Why L1 for a staging point?

[samkent]

I keep reading in various posts where people suggest using the L1 or L2 points as a good place to launch missions from. I understand the benefits of L1 for a mission such as SOHO, but not as a staging point.<br /> <br />What would you gain from this over LEO?<br /><br />Wouldn’t you have to make a burn to get to L1 and make another burn to stop at L1? And then another burn to leave L1 for your destination? Isn’t that a waste of propellant?<br />

 

[venator_3000]

I think it comes down to the type of spacecraft you are using and where, exactly, you might want to go.<br /><br />If you were going to the asteroids or Mars it might be better to leave from the Earth-Sun L2 point. This is because your space cruiser would need less energy (delta-V) to get to your destination from L2 than from LEO. If you left from L2 in your cruiser you could use a series of lunar gravity-assists ("slingshots") and a perigee rocket maneuver to enter on the interplanetary flight path to say, Mars. <br /><br />That isn't to say it wouldn't be without drawbacks. You would need a vessel to get the crew from Earth to the ship waiting at L2. This would add cost as well as time. Also, on the return trip you would need to rendezvous with a ship to get you home from L2 and back to Earth. <br /><br />Still, if you planned to use your space cruiser again and again L2 could serve as a hub for a cruiser that repeatedly cycled between earth and Mars. This might also be a safer port of call than LEO if the space cruiser had a nuclear engine.<br /><br />So there would be pluses and minuses to this concept.<br /><br />v3k <div class="Discussion_UserSignature"> </div>

 

[samkent]

But getting the fuel and oxidixer to the L1 point to serve as a depot, requires fuel as well. I'd be interested to see the delta v numbers for LEO to Mars direct vs LEO to L1 then Mars. It still sounds like it would take more energy than a direct path.<br /><br />Has Nasa used L1 as a stop over for any planetary missions?

 

[venator_3000]

I believe the first may have been the International Sun–Earth Explorer. It was launched in the late (70s?) and used a halo orbit around the L1 point and was the first libration point facility. It later used a bunch of lunar gravity-assist maneuvers to help its voyage of mapping the trailing edge of the Earth's magnetic field. The mission planners really extended the mission as it used a lunar gravity slingshot to send it into an orbit to encounter a comet. This was before Giotto to Halley, etc. <br /><br />I believe many other satellites have since used this location including SOHO and the upcoming JWST. However, this orbit will be their home and they will not go on planetary voyages.<br /><br />I don't have exact numbers for you but the energy budgets from a libration point are typically 1/2 of the delta v needed from LEO. But again, getting the cargo, propellants, and people to L1 is no easy feat, and the delta vee budgets for getting people there quickly vs getting cargo there slowly will be different.<br /><br />v3k <div class="Discussion_UserSignature"> </div>

 

[josh_simonson]

EML1 is gravitationally a hilltop (actually more like a mountain pass), which is why the fuel needed to stop at L1 is small. The benefit is that the Delta-V to most places from there is also small. This is particularly attractive for an L1R architecture because most of the Delta-V comes in getting to L1, and the bulk stuff can take a high-isp route while people in a minimal vehicle can take a fast but low ISP route. <br /><br />L1 has some other benefits over LLO, it's fairly stable, whereas LLO orbits decay, it's colder there which makes cryogenic propellant storage easier (less reflection from moon/earth), but it also has continuous sunlight for power. It always has line-of-sight to earth for radio communications, and it provides access to landing points anywhere at the moon at any time (whereas LLO orbits will have windows to certain areas and require polar orbits for polar access). LL1 can also be used to make a lunar space elevator someday, and the moon is light enough that it could be made of materials that actually exist.<br /><br />Benefits of LLO are that the instability of the orbit eliminates space junk rapidly, proximity to the moon reduces many types of radiation by half (covers half the sky), and it's a closer platform from which to observe the moon. Delta-V to the surface is also lower.<br /><br />Soho is at the sun-earth L1 point. The one mentioned for lunar missions is the Earth-Moon L1 point.

 

[h2ouniverse]

To all:<br />Don't forget a huge advantage for L2 over L1: L2 is still somewhat shielded by Earth radiation-wise! (in the magnetotail...)

 

[no_way]

<blockquote><font class="small">In reply to:</font><hr /><p>That isn't to say it wouldn't be without drawbacks. <p><hr /></p></p></blockquote><br />the most obvious one: lots of launchers that can get to LEO, cant get to L1. Which severely limits your freedom in choosing launchers and thus also maximum launch rate.

 

[comga]

"Don't forget a huge advantage for L2 over L1: L2 is still somewhat shielded by Earth radiation-wise! (in the magnetotail...)"<br /><br />Huh? All Earth-Moon Lagrange points rotate about the Earth-Moon barycenter, which is located within the Earth, with the Moon. L2 is actually on the other side of the moon, can't be seen from Earth, and is in the magnetotail for a slightly smaller fraction of the month than is L1, or L4 and L5 for that matter, not that any difference is significant. Unless I am terribly mistaken, all five rotate through the magnetotail on a monthly basis, but this is not very useful or important.

 

[solarspot]

I believe he was referring to the Earth-Sun L2 point; not the Earth-Moon L2 point. That would make a noticeable difference...

 

[h2ouniverse]

Indeed. I was talking about Sun-Earth L2.

 

[willpittenger]

Unfortunately, the posters in this thread are using the Earth-Moon and Sun-Earth points too interchangeably. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[willpittenger]

<blockquote><font class="small">In reply to:</font><hr /><p>I believe many other satellites have since used this location including SOHO and the upcoming JWST. However, this orbit will be their home and they will not go on planetary voyages.<p><hr /></p></p></blockquote>JWST might be intended to go <font color="yellow">through</font>L1, but last I knew, its destination was either L4 or L5. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[lampblack]

According to this link, they plan to park the JWST at the Sun-Earth L2. <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>

 

[venator_3000]

Yes, I guess it would be the same whether you spent a long time getting there or made a "quick" dash. Isn't it about 1.5 million miles from Earth? (Sun-Earth L1 or L2).<br /><br />And what would be a reasonable flight time for people to get there? Assuming a 3 day trip to the Moon, you might need 15 days to get to L1. The early excursions would certainly be a challenge but also a good way to test hardware for more extended (non-landing) missions.<br /><br />v3k <div class="Discussion_UserSignature"> </div>

 

[h2ouniverse]

Yes, but going from Earth to Earth-Sun L2 you are in magnetotail most of the time. Better to spend 15 days protected rather than 3 under radiations... <br />And in history of travel, passes have always been ideal stops. And that is what L1, L2 are (passes).

 

[willpittenger]

Would orbital mechanics allow the spacecraft to stay in the tail for the entire trip? I don't know. The trajectory would probably be an arc -- not a straight line. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[h2ouniverse]

Depends.<br /><br />If you have most of your boost initially (especially from the launcher, or from chemical prop from LEO), you would get a highly elliptical transfer orbit that would fit inside it the mag tail. If you reach L2 with a slow approach, (e.g. ion propulsio), then indeed you would spiral up and get outside the mag shield.<br /><br />Note that the tail is relatively wide, about one fifth of Moon's orbit.

 

[j05h]

<i>> But getting the fuel and oxidixer to the L1 point to serve as a depot, requires fuel as well. I'd be interested to see the delta v numbers for LEO to Mars direct vs LEO to L1 then Mars. It still sounds like it would take more energy than a direct path.<br /><br />Has Nasa used L1 as a stop over for any planetary missions?</i><br /><br />NASA has proposed an L1 general-purpose station as a follow-on to ISS. It would be in an Earth-Moon L1 halo orbit, as is the propellant depot I am proposing for "base camp" operations leading to the Moon and beyond. Generally when "L1" or "L5" is referred to it means Earth-Moon Lagrange points, EML1 and EML5 compared to other Lagrange points such as SEL1 or SJL4 (Jupiter trojan orbit).<br /><br />"The Earth–Moon L1 allows easy access to lunar and earth orbits with minimal change in velocity and would be ideal for a half-way manned space station intended to help transport cargo and personnel to the Moon and back."<br /><br />http://en.wikipedia.org/wiki/Lagrange_point<br /><br />Getting propellant to L1 is not as expensive as you suggest. Payloads coming from interplanetary trajectories can aerobrake using Earth's atmosphere, entering highly elliptic orbit. This is followed by successive aerobraking passes that refine the orbit to near L1, then a small burn to enter the halo orbit. This also works for entering LEO, but requires that much more deceleration. <br /><br />http://en.wikipedia.org/wiki/Delta-v_budget<br /><br />Check out the chart halfway down the page. There is a lot of free aerobraking returning to Earth and going to Mars. <br /><br />LEO to Mars is 4.7km/s and L1 to Mars is 1.6 km/s, assuming aerobraking at Mars. L1 requires about 3.7km/s from LEO, so all told getting to Mars costs slightly more energy with the stopover. L1 stations could still tug propellant down to LEO, too. L1 opens <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[samkent]

If we used the same craft for repeated missions from L1 to the solar system, then you are correct. But as far as I can tell all of our missions will be from LEO, with different crafts. That would make L1 a waste of delta v. Plus I can’t see any country spending money for a depot at L1.

 

[h2ouniverse]

ESA is considering a man-tended station at one Laagrange point (without the money to make it).<br />

 

[j05h]

<i>> If we used the same craft for repeated missions from L1 to the solar system, then you are correct. But as far as I can tell all of our missions will be from LEO, with different crafts. That would make L1 a waste of delta v. Plus I can’t see any country spending money for a depot at L1.</i><br /><br />L1 makes the most sense as the central transportation node in a larger network. Any orbital inclination access up and down through LEO, easy transfer and slow orbits to interplanetary locations. It really makes sense when propellant is being brought back into cis-Lunar space (from Moon, Mars, NEOs) from away instead of only from Earth. The first step is definitely a reliable, reusable tug to do LEO-L1. <br /><br />I'm proposing a new, dedicated corporation build the L1 station. They would host NASA and the propellant depot operator as anchor tenants. Ideally they would use existing/near-term hardware to do this instead of developing it all from scratch: BA-330 & SunDancer, Dragon and Soyuz, Alenia nodes and hard modules, whatever else is available. An excellent leverage will be long-duration, "smart" upper stages that serve as tugs/depot-parts. Commercially-sourced space stations will be available in the next 5-8 years. L1 is in a unique position. It's more than half-way to anywhere. <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[venator_3000]

I found this old article that is from 2002. There was some talk of this and an introductory overview of NEXT at an AIAA conference I attended, but memory fades over time...<br /><br />There is also a reference to the space reporter here at SDC.<br /><br />It is interesting that an L1 station was referred to as a stepping stone. And also a pity this sort of plan has been all but lost in the post-Columbia shuffle for VSE. <br /><br />Link is here: http://technology.newscientist.com/article/dn2955-nasa-prepares-to-boldly-go.html<br /><br /><br /><br /><br />NASA prepares to boldly go<br />19:00 23 October 2002 <br />Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues <br />Pat Dasch, Houston <br /><br />Ever since astronauts last set foot on the Moon in 1972, the world has been waiting for a grand vision of humanity's next foray deep into space. Our visits have been restricted to the space stations barely 400 kilometres above the Earth's surface and, burdened with the spiralling costs of the International Space Station, NASA has kept silent about the future.<br /><br />But now it has become clear that the agency never stopped dreaming of sending people into the unknown. Last week, without fanfare or any grand announcement, it quietly unveiled its blueprint for the future. It calls for a space station close to the Moon that will ultimately serve as a gateway for missions all over the Solar System.<br /><br />The NASA Exploration Team (NEXT), which was set up by NASA three years ago to dream up a future for human space exploration, shared its vision with aerospace experts at the World Space Congress in Houston, Texas. The plan could not be in starker contrast to the politically motivated Apollo missions of the 1970s, or the aimless, cash-guzzling International Space Station. This time the science will come first, promises Gary Martin, NASA's Future Technology <div class="Discussion_UserSignature"> </div>

 

[josh_simonson]

>>LEO to Mars is 4.7km/s and L1 to Mars is 1.6 km/s, assuming aerobraking at Mars. L1 requires about 3.7km/s from LEO, so all told getting to Mars costs slightly more energy with the stopover. L1 stations could still tug propellant down to LEO, too. L1 opens up the Inner Solar System in ways that LEO and Shackleton simply fail. <br /><br /> />If we used the same craft for repeated missions from L1 to the solar system, then you are correct. But as far as I can tell all of our missions will be from LEO, with different crafts. That would make L1 a waste of delta v. Plus I can’t see any country spending money for a depot at L1.<br /><br />It is not true that L1 is only useful if spacecraft are re-used. This is because ANY mission beyond earth/moon space will require multiple launches. Assembling the craft in EML1 allows these launches to get to EML1 by different means, with non-perishable hardware going a slow but high ISP route while humans and perishable payloads take a high-ISP route. Since />2/3 of the DV is used to get to EML1 the fuel savings of this strategy are enormous and more than makes up for the 0.6km/s difference in DV.<br /><br />The biggest drawback of EML1, IMO, is that it is a pretty harsh radiation environment, comparable to interplanetary space. But that'll have to be solved anyway if a Mars mission is ever to happen, and the L1 depot probably should not be continuously manned anyway.

 

[samkent]

What about the delta v to get all of the parts to EML1? Not to mention the resources (crew/hardware) stationed at EML1. They had to get there before assembly. Which requires more delta v. <br />In the end it's cheaper to launch from LEO than mess with all the other hurdles of EML1.<br /><br />It's like a situation close to home.<br />I can drive 2 miles to a parking lot and catch the bus. But by the time you add the cost of parking on top of time delays, it's cheaper just to drive the 10 miles.

 

[josh_simonson]

The delta-V to get all the parts to EML1 are what make it attractive. Most of, say a Mars stack, could spiral out to EML1 with an ion drive over 6 months needing only a fuel fraction of 10%. Then the crew can fly up there with a J-2X engine with a fuel fraction of around 50%, dock and go. Considering the hardware for a mars mission will weigh 500-1000t for an LEO based mission, staging in L1 can save nearly 40% - 200-400t. That's $2-4B in savings and also enhanced reliability due to reducing the number of launches. <br /><br />A more realistic comparison than your bus one is that you're in singapore with 10 tonnes of Plasma TVs and need to get you and the TVs to your house. Do you 'carry on' the 10 tonnes of plasmas on the plane, or send it in a shipping container that takes a couple weeks and travel separately, meet it at the port, load it on a truck and drive it the last mile.