By shipping material and fuel to those points through various efficient means such as high delta t low delta v ballistic trajectories or ion propulsion and doing your in space assembly there, delta v can be accumulated and stockpiled at lower cost than simply building and fueling a big Earth Departure Stage (EDS) in LEO and lighting it off to go on your way to a NEO or Phobos or Mars.
Also, as Robotbeat says, an EML assembly point is inclination neutral for purposes of terrestrial launch sites. Therefore, less politics is involved.
Jason,Do you have any idea how difficult it is to build a 100% reusable rocket engine that you can trust crews lives to, that CAN'T BE INSPECTED between uses? That's what you're assuming all-too-casually there.Also, cryogenic pressure tanks (LOX, LCH4 and LH2 alike) have to go through far more heat and load cycles in orbit than they ever would for a launch vehicle. Shuttle ET's are a case in point -- they can only go through, what, 13 thermal/pressure cycles? Yet a stage on orbit is going to have to go through those, plus long durations of cold-storage interspersed with intense sunlight. And radiation, molecular oxygen and other factors that aren't immediately apparent. The requirements for that are far more demanding than anything we've done to date.Don't get me wrong, I'm all for an in-space infrastructure to reduce our dependence on Earth-launched materials and equipment, But its unrealistic to expect to use any existing hardware. NOTHING has so far been built to do any of these things with humans in the mix -- and its going to take a MAJOR effort to develop the necessary systems to a safe enough level that we can depend on them "unseen".Ross.
the exploration vehicles mass is a small fraction of propellant, supplies, crew, and any auxiliary vehicles (sats, landers, etc.)
1. how difficult it is to build a 100% reusable rocket engine that you can trust crews lives to, that CAN'T BE INSPECTED between uses? ...2. cryogenic pressure tanks (LOX, LCH4 and LH2 alike) have to go through far more heat and load cycles in orbit than they ever would for a launch vehicle. Shuttle ET's are a case in point -- they can only go through, what, 13 thermal/pressure cycles? Yet a stage on orbit is going to have to go through those, plus long durations of cold-storage interspersed with intense sunlight. And radiation, molecular oxygen and other factors that aren't immediately apparent. The requirements for that are far more demanding than anything we've done to date.
Very likely, if you're going to Mars (since very-low-boiloff is already a requirement for a cryogenic chemical or NTR Mars mission for the return). Also, the amount of delta-v you need once at EML1/2 is much smaller, meaning that hypergolic or other non-hydrogen stages would be not such a big hit.... And besides, you're going to be hanging out in LEO for any kind of Mars mission because it'd take multiple launches (i.e. likely months) of even the monster Ares V to get enough propellant. So yes, you still need very-low-boiloff technology.
Your assumption, then, is that all the industrial infrastructure will always be on Earth (not counting the far future). Then, your conclusion is that we should not build industrial infrastructure (multi-mission depot, etc) off of Earth? Interesting logic. If I agree with your assumption, then of course I'd agree with your conclusion.
...(by the way, if you're using chemical rockets, you can use the Oberth effect twice, thus giving you more benefit... like as if your rockets had twice as much thrust... and you can use both the Earth and the Moon as gravitational levers... so there IS some fundamental benefits in the idealized sense)
An EML1/2 gateway or depot or rendezvous point or whatever does have the benefit that we can take advantage of all the launch infrastructure on Earth instead of just the launch infrastructure at a certain latitude, while also avoiding the cost of launching to a high-inclination (say, ISS) orbit from a mostly-equatorial launch site... everyone can launch with the most efficiency. Unless, of course, you're assuming that we launch a single enormous HLV straight to Mars, we're going to have to do some assembly in orbit. Apollo was built assuming NO infrastructure in orbit because space was entirely new and there was a race. That is no longer true... we have many satellites for communication, for instance.
Launch efficiency (expressed in mission-total IMLEO) isn't the only metric that matters (and even then, there are some advantages to EML rendezvous/depot/etc).
QuoteLaunch efficiency (expressed in mission-total IMLEO) isn't the only metric that matters (and even then, there are some advantages to EML rendezvous/depot/etc).Only with technological and budget assumptions that I find wildly optimistic.
Let me just point out here that delta-V doesn't add linearly. If you fully fuel a transfer vehicle at an L-point, the delta-V bonus from an Earth swingby can be larger than you could ever get starting from LEO. This could increase the achievable transit speed.
Alternately, you could achieve Earth escape with an electric stage without the weeks-long spiral out of LEO, which also improves mission time.
There's also the possibility of mining the lunar polar craters for ice (current indications are that there is a lot of it, and relatively pure), cracking it into LOX and LH2, and sending it to an L-point depot on a reusable lander for a lot less delta-V than would be required from Earth. This is a much nearer-term possibility than you seem to think.
On the subject of reusability, wasn't that part of the idea of the NAUTILUS-X? I suppose using the same ship for multiple Mars missions would be further down the line...
You're saying that mass is more important to minimize than total integrated mission cost? I think everyone here would disagree.
True, but propellant and supplies are extremely cheap. I should have stressed in my last post that reusing value is important, not mass alone.
Guys...stick with me -- we are not going to have an industrial base anyware except the Earth for the deep foreseeable future.
The exploration vehicle would have to be reconditioned after each mission.
...that infrastructure costs real shekels and real payload space...
Quote from: Robotbeat on 07/27/2011 05:49 pmVery likely, if you're going to Mars (since very-low-boiloff is already a requirement for a cryogenic chemical or NTR Mars mission for the return). Also, the amount of delta-v you need once at EML1/2 is much smaller, meaning that hypergolic or other non-hydrogen stages would be not such a big hit.... And besides, you're going to be hanging out in LEO for any kind of Mars mission because it'd take multiple launches (i.e. likely months) of even the monster Ares V to get enough propellant. So yes, you still need very-low-boiloff technology.Low boiloff technology in a small stage (i.e. Earth return stage) is probably more accessible than in a large one (i.e. EDS).
As for the delta-v you need from an EML, to borrow a phrase, so what? You still had to spend the energy to get there. TANSTAAFL.
Quote from: Jason1701 on 07/27/2011 06:12 pmYou're saying that mass is more important to minimize than total integrated mission cost? I think everyone here would disagree.I'm saying that, given a certain lifting capacity, payload to objectives is more important to maximize. And I'm simply not seeing how the costs are going to be less for using the lifting capacity to goof around with infrastructure projects than it will be to use it to go places. That's what the lunar resources and EML1/2 people miss -- that infrastructure costs real shekels and real payload space. It would take an enormous continuous exploration investment to make it pay off.
It would take an enormous continuous exploration investment to make it pay off.
Quote from: Jason1701 on 07/27/2011 06:07 pmTrue, but propellant and supplies are extremely cheap. I should have stressed in my last post that reusing value is important, not mass alone.You're already committed to expending the value of propellant, cargo, and crew shuttles. Those are not cheap.
So, you just design the stage for more thermal cycles. Big deal. It only adds very marginally to the mission mass for an order of magnitude increase in number of thermal cycles.
This is well known. Everything is built on Earth, dry. The only possibility of a new technique is to make prop in the dark lunar craters, and drop it to a convenient point; L1 or LEO. This would help the very massive issue of propellant launch from Earth's surface, altho such a scheme is only in the early design stages.
Orbital assembly is a given as well. Should that be in LEO, and if so, at ISS, or a better inclination for lunar or martian orbit injection. Should the assembly take place at L1 or L2?
I'm not sure what your objection is.
While this is certainly true, the only alternative would be to construct a new one for each mission. WRT Mars, unless official strategies can be changed, there will only be one photographic mission, and the flag will cost a bit more, but it would probably be within that budget. WRT the Moon, reusability seems to be desirable.
IMLEO is not the only metric that matters.
This is quite well known, too. One of the biggest complaints about SLS is that it is a huge rocket for no known or budgeted missions.
Again, it's not clear where you're headed with your line of reasoning.
...hmmm... big on Mars... fan of HLVs... not a fan of in-space infrastructure... Robert Zubrin, is that you?
The technology needed is the same, and because of the larger surface-area-to-mass ratio of the smaller stage (and the longer time it needs to have low-boiloff), design of a larger stage at a certain boil-off rate should actually be a lot easier than for a smaller one, so actually the opposite of what you said.
Yes, there is such a thing as a free lunch. You can leverage incredibly high SEP (which has the benefit of the enormous improvements in specific power for solar arrays) for a huge portion of the mission delta-v if you so decide. If you don't do EML (or, I suppose, high Earth-orbit) rendezvous, you can't really take advantage of this unless you intend to buy into the difficulty of an entirely SEP (or NEP)-propelled spacecraft.
Another large benefit is that of safety. Most spacecraft failures occur at the beginning of the mission, and the rate of failure drops gradually as the time in space increases. Thus, by rendezvous just at the edge of Earth-escape, you don't have to commit to the mission until most of the things that are going to fail have already failed and you can plan accordingly (or fix them before leaving Earth-Moon). The environment at an EML point is much closer to deep space than it is in LEO (where you have much quicker and more intense thermal cycles, atomic oxygen, and greater drag which may mean you have to wait to deploy large structures like solar arrays, radiators, etc), so it gives you the opportunity to test your craft before leaving the Earth-Moon system. You get rid of all the infant-mortality failures and are left mostly with the better-understood-and-modeled-and-predicted fatigue-type failures.
sergeante, I agree with this:QuoteIt would take an enormous continuous exploration investment to make it pay off.However I am not persuaded human spaceflight is worth funding UNLESS we intend to make a continuous exploration investment.Unless we are going to stay (at least eventually) just send robots.
Also too, EML assembly would not require massive infrastructure or permanent human presence. Think human tended, not human occupied.
Also, also too - - despite being a DIRECT / SLS supporter I would not be confident in a single launch Mars or NEO mission and once we start doing on orbit assembly of missions using EML to assemble just makes more sense to me than using LEO to assemble.
In-space infrastructure seems to be the only proven way of "making sure those boundaries stay put once pushed back."