Orbits Q&A

[baldusi]

I can't really understand the Lissajous orbit, so I'm going to ask. Can you bias a Lissajous orbit around EML2, so that you spend more time looking to the moon poles than the equator? In other words, can you make it have an "apogee" perpendicular to the moon orbit plane and the "perigee" when it intersect it?
I'm thinking of having three satellites on such an orbit for permanent LOS to the poles, sort of like the Russians do with their Molnyia orbits.

[MP99]

not exactly sure where to ask it so i will ask it here.
I am going to be talking to my physics students about using EML-1 as a hypothetical spot to build a spacestation and then launch a probe to mars.  To get the probe to mars do they do a small burn to get it into a large elliptical orbit around earth (apogee of EML1 distance and perigee altitude of about 200km(?) ) and then do a burn at perigee to get the probe into the earth mars transfer orbit?
is 1) this right way to do it..

The answer here is "it depends".

If your probe has a very weak but high-Isp engine (IE Solar Electric Propulsion, like Dawn), then your probe doesn't want to struggle out of Earth's gravity well. Dawn only fired it's SEP once Delta II had injected it past escape velocity.

This mode would also work spiralling away from EML, after you'd perhaps used a chemical stage to deliver your probe to the Moon, though the Dawn mode (chemical injection past escape, then SEP in interplanetary space) is probably better.



If your probe has a powerful rocket engine, then you get an advantage from the Oberth effect if it perform it's TMI burn as close as possible to the Earth. If it starts in the vicinity of the Moon, a relatively small burn will leave the Moon and put the probe into an elliptical orbit that has it's perigee at LEO. The Oberth benefit at perigee when it does the TMI is bigger than the cost of the departure burn from the Moon.

But, it's worth noting that EML is not a free lunch. Imagine your spacecraft was launched through TLI, but the Moon happened to be in the wrong part of the sky. The s/c would end up in an elliptical orbit around the Earth with it's apogee near the Moon's orbit and it's perigee at LEO. The s/c could perform it's TMI burn at any subsequent perigee and gain the Oberth benefit mentioned above.

In order to go to EML you need to add some velocity to circularise the orbit. Then, you need another burn to reverse that and leave EML again, and further dV to reduce the orbit's perigee down to LEO, and that's just to put you back into essentially the same orbit you were in just after you'd finished TLI. That's quite a lot of extra dV, and is badly counter-productive if all the manoeuvres are performed by a single stage. Far more efficient if your stage just performs a simple TMI directly from LEO immediately after it's launched, exactly like all current Mars missions have done.



To get any benefit from EML you need a more complicated mission plan - some form of assembly of components:-

A chemical stage that boosts your stage through TLI, docks and gets refueled at EML, before leaving, reducing perigee and performing TMI (via the Lunar flyby).

Or, the probe and it's TMI stage delivered separately to EML, docking together and then performing TMI. This mode would work for either a chemical or SEP TMI stage.



As far as getting to EML, if the element can survive a long transit (months) then it can can get to EML quite efficiently using a ballistic capture trajectory. For quick transits, EML2 needs less total dV. This may allow crew to quickly transit to a hab or TMI stage that is bigger because it was able to get there via a slow-boat trajectory.

For chemical propulsion, I believe you can get from EML2 into that elliptical orbit that takes advantage of the Oberth effect with less dV than from EML1.

cheers, Martin

[MP99]

B) Even if reality met the assumptions of the CR3BP model it would be essentially impossible to balance exactly at a lagrange point without some station-keeping, as even the tiniest deviation leads onto trajectories that head away from the lagrange point. (Thus the expectation is that a spacestation would instead be in a stable "orbit" around a lagrange point.)

did not realize that it could not actually stay there.  I realized they could orbit though.  I'm assuming the"field lines" for the diagram attached are gravitational potential field lines?  Or am I off base there as well.

See http://en.wikipedia.org/wiki/Halo_orbit.

For EML2 this is actually an advantage, because the probe is not eclipsed by the Moon, so can communicate with the Earth.

cheers, Martin

[jabe]

As far as getting to EML, if the element can survive a long transit (months) then it can can get to EML quite efficiently using a ballistic capture trajectory. For quick transits, EML2 needs less total dV. This may allow crew to quickly transit to a hab or TMI stage that is bigger because it was able to get there via a slow-boat trajectory.

For chemical propulsion, I believe you can get from EML2 into that elliptical orbit that takes advantage of the Oberth effect with less dV than from EML1.

cheers, Martin

some great points i hadn't thought of, Ion vs chemical.
I have both of Bruno's books. his Fly me to the moon  is excellent.  A MUST read for anyone who has interest in non hohmann orbits..VERY easy read. His other one is just a tad over my head but interesting to sorta follow.
My premise to the discussion is that  station is there and we have a probe ready to fly..so what best to do..  For high school can't get too much into math..just the general train of thought.

I appreciate everyones input..

jb

[aero]

What is BEO?

[mmeijeri]

What is BEO?

Beyond Earth Orbit, often confused with beyond Low Earth Orbit. The moon is beyond LEO, but not BEO.

[truth is life]

Stimulated by some comments made [topic=29081.150]here[/topic] simonbp and jcm, I was wondering some things about the orbits into which space observatories are put:

1: Clearly, a lot of observatories benefit from being outside of LEO, in highly elliptical orbits or even at SEL1/2. Just as clearly, a lot of observatories don't, or at least don't benefit enough for it to be worthwhile paying for the bigger rocket to escape LEO. What sorts of factors lead to the selection of the observatory's orbit? Clearly cost and launch vehicle capability is one of them, but are there any frequency bands which benefit particularly from certain locations, for example?

2: At what point were the possible advantages of these alternative orbits, especially SEL-2, identified? The 1960s, the 1970s, later? Because of the Shuttle and the possibility of repair and refurbishment, in the 1970s and 1980s many observatories were going to be placed into LEO, correct? (I know at least three, Hubble, Compton, and SIRTIF were). Do you think that without Shuttle there might have been more consideration of alternate orbits for these vehicles?

(This seemed to belong here, since it *is* about orbits...)

[Jim]

2: At what point were the possible advantages of these alternative orbits, especially SEL-2, identified? The 1960s, the 1970s, later? Because of the Shuttle and the possibility of repair and refurbishment, in the 1970s and 1980s many observatories were going to be placed into LEO, correct? (I know at least three, Hubble, Compton, and SIRTIF were).

Where did SIRTIF eventually go?

GRO was gamma rays which are not affected by anything.

[mmeijeri]

At what point were the possible advantages of these alternative orbits, especially SEL-2, identified?

Interesting question. The Lagrange points have been known for a very long time, having been discovered by Euler (hence the name ), but to the degree early exploration plans used high energy staging orbits, they focused on highly elliptical ones, or perhaps general high altitude circular orbits, not Lagrange points specifically.

[truth is life]

Where did SIRTIF eventually go?

SIRTIF eventually became Spitzer and ended up in a heliocentric orbit. But it was originally supposed to be a Spacelab pallet and get carried up by Shuttle for week-long missions. Obviously the Challenger disaster killed any idea of Shuttle rapidly flying astronomical payloads, and then Dan Goldin's tenure led to a lot of changes which delayed the first flight. But way back in the early '80s, which is what I was asking about, it was supposed to be in LEO, as a Shuttle payload.

GRO was gamma rays which are not affected by anything.

Well, the Earth is certainly opaque to them...

I guess I'm not quite understanding what you're trying to say here? There isn't a lot in space (on Solar System levels, ignoring interstellar extinction and the like) which affects EM radiation. There's zodiacal light, especially for infrared telescopes; there's not burning out your delicate scope by pointing it at an excessively intense source, like the lit Earth or the Sun; and there's not exposing your telescope to too much radiation and screwing with the electronics. There's nothing exceptionally special about gamma rays in that regard, except possibly for the electronics being less sensitive.

The off-hand comment was just pointing out that Hubble and Compton were put in LEO, and could not have been put into a higher orbit, and that SIRTIF was originally just a Spacelab pallet, and not capable of independent flight (although they had some notion of possibly adapting it to free-flight later).

@mmeijeri: Lagrange, not Euler Sorry for being a tad pedantic, but...eh, I'm a physicist, so...

[mmeijeri]

@mmeijeri: Lagrange, not Euler Sorry for being a tad pedantic, but...eh, I'm a physicist, so...

The collinear ones had already been discovered by Euler. Apparently Euler already had so many things named after him that they decided to give the honour to Lagrange... Very often these things aren't named after their original discoverer.

[Jim]

Where did SIRTIF eventually go?

SIRTIF eventually became Spitzer and ended up in a heliocentric orbit. But it was originally supposed to be a Spacelab pallet and get carried up by Shuttle for week-long missions. Obviously the Challenger disaster killed any idea of Shuttle rapidly flying astronomical payloads, and then Dan Goldin's tenure led to a lot of changes which delayed the first flight. But way back in the early '80s, which is what I was asking about, it was supposed to be in LEO, as a Shuttle payload.

GRO was gamma rays which are not affected by anything.

Well, the Earth is certainly opaque to them...

I guess I'm not quite understanding what you're trying to say here? There isn't a lot in space (on Solar System levels, ignoring interstellar extinction and the like) which affects EM radiation. There's zodiacal light, especially for infrared telescopes; there's not burning out your delicate scope by pointing it at an excessively intense source, like the lit Earth or the Sun; and there's not exposing your telescope to too much radiation and screwing with the electronics. There's nothing exceptionally special about gamma rays in that regard, except possibly for the electronics being less sensitive.

The off-hand comment was just pointing out that Hubble and Compton were put in LEO, and could not have been put into a higher orbit, and that SIRTIF was originally just a Spacelab pallet, and not capable of independent flight (although they had some notion of possibly adapting it to free-flight later).

What was saying is that GRO went into LEO because a higher orbit would not have brought any real benefits, where SIRTF did.  There were many other things than Golden that led to its many delays.  One was affordability as a Titan IV Centaur payload

The shuttle paradigm thinking also limited what was to be done.

[Robotbeat]

@mmeijeri: Lagrange, not Euler Sorry for being a tad pedantic, but...eh, I'm a physicist, so...

The collinear ones had already been discovered by Euler. Apparently Euler already had so many things named after him that they decided to give the honour to Lagrange... Very often these things aren't named after their original discoverer.

Freaking Euler. Is there anything he didn't do?

[truth is life]

On a completely different note, I was looking at the proposed Exploration Platform at EML-2 the other day, and got to thinking about the powered swing-by maneuver that really helps L-2's economics (compared to L-1). It occurred to me that during the powered swing-by, you have to pass quite close to the Moon, so that you could take the lander, undock, perform an LOI (to a phasing orbit; I assume that the swingby is not necessarily positioned perfectly for the desired landing site) and land, all the while Orion goes on and does the swing-by, then rendezvous and docks with the platform. It seems that this could provide a somewhat better performance than taking the lander all the way to L-2 then returning it to the Moon, albeit at a (small, I suspect, due to the presence of the platform) safety risk. It would also reduce the time spent in deep space, since the crew would be on the lunar surface while Orion was climbing towards L-2 (a saving of about three days), and would not need to undergo a transit from the platform to the Moon's surface (another three days less).

Assuming, however, that global access is a requirement, would the LOI burn require a large plane change? Or could the departure trajectory from Earth be selected to allow coplaner injection into whatever phasing orbit you want? Put another way, to what extent are the large plane changes demanded by ESAS-type all-LOR architectures determined by global access requirements versus a desire for "anytime return" that isn't relevant in an L-2 based architecture?

[baldusi]

Do Mars missions that land do a direct insertion? In other words, they come at planetary speeds and brake directly to the atmosphere? Thus, do the Mars orbiters need to spend fuel to break into orbit?
If that is the case, a Mars Synchronous Orbit would require more delta-v than a lander, but less than Low Mars Orbit orbiter?
Any idea of how much extra delta-v after a TMI for a MSO orbit?