...The flip side of that is that you trade infant mortality for extra operational service time in space in which the equipment can fail. I'm not sure you could justify that with an honest trade study.
The majority of the failures happen earlier... deploying antenna, radiators, etc, plus just general infant mortality problems. For a long mission to, say, Mars, you're trading a proportionally small increase in operational time versus going on the mission with all the infant mortality failures. We know how to make spacecraft last for decades. Any failures which occur during the extra operational time can be repaired before the mission even occurs.
I'm not convinced that in-space infrastructure is always the answer, but your dismissal seems to be premature.
Quote from: Bill White on 07/27/2011 06:30 pmAlso too, EML assembly would not require massive infrastructure or permanent human presence. Think human tended, not human occupied.Docking type and maybe a few EVAs type assembly is not going to require a lot of human involvement. A spacecraft refurbishment architecture will.
Also too, EML assembly would not require massive infrastructure or permanent human presence. Think human tended, not human occupied.
Quote from: Bill White on 07/27/2011 06:30 pmAlso, 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.Docking is docking, wherever you do it.
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.
Quote from: 93143 on 07/27/2011 05:55 pmLet 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.You still had to spend the energy to lift the propellant uphill. You can't get more energy into propellant by dropping it from a high orbit than you spent to put it there.
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.
Quote from: Robotbeat on 07/27/2011 07:19 pmThe majority of the failures happen earlier... deploying antenna, radiators, etc, plus just general infant mortality problems. For a long mission to, say, Mars, you're trading a proportionally small increase in operational time versus going on the mission with all the infant mortality failures. We know how to make spacecraft last for decades. Any failures which occur during the extra operational time can be repaired before the mission even occurs.You can do all of that in LEO too, if you insist on on-orbit checkout prior to departure. In fact, that's pretty much assumed from the beginning in a lot of mission designs.
It's a more complicated trade than you're making out.Let's say you have an all-chemical transfer vehicle that can reasonably spare 5 km/s departure delta-V. Hydrolox, with 4500 N·s/kg; that's a mass ratio of 3.This vehicle can go to L2 on its own, using 80% of its departure propellant. Tanking up then requires the use of dedicated tankers delivering about 40% (cf. Zegler, Kutter, and Barr 2009) of their initial propellant load to L2. This requires twice the departure propellant allocation to fill the transfer vehicle back up, so the propellant requirements triple.You could do the same v_inf (~11,500 m/s) with a mass ratio of 6 from LEO, requiring a little over 80% as much total propellant. But look what happened - you just doubled your departure mass ratio. The transfer vehicle's IMLEO now includes a giant EDS, which eats into the available payload.If you want to reuse a transfer vehicle that's already at L2, all the propellant goes by tanker, and the 20% advantage (in total propellant mass) of starting in LEO disappears... and if you want more initial delta-V, the mass ratio for the LEO option climbs rapidly...
Or you could use electric propulsion to tank up the L2 depot, which saves immense mass. In this case, an Earth swingby burn on the order of 100 m/s or so would put you on an escape trajectory, and your transfer vehicle could use electric propulsion the rest of the way, making a large chemical EDS entirely unnecessary.
Also, it seems a bit odd that you would complain about orbital plane misalignment being an insurmountable obstacle for a space-based exploration depot, and then turn around and complain that an L-point architecture offers no real advantages while ignoring the fact that it solves your plane change issue...
Quote from: 93143 on 07/27/2011 07:32 pmIt's a more complicated trade than you're making out.Let's say you have an all-chemical transfer vehicle that can reasonably spare 5 km/s departure delta-V. Hydrolox, with 4500 N·s/kg; that's a mass ratio of 3.This vehicle can go to L2 on its own, using 80% of its departure propellant. Tanking up then requires the use of dedicated tankers delivering about 40% (cf. Zegler, Kutter, and Barr 2009) of their initial propellant load to L2. This requires twice the departure propellant allocation to fill the transfer vehicle back up, so the propellant requirements triple.You could do the same v_inf (~11,500 m/s) with a mass ratio of 6 from LEO, requiring a little over 80% as much total propellant. But look what happened - you just doubled your departure mass ratio. The transfer vehicle's IMLEO now includes a giant EDS, which eats into the available payload.If you want to reuse a transfer vehicle that's already at L2, all the propellant goes by tanker, and the 20% advantage (in total propellant mass) of starting in LEO disappears... and if you want more initial delta-V, the mass ratio for the LEO option climbs rapidly...With all due respect, you're being disengenuous. Mass ratio is not a figure of merit. You have a higher mass ratio from LEO, but you didn't spend any mass on propellants used to deliver more propellant to L2.IOW, you never get something for nothing....
You can't do all that in LEO... not everything is deployed. For instance, in the case of a primarily-SEP-propelled Mars Transfer Vehicle, you don't want to deploy the entire very large array in LEO because of drag. Also, the thermal environment in LEO is different than deep space.... And suppose a problem IS found? Since you were lined up in that exact inclination for only your little window, you have very few options if you need more time to repair.
And, as 90210 pointed out, if you launch straight from LEO, you have a much, much heavier dry mass for the EDS which ends up being put through Trans-Mars-insertion than if you launched from EML1/2.
And if a gateway (which I'm not 100% sold on, BTW) is considered much different than what we know how to do today, then perhaps it's not wise we go beyond cislunar space (running) until we've mastered operations in cislunar space (walking). After all, we may well need to do repair and inspection at Mars orbit, too, so it's good to get some practice in beyond-LEO repair and inspection operations.
Quote from: Robotbeat on 07/27/2011 07:41 pmYou can't do all that in LEO... not everything is deployed. For instance, in the case of a primarily-SEP-propelled Mars Transfer Vehicle, you don't want to deploy the entire very large array in LEO because of drag. Also, the thermal environment in LEO is different than deep space.... And suppose a problem IS found? Since you were lined up in that exact inclination for only your little window, you have very few options if you need more time to repair.If you have to fold your solar pannels whenever you're close to Earth, then obviously you'll have to fold them when you make your gravity well maneuver....
No, you're wrong. By using a single EDS straight from LEO, you have more dry mass put through the same delta-v than if you have a smaller single EDS launched from EML1/2. This would only change if you did a staging event part-way through the trans-Mars-insertion burn.
Quote from: Robotbeat on 07/27/2011 07:56 pmNo, you're wrong. By using a single EDS straight from LEO, you have more dry mass put through the same delta-v than if you have a smaller single EDS launched from EML1/2. This would only change if you did a staging event part-way through the trans-Mars-insertion burn.As explained in another post, if you fuel up at EML1/2, you accelerate all of the propellant mass almost to Earth escape anyway. And you expend the inert mass of the tankers, along with the propellant to get to EML1/2.IOW, you can't get something for nothing.
No, the integrated drag wouldn't be high for just a single maneuver lasting minutes versus many orbits lasting hours or days, even with the speed difference.
Quote from: Robotbeat on 07/27/2011 08:05 pmNo, the integrated drag wouldn't be high for just a single maneuver lasting minutes versus many orbits lasting hours or days, even with the speed difference.You have a point, but then you're talking about a mission design decision that has no EML1/2 infrastructure requirement per se. All you're doing is prepositioning and testing assets at a very high Earth orbit (remember, EML1/2 is an orbit, with all that implies), then going from there. What's the infrastrucure of a hab and 3-6 astronuts there for?
Again, you're wrong because the tankers don't get put through the full delta-v. Staging your dry mass at a lower delta-v is more efficient, thus it's something you get for "nothing."
Quote from: Robotbeat on 07/27/2011 08:10 pmAgain, you're wrong because the tankers don't get put through the full delta-v. Staging your dry mass at a lower delta-v is more efficient, thus it's something you get for "nothing."Nope. They do however get put through almost all of it, you have to set aside propellant for EML1/2 orbit insertion (you just don't get there and stop), you have to set aside propellant for insertion into disposal orbit (only a couple hundred miles per hour from LEO, how much from EML1/2?), and probably other things I can't think of right now.And you still have the mass of the EML1/2 departure stage to figure, which is going to Mars (or somewhere in that direction).And you still have to deal with the possibility of the tankers not being as mass efficient as a large EDS.I'm not seeing where you're gaining anything.
I'm sort of enjoying the back and forth here, but how is it related to the Full Committee Hearing thread topic? Isn't there a more topical place for your "TANSTAAFL/is too/orbital mechanics" debate?
One final comment ... If an Oberth effect leveraged TMI burn were executed on a dark clear night by a large spacecraft coming from EML-2 for a terrestrial gravity assist, that would be one heck of a photo opportunity. Just saying . . .
Quote from: Bill White on 07/27/2011 08:49 pmOne final comment ... If an Oberth effect leveraged TMI burn were executed on a dark clear night by a large spacecraft coming from EML-2 for a terrestrial gravity assist, that would be one heck of a photo opportunity. Just saying . . . Sorry, but I can't resist.An LH2/LOX engine burning at a 100 mile altitude would be invisible, unless you were looking right at the engine bell. And then it wouldn't be much more than a small dot of bluish-white light.