should I choose to use said delta-V capability to the utmost?
Proponent
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 « Reply #118 on: 11/05/2012 03:27 AM »

The quantity c3 is just twice the specific mechanical energy, i.e., twice the sum of the kinetic energy per unit mass and the potential energy per unit mass.  At radius r and speed v the value of c3 is

v2/2 - GM/r = c3/2 .

Solve this for v using the desired c3, and that tells you the speed needed at radius r.  Subtract the initial speed, and you've got the necessary delta-V, assuming no losses.
baldusi
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 « Reply #119 on: 01/28/2013 09:39 PM »

There's something I don't get about GTO launch windows. GSO should be the only orbit that doesn't care about your RAAN, thus, only your geographic latitude should matter. But I've seen lots of times launch windows. Even on the Ariane 5 manual shows the payload penalties around the year. Why is that? Are solar and thermal issues important (like the recent Briz-M failure)? something to do with GTO and the desired orbital slot?
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