Why L1 for a staging point?

[spacester]

I would just note that SG's newfound reference has been known to me for many years and in fact I pointed Keermalec to it in the first place. <img src="/images/icons/tongue.gif" /> <div class="Discussion_UserSignature"> </div>

 

[CalliArcale]

*mod hat on -- general post, not to SG specificially, but to all*<br /><br />Just a friendly reminder to everybody -- let's try to stay nice here, okay? It doesn't have to be an absolute "I'm right, you're wrong", and it doesn't have to be a matter of pride. It also doesn't do to assume that your opposition must think you're an idiot or is cavalierly dismissing you just because they disagree. Don't assume that they're putting you down, and don't respond in kind, especially when you can't be sure how they meant their post to be taken anyway. You don't have to preemptively attack people with differing opinions; just say you think they're wrong, and let the facts show. Don't hide your facts behind unattractive bickering. You'll just make people suspect your case to be weak, and potentially spark a flamewar.<br /><br />*mod hat off* <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[j05h]

(thanks Calli, we all need a reminder once in a while)<br /><br />shuttle_guy, Calli etc - what do you think of the idea of a "truck stop" or base camp at EML1? Obviously it requires more demand to make sense, how to get to that? Are my proposals reasonable?<br /><br />josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[josh_simonson]

I have a spreadsheet that calculates delta-V via the rocket equation as DV=9.8*ISP*LN(Mi/Mf), for a four stage rocket (provided by Dr. Elias on NSF in the example of the Pegasus). I fiddled with the numbers on one stage of that to find the fuel fractions for LH2 and Methane fuels, ISP 450 and 380 respectively, using delta-V's of 3.77km/s from LEO to EML-1 and 2.52km/s from EML-1 to/from the moon.<br /><br />That doesn't take into account differences in DV due to trajectory and shifting gravitational fields due to the position of the earth, moon and sun, but should be a close enough approximation to highlight the benefits/penalties of an architecture choices discussed.

 

[h2ouniverse]

Hi spacester,<br /><br />in reply to:<br />------<br />you guys seem to think that a spiraling trajectory is a loss-free trajectory<br />------<br /><br />Actually this is not what I said. Spiraling is less efficient than Hohmann transfer. What I said is that when you spiral slowly, you can aproximate your fuel by applying the rocket equation to a "delta-V" that is the difference between the velocities associated to the two circular orbits.<br /><br />This is true that there is confusion of words between "spacecraft instant delta-V" and "delta-V as difference in orbits velocities".<br />But this is as things go... In system engineering you need to grasp orders of magnitude first, before going to the heavy analyses...<br /><br />The two notions coincide in the specific case of a slow spiral since with a slow transfer, you can neglect the eccentricity effect (Oberth effect) and consider at any instant t of the trajectory that the orbit remains circular. <br /><br />Best regards.

 

[spacester]

Thanks for the info, josh. Unfortunately I'm not going to have time to go back and review your post. Certainly nothing jumps out at me as being inaccurate.<br /> <div class="Discussion_UserSignature"> </div>

 

[spacester]

<font color="yellow">Spiraling is less efficient than Hohmann transfer. What I said is that when you spiral slowly, you can aproximate your fuel by applying the rocket equation to a "delta-V" that is the difference between the velocities associated to the two circular orbits. </font><br /><br />Yes, but of course you realize that this assumption is not conservative on the side of safety (negative dV margins). When we engineers make simplifying assumptions, we like them to model a situation safer than the actual. Orbital Mechanics is a harsh mistress <img src="/images/icons/smile.gif" /> so we have to take what we get, but it spurs me to investigate the magnitude of the gravity losses, not just the ideal numbers. This thing we do here is IMO all about trying to 'out-wit the rocket equation' and I like to go straight to the numbers we need to see how close to the ideal we can get.<br /><br /><font color="yellow">The two notions coincide in the specific case of a slow spiral since with a slow transfer, you can neglect the eccentricity effect (Oberth effect) and consider at any instant t of the trajectory that the orbit remains circular.</font><br /><br />Again, it's a first approximation and it is an ideal value which cannot be achieved. <br /><br />Plus I am biased against slow spirals. I like to think it's an objective bias, but still . . . IMO it's not a good way to explore the solar system and not a good way to traverse the Van Allen belts. These 'close enough to loss-free for practical purposes' slow spirals are hugely time-consuming, which means long-term exposure to space. Even for cargo and resupply, etc. I don't see them as practical choices for practical missions.<br /><br />I jumped into this thread on this subject because I didn't want people using non-conservative assumptions based on a statement that used the "exactly . . . . no matter what" format. I've made my case. If I may be allowed to drift OT for a sec . . . <br /><br />I am just not willin <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

<Thank you for the link to the very interesting discusion. It will take me a lot of time to go through that thread.><br /><br />You're welcome. Once you have read through, I'm interested in your opinion of the thread.<br /><br /><[Keermalec] agrees with spacester that more delta V is required for the LEO to LMO case however the difference in the delta V required to go from LEO to Low Mars Orbit does not apply to orbit transfers in Earth orbit /><br /><br />Well, that wasn't my understanding of that thread. There is a low-thrust penalty when in Solar orbit AND when in Earth orbit. I'd always assumed before that thread that low-thrust propulsion didn't suffer any penalty so long as a spacecraft was in orbital free-fall (whether in Earth or Solar orbit). But Keermalec convinced me otherwise, despite how much I might wish it wasn't so. <br /><br />In fact the numbers in one paper I dug up on a proposed SOTV tug only made sense when taking into account the penalty from the SOTV low-thrust propulsion.

 

[j05h]

(just attaching to end of thread) <br /><br />Great post, Kadetken. Sam's "star trek" dig got my goat because the station we are discussing is closer to reality than any other space structure beyond LEO. Neutral truckstop is the way to go! <br /><br />I posted a question in "Ask the Astronomer" about the visibility of a 1km EML1 transfer station. The universal answer is that it would be a point-source of light, largely just another twinkle in the sky. It wouldn't block the view and would hardly be noticed by anyone but avid skywatchers. <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[samkent]

kadetken<br /><br />While the idea of the truck stop may make sense if we expect to have a lot of passers by, we simply don’t. The car analogy works quite well. We need the gas stations since we have so many cars traveling long distances. We just don’t have that many space crafts doing the same. If this method of refueling was the best way, the airlines would be using it. Think of how much more cargo they could haul, if they didn’t have to haul the total amount of fuel for the entire flight, right from takeoff. It must not be as practical as the concept.<br /><br />It may be 20 times easier to lift O2 from the Moon, but what did it take to get to that point in the first place. You have to launch and land all of the stuff necessary for people to live there. Then you have to L&L the hardware to make the O2. I suspect there is consumables used in the process besides the electricity. So you would still have to L&L those consumables. Not to mention a totally reusable craft to deliver the O2 to the truck stop. And then there’s the basic fuel, beyond the O2, to fuel this space tanker. Is it found on the Moon also?<br /><br />This “Interplanetary Superhighway” you envision just doesn’t exist today and won’t for a great many decades. Smart people in the space community may have been talking about it for years but the people paying the bills aren’t listening. And most don’t care. It’s just too much money for zero return.<br /><br />As per the Bigelow story this week. How many rich people would really spend the 15mil to stay in orbit for four weeks? The number of people worth $30 mil or more in the US is only 38,000acording to wiki. And that’s total assets. I would think that less than 10% would have the disposable $15mil for a space trip. What bank would loan the needed money to build the whole thing, when you factor the number of potential customers?<br /><br />Yes XM may have been willing to fork over another 50mil to fix their satellite, but how many are worth fixing. Most

 

[h2ouniverse]

Spacester,<br /><br />The approximation is more than fair enough(*) in the case of the slow spiral. <br />If you are frustrated with low thrust ion/plasmic propulsion, then please provide me with a qualified or high-TRL high thrust high Isp mean of at least 4000s.<br /><br />A satisfactory conservative character can be recovered after simplified calculations like this by using a common practice we call "budget margins". The equivalent of the different safety margins a mechanical engineer like you has to ensure, yield, resistance, fatigue, hammering...<br /><br />For your information, a phase 0/A budget includes a compulsory 20% system margin, on the top of all others. The margin is even higher when doing the kind of rough calculation we discuss about. That should reassure you on the cautious character of approaches.<br /><br />Engineering assumes the capability to trade technical ideas with increasing levels of accuracy as you focus your trade-off tree. Running a detailed analysis anytime an idea is raised is not achievable in real life. You have to begin with this kind of first approach ranking after a brainstorm. If you have now accumulated some experience as an engineer, you know it already...<br /><br />Best regards.<br /><br />(*) enough = enough for a discussion on a forum

 

[josh_simonson]

I don't see the radiation belts being too big of a problem because it's already been done a number of times without issue, yet if it becomes a problem, fuel alone could still be delivered this way while retaining 90% of the benefits.<br /><br />Incidentally, the round trip, DV of LEO-Rendevous-Moon-Rendezvous-Earth is 9.09km/s, 9.58km/s and 8.81km/s, where the rendezvous point is LLO, EML-1 and EML-2 respectively. There is a lunar gravity assist that makes EML-2 a bit more efficient, but would not be possible with SEP cargo. That's from the DV numbers in a couple NASA documents quoted in wikipedia: http://www.nasaspaceflight.com/_docs/haloOrbitLunarStation.pdf<br />and http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/DOCS/EIC042.HTML<br />This makes EML-2 look a little better than EML-1 (and even LLO) DV-wise for even a basic, bare bones lunar mission.

 

[willpittenger]

<blockquote><font class="small">In reply to:</font><hr /><p>We need the gas stations since we have so many cars traveling long distances. We just don’t have that many space crafts doing the same.<p><hr /></p></p></blockquote><br />It's called the "Chicken Before the Egg problem." <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>

 

[j05h]

<i>>> We need the gas stations since we have so many cars traveling long distances. We just don’t have that many space crafts doing the same.<br /> /> It's called the "Chicken Before the Egg problem."</i><br /><br />That's why any transfer station has to aggregate customers - you get a bunch of two-legged dinosaurs together and eventually a stable chicken evolves. Aggregating interests, either in a classic consortia or prime-client model, should help make the case. <br /><br />World launch is several dozen/year now, and roughly stable at current launch prices. Efforts such as the Bigelow demonstration station look to add at least another 16/yr. To-LEO flight rates will climb, ideally Bigelow will be building a couple dedicated stations in the later 2010s. These along with perhaps a digital-TV provider and NASA participation might be enough to make the business case for L1. <br /><br />EML1 (or L2) makes a good location for human servicing of GEO comsats. Consolidating outbound traffic to a Lagrange transfer station would enable maintainenance of massive GEO platforms - imagine what satellite TV would be like based on a truss, huge inflatable antennae and hot-swappable transmitters. If a company like Hughes got onboard with an L1 project, they could send specially trained crew to service 6 or 8 massive GEO platforms that provided broadband/TV/VOIP to the world. Imagine what could be done with a 50t satellite. The same would be enabled for weather, earth-obs and SAR platforms. There are ways to do this from LEO, but Lagrange points have the advantage of similar access from multiple inclinations.<br /><br />The "gas station" aspect of the transfer point makes the most sense with non-Earth propellants, but might be competitive for starters with some kind of lower-cost/reliability small launcher flying often, propping tugs in LEO that transfer payloads and any extra prop to the L1 station. Any tank-farm has to be able to start small.<br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[samkent]

>>If a company like Hughes got onboard with an L1 project, they could send specially trained crew to service 6 or 8 massive GEO platforms that provided broadband/TV/VOIP to the world.<br /><br />Did I wake up in a different decade?<br />I thought we had sats that provided tv and broadband (downlink), at lower cost, in a orbit that’s closer, with a designed lifetime of a decade. What is the point of repairing a ten year old sat?<br /><br />Consider the orbital logistics of getting from eml1 to the geo stationary sats for a service call. You would leave the space station that has an orbital period of about 28 days (give or take). Retro burn to an orbit 140,000 miles closer to the earth, with a period of 24 hours. Then you will have a few short hours to make the repairs and test things. If you stay more than a day, you will have a rather high energy burn to get back to the station. Unless you hang out for a month.<br /><br />These dreams are about the same as the flying car of yesteryear. When you add up the costs and the benefits compared to the problems, it just doesn’t make sense.<br />

 

[josh_simonson]

><br />Consider the orbital logistics of getting from eml1 to the geo stationary sats for a service call. You would leave the space station that has an orbital period of about 28 days (give or take). Retro burn to an orbit 140,000 miles closer to the earth, with a period of 24 hours. Then you will have a few short hours to make the repairs and test things. If you stay more than a day, you will have a rather high energy burn to get back to the station. Unless you hang out for a month. <br /><br />No, you would have a window roughly ever 24 hours. The moon rises and sets over points on earth (and their corresponding GEO points) ~daily.

 

[samkent]

But you want to get back to the eml1 station which has an orbital period of a month. For each day you spend at the geo stationary sat the eml1 station would move 13 degrees farther in it’s orbit. I imagine some of the burns for the return trip would be pretty energy intensive.

 

[j05h]

1.38 km/s each way between GEO and EML1. <br /><br />http://en.wikipedia.org/wiki/Delta-v_budget<br /><br /><br />The idea for a comsat provider would be to consolidate onto a few GEO platforms and use lessons from the PC industry for designing the new systems. It's not unreasonable to expect periodic servicing to require several weeks per satellite. <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[josh_simonson]

Nope, nope, nope. If you consider the earth/moon system with the sun as a stationary point of reference, the earth rotates once daily, GEO orbits go about the earth once daily, and the moon goes around the earth once every 28 days. This means GEO will pass under the moon about daily. The inclination of the moon may change somewhat and result in some DV variation, but the moon won't be on the other side of the earth for two weeks.

 

[samkent]

>>but the moon won't be on the other side of the earth for two weeks<br /><br />My point exactly. <br />Unless you plan to stay in the GEO orbit for a long time (a few weeks), you won't have a low energy burn to get back to the L1 station. The Moon will have moved quite a bit.

 

[josh_simonson]

GEO travels around the earth faster than the moon and the two are in conjunction every 25 hours or so, providing a launch opportunity. Just because GEO seems stationary to us on earth here doesn't mean it isn't actually a pretty fast orbit relative to the moon's.

 

[keermalec]

Hey all,<br /><br />I'm actually on the beach in Brazil on vacation but as this is an interesting discussion I will bring in my two cents worth:<br /><br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>shuttle-guy: The Delta V required is exactly the same no matter what propulsion technique is used.<p><hr /></p></p></blockquote><br /><blockquote><font class="small">In reply to:</font><hr /><p>spacester: Not true.<p><hr /></p></p></blockquote> <br /><br />Like someone said before, you are not talking about exactly the same thing. spacester is a ssuming a [underline]continuous thrust spiral trajectory[/underline] which effectively costs much more delta-v than short burns at periapse.<br /><br />However, most continuous thrust missions to date have not used this type of propulsion. Instead they have made many short bursts at periapse, thence slowly lifting the altitude of the orbits apoapse. Once the apoapse reachs L1, a minor burn allows the vehicle to be captured by the moon (ex: SMART-1). A greater burn should allow for stabilisation at L1.<br /><br />If a low thrust vehicle should do 50x 30 minute burns at periapse, I see no reason why it should loose more delta-v thru gravity losses than a high thrust vehicle doing a single 30 minute burn at periapse. Even though gravity losses are proportional to burn duration, they are also proportional to delta-v and angle wrt to the gravity field, and these latter are much lesser in the case of low thrust.<br /><br />to conclude: low thrust to L1 WILL cost more delta-v if it is done continuously in a spiral trajectory, so spacester would be right in this case. However, low thrust delta-v will probably be the same as high thrust if it is accomplished in several short burns at periapse, so shuttle-guy is right too.<br /><br />And now you will excuse me the beach awaits. <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

 

[gunsandrockets]

<I'm actually on the beach in Brazil on vacation><br /><br />Lucky guy! There is a cold snap right now even here in sunny San Diego.<br /><br /><but as this is an interesting discussion I will bring in my two cents worth: /><br /><br />Thanx for clearing things up.<br /><br /><br />To Shuttle Guy: Go and read the thread I first referred you to. Keermalec and I had a lot of back and forth trying to come up with schemes to minimize the drawbacks of low-thrust propulsion and I think with a lot of success. <br /><br />But the simple fact is low-thrust does have some drawbacks even for LEO tug missions. The good news is that despite the drawbacks, low-thrust ion or plasma propulsion is superior to conventional LOX/LH2 for cargo missions.<br /><br />

 

[HopDavid]

Re:

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.

What would you gain from this over LEO?

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?

Sorry about resurrecting a dead thread but it is one of my interests. Found the forum and thread via Google.

I had answered similar questions elsewhere with pics. Pics seem more accessible than throwing out a mass of text and numbers.

TMI from LEO is 3.6 km/sec. TMI from EML1 totals about 1.1 km/sec

The usual response to this is "Yes, and to get propellent to EML1 you must climb earth's gravity well. It would take 13 km/sec to get propellent to EML1. Eyup, that's a big savings of resources and money, alright."

This argument ignores the moon.

The moon is rich in oxygen which counts for most of the mass in a typical rocket's propellent. The oxygen can be extracted with pyrolisis.

The moon has no atmosphere. So It would be possible to do the 2.3 km/sec portion with a mag rail.

.65 km/sec suffices to drop cargo from EML1 to an atmosphere grazing orbit. Using repeated drag passes, the cargo can brought down to LEO.

Thus Lunar Oxygen (lunox) can be sent from EML1 to LEO with possibly as little at .65 km/sec. Certainly not more than 3.75 km/sec.

A ship in LEO could fuel at this LEO depot filled with Lunox. All it would need is enough fuel for the 3.8 km/sec trip to EML1. From EML1 it could refuel. Remember, TMI from EML1 is 1.1 km/sec. TMI from LEO is 3.6 km/sec.

Developing our moon, Phobos and Deimos as propellent would enable short hops between depots.

For decades a Single-Stage-To-Orbit Reusable-Launch-Vehicle (SSTO RLV) was the holy grail for rocket engineers. Whether such an animal is possible seems doubtful. If it is, it would probably be more expensive than Expendable-Launch-Vehicles (ELVs)

However Single Stage reusable vehicles designed for trips between depots are a completely different animal. Instead of a delta V budget of 9 or 10 km/sec, these would have a max delta V of 5 km/sec, more often in the neighborhood of 3 or 4 km/sec. They would not have to endure the trauma of re-entry. They need no thermal protection, ablation shield parachutes, robust structure etc.

Reusable Mars Ascent Vehicles which double as Mars Landers are perhaps doable if they move between Phobos and Mars surface. Reusable Lunar ascent/Lunar landers are perhaps doable if they move between EML1 and the moon's surface.

It is true developing the moon, Phobos and Deimos would take a greater initial investment. However doing so would enable travel with smaller, simpler reusable vehicles rather than larger, more complicated, more expensive, more failure prone, expendable vehicles.

This architecture would make the difference between flags and footprints and a sustainable architecture.

 

[samkent]

But doesn’t the fuel spent dropping all of that hardware on the Moon, not to mention the price of the hardware, negate any fuel savings you get back out?
Especially when we still haven’t come up with super good reasons to be out there.
We don’t send man into space for logical reasons. We send them for emotional reasons.