Author Topic: Orbits Q&A  (Read 84768 times)

Offline Danderman

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Re: Orbits Q&A
« Reply #20 on: 03/07/2011 05:42 PM »
Re-stating my question would be helpful.

Let's say you have a spacecraft (MAVEN) and for whatever reason you want it to leave the vicinity of Earth on a specific date: December 7, 2013, no matter what. Therefore, a backup probe and rocket are irrelevant unless magic intervenes and they can be launched on December 7, as well if the prime mission fails (so let's ignore the possibility of a backup).

The other requirements are that the trans-Mars burn (or at least the major part of the burn) occur on the day of launch, and that transits through the Van Allen belt should be minimize. This means that long duration parking orbits in LEO or HEO really aren't useful.

What I am looking for is to see if the spacecraft could be launched weeks or months ahead of time into a heliocentric orbit that would later intercept the Earth's orbit while the Earth is there, and use the gravity of the Earth for a swingby on December 7 2013 to go to Mars.  If this is possible, what are the drivers for the duration of the loop away from the Earth? Its mentioned above that some fraction of a year is required, but what determines the size of the fraction? Is that fraction actually a full year?

 ??? ??? ??? ???

Offline mmeijeri

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Re: Orbits Q&A
« Reply #21 on: 03/07/2011 05:49 PM »
What's wrong with using a Lagrange point? The penalty is no more than tens of m/s, and that doesn't count any reduction in gravity losses through lower thrust requirements.
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Offline Danderman

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Re: Orbits Q&A
« Reply #22 on: 03/07/2011 05:51 PM »
What's wrong with using a Lagrange point? The penalty is no more than tens of m/s, and that doesn't count any reduction in gravity losses through lower thrust requirements.

Maybe that's a good choice if the actual delta v requirements to enter and depart the LaGrange point is that low. And assuming that a return to the vicinity to Earth for a gravitation swingby could be achieved from that location.

Offline IsaacKuo

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Re: Orbits Q&A
« Reply #23 on: 03/07/2011 05:51 PM »
Let's say you have a spacecraft (MAVEN) and for whatever reason you want it to leave the vicinity of Earth on a specific date: December 7, 2013, no matter what.
[...]
The other requirements are that the trans-Mars burn (or at least the major part of the burn) occur on the day of launch, and that transits through the Van Allen belt should be minimize. This means that long duration parking orbits in LEO or HEO really aren't useful.

Why not?  Neither would imply spending more time in the Van Allen belts.

Quote
What I am looking for is to see if the spacecraft could be launched weeks or months ahead of time into a heliocentric orbit that would later intercept the Earth's orbit while the Earth is there, and use the gravity of the Earth for a swingby on December 7 2013 to go to Mars.

No.  It is not possible.  In order to intercept the Earth again in such a short time, the aphelion must be small.  This lacks the orbital energy required to reach Mars.  An unpowered gravity assist will not help.

You could use a powered gravity assist, but this will not save you any fuel compared to simply going straight for Mars on the desired date (either from LEO or from Earth's surface).

But my big question for you is--what is the point?  What are you trying to accomplish with this idea?  Why not simply launch during the normal launch window?

Offline sdsds

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Re: Orbits Q&A
« Reply #24 on: 03/07/2011 06:09 PM »
Tha ballistic transfers described by Parker (http://ccar.colorado.edu/nag/papers/AAS%2006-132.pdf) are your friends.  They just barely enter the realm of solar influence before returning to e.g. the L1 attractor.
« Last Edit: 03/07/2011 06:09 PM by sdsds »
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Offline Danderman

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Re: Orbits Q&A
« Reply #25 on: 03/07/2011 06:25 PM »
Quote
What I am looking for is to see if the spacecraft could be launched weeks or months ahead of time into a heliocentric orbit that would later intercept the Earth's orbit while the Earth is there, and use the gravity of the Earth for a swingby on December 7 2013 to go to Mars.

No.  It is not possible.  In order to intercept the Earth again in such a short time, the aphelion must be small.  This lacks the orbital energy required to reach Mars.  An unpowered gravity assist will not help.

You could use a powered gravity assist, but this will not save you any fuel compared to simply going straight for Mars on the desired date (either from LEO or from Earth's surface).

I think you are getting close to nailing the problem - since the requirement is for most of the trans-Mars burn to occur on the day of launch (for various reasons), then the duration of the loop away from the Earth will be a function of the magnitude of that burn. If the burn is Earth C3 + a small delta-v, yep, the aphelion will indeed be small.

On the other hand, if the Earth escape burn is, say, 100 m/s under what is required to reach Mars, then the loop away from the Earth will take years.

So, if the initial burn is small enough that the loop away from the Earth is short, the resulting Earth swingby on December 7 2013 must incorporate a fairly significant burn to enable the spacecraft to actually get to Mars.  This is not an "extra" burn in the sense of requiring more propellant than a direct injection to Mars on December 7 2012, but it does entail the risk of splitting the trans-Mars injection into two distinct burns, precisely what killed the Nozomi mission.
« Last Edit: 03/07/2011 06:27 PM by Danderman »

Offline mmeijeri

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Re: Orbits Q&A
« Reply #26 on: 03/07/2011 06:34 PM »
Maybe that's a good choice if the actual delta v requirements to enter and depart the LaGrange point is that low. And assuming that a return to the vicinity to Earth for a gravitation swingby could be achieved from that location.

Based on the delta-v's for ballistic transfers to L1/L2 and the cost of an insertion burn into an L1/L2 orbit on a fast transfer trajectory (which should be equal to the cost of a fast Earth swingby) it would be that cheap. Huntress' exploration architecture was based around use of Lagrange points. You could do a swingby from EML1/2 or, as Farquhar figured out, even more efficiently (but less flexibly as Kirk Sorensen pointed out) both an Earth and a moon swingby from SEL1/2.
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Offline IsaacKuo

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Re: Orbits Q&A
« Reply #27 on: 03/07/2011 06:39 PM »
I think you are getting close to nailing the problem

What is the problem?

Quote
- since the requirement is for most of the trans-Mars burn to occur on the day of launch (for various reasons), then the duration of the loop away from the Earth will be a function of the magnitude of that burn. If the burn is Earth C3 + a small delta-v, yep, the aphelion will indeed be small.

Huh?  You can make the aphelion pretty much whatever you want.  The reason I said you would need a small aphelion is because you want to return to Earth for some bizarre reason within a few weeks or months.  You can't go far away from Earth and expect to return so soon.

Quote
So, the Earth swingby must incorporate a fairly significant burn to enable the spacecraft to actually get to Mars.  This is not an "extra" burn in the sense of requiring more propellant than a direct injection to Mars on December 7 2012, but it does entail the risk of splitting the trans-Mars injection into two distinct burns, precisely what killed the Nozomi mission.

Actually, this second burn would be relatively small.  You already have greater than escape velocity--meaning you come in with a positive C3 already.  You only need to boost your C3 by a small amount to get to the desired transfer orbit to Mars.  Maybe 300m/s delta-v or less, depending on the specifics.  It's small change compared to the 11+km/s you needed to escape Earth in the first place.

Anyway, the Nozomi mission involved using the Moon for gravity assist maneuvers.  Like I said, the existence of the Moon allows for various possibilities.

So again--what is the point of all of this?  You keep saying there is some "problem", but what is the "problem" you are concerned with?

Offline Danderman

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Re: Orbits Q&A
« Reply #28 on: 03/07/2011 06:40 PM »
You could do a swingby from EML1/2 or, as Farquhar figured out, even more efficiently (but less flexibly as Kirk Sorensen pointed out) both an Earth and a moon swingby from SEL1/2.

What would you gain from the lunar swingby?

Offline mmeijeri

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Re: Orbits Q&A
« Reply #29 on: 03/07/2011 06:43 PM »
What would you gain from the lunar swingby?

I got the impression the idea was to gain efficiency through use of two powered swingbys.
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Offline Danderman

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Re: Orbits Q&A
« Reply #30 on: 03/07/2011 06:44 PM »
What would you gain from the lunar swingby?

I got the impression the idea was to gain efficiency through use of two powered swingbys.

You are correct, but I am always on the lookout for a free lunch!

If you go back to the premise of this thread, the requirement for all this is to absolutely positively ensure an Earth departure date for Mars on December 7 2013. Gaining efficiencies as part of the architecture is a desirement, not a requirement.
« Last Edit: 03/07/2011 07:02 PM by Danderman »

Offline Danderman

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Re: Orbits Q&A
« Reply #31 on: 03/07/2011 07:47 PM »
You could do a swingby from EML1/2 or, as Farquhar figured out, even more efficiently (but less flexibly as Kirk Sorensen pointed out) both an Earth and a moon swingby from SEL1/2.

It appears that the Delta-V required to achieve EML1 or EML2 is fairly high for a direct injection (3.8 meters/second) - I am not sure how much of this could be recaptured after an Earth swingby for a Mars trajectory.

Offline mmeijeri

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Re: Orbits Q&A
« Reply #32 on: 03/07/2011 07:59 PM »
It appears that the Delta-V required to achieve EML1 or EML2 is fairly high for a direct injection (3.8 meters/second) - I am not sure how much of this could be recaptured after an Earth swingby for a Mars trajectory.

The injection into a fast transfer trajectory is a little under 3.2km/s while the injection into a slow one is a little over 3.2km/s. The fast trajectory requires a ~0.6km/s insertion burn, while the slow one doesn't require one at all, instead relying on perturbation of the transfer orbit by the Sun. After a 0.6km/s perigee lowering burn for the swingby you should recover the 3.2km/s over LEO orbital velocity at perigee. This suggests the penalty is ~0.6km/s and not the mere tens of m/s I mentioned above. Of course in the case of an MTV that still disregards the large savings of cycling between the edges of gravity wells instead ascending / descending them completely.

EDIT: I suspect you could use the ballistic trajectory in reverse too, in which case the penalty should really be mere tens of m/s be zero(?), but at the cost of another 100 days travel time, which may or may not be an advantage for a manned mission. This would be similar to Farquhar's SEL1/2 scheme.
« Last Edit: 03/07/2011 08:09 PM by mmeijeri »
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Offline Danderman

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Re: Orbits Q&A
« Reply #33 on: 03/07/2011 09:57 PM »
It appears that the Delta-V required to achieve EML1 or EML2 is fairly high for a direct injection (3.8 meters/second) - I am not sure how much of this could be recaptured after an Earth swingby for a Mars trajectory.

The injection into a fast transfer trajectory is a little under 3.2km/s while the injection into a slow one is a little over 3.2km/s. The fast trajectory requires a ~0.6km/s insertion burn, while the slow one doesn't require one at all, instead relying on perturbation of the transfer orbit by the Sun. After a 0.6km/s perigee lowering burn for the swingby you should recover the 3.2km/s over LEO orbital velocity at perigee. This suggests the penalty is ~0.6km/s and not the mere tens of m/s I mentioned above. Of course in the case of an MTV that still disregards the large savings of cycling between the edges of gravity wells instead ascending / descending them completely.

EDIT: I suspect you could use the ballistic trajectory in reverse too, in which case the penalty should really be mere tens of m/s be zero(?), but at the cost of another 100 days travel time, which may or may not be an advantage for a manned mission. This would be similar to Farquhar's SEL1/2 scheme.

Yeah, I was looking for a free lunch. I guess the point solution for my particular question is a heliocentric orbit that returns to the vicinity of the Earth when the Earth is there, that's the quick and easy approach.

Offline Proponent

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Re: Orbits Q&A
« Reply #34 on: 03/08/2011 07:19 AM »
You can't use an unpowered gravitational assist to gain more orbital energy than you could have started with.

Actually, it is possible and is done.  Here's Wikipedia's explanation.  Basically, viewed in the frame of the planet, the spacecraft's energy is conserved, and it arrives and departs at the same speed though different directions.  In the sun's frame, however, the spacecraft can gain or lose energy.  Energy is still conserved, because in the sun's frame the planet's speed changes a (very tiny) bit too.
« Last Edit: 03/08/2011 07:21 AM by Proponent »

Offline IsaacKuo

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Re: Orbits Q&A
« Reply #35 on: 03/08/2011 01:04 PM »
You can't use an unpowered gravitational assist to gain more orbital energy than you could have started with.

Actually, it is possible and is done.  Here's Wikipedia's explanation.  Basically, viewed in the frame of the planet, the spacecraft's energy is conserved, and it arrives and departs at the same speed though different directions.  In the sun's frame, however, the spacecraft can gain or lose energy.  Energy is still conserved, because in the sun's frame the planet's speed changes a (very tiny) bit too.

That's true in general, but this was in reference to a specific situation--using Earth only (and pretending that the Moon doesn't exist).  You could only use it to gain energy up until what you could have started with--by thrusting directly along the path of Earth's orbit.  You can change your direction relative to Earth, but not your speed.  So your maximum speed relative to the Sun is equal to your speed relative to Earth plus Earth's orbital speed.

Offline baldusi

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Re: Orbits Q&A
« Reply #36 on: 03/18/2011 03:19 PM »
If you were in the equator, in the middle of the Atlantic. Would it be easier, worse or the same than being in Antartica for launching polar orbits? Or SSO?

Offline kevin-rf

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Re: Orbits Q&A
« Reply #37 on: 03/18/2011 04:02 PM »
Polar orbits are unable to use that extra bump from the earths rotation, and actually have to counter it. So yes you take a payload hit. But the name of the game is logistics, not the optimal launch profile. Antarctica would be darn expensive to operate out of.
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Offline baldusi

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Re: Orbits Q&A
« Reply #38 on: 03/18/2011 04:16 PM »
Polar orbits are unable to use that extra bump from the earths rotation, and actually have to counter it. So yes you take a payload hit. But the name of the game is logistics, not the optimal launch profile. Antarctica would be darn expensive to operate out of.
what about Ushuaia? It's got an airport, a deep water port and it's home to electronics manufacturing. They are also developing a deep space radar station around there.
« Last Edit: 03/18/2011 04:17 PM by baldusi »

Offline Jim

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Re: Orbits Q&A
« Reply #39 on: 03/18/2011 04:35 PM »
Polar orbits are unable to use that extra bump from the earths rotation, and actually have to counter it. So yes you take a payload hit. But the name of the game is logistics, not the optimal launch profile. Antarctica would be darn expensive to operate out of.
what about Ushuaia? It's got an airport, a deep water port and it's home to electronics manufacturing. They are also developing a deep space radar station around there.

Plesetsk is closer to a pole

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