Author Topic: LIVE: Full Committee Hearing - A Review of NASA's Space Launch System  (Read 475780 times)

Offline Robotbeat

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...
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 (if it can survive launch, etc). 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.

The idea is to improve the in-space infrastructure, lower its overhead. This is partially what transitioning from Shuttle to CRS/CommercialCrew is all about. This is what improved ECLSS and water-recovery on ISS is about. We must learn how to lower its cost in order to allow NASA (and the other space agencies) to do more with the same funding, to keep expanding without going back.
« Last Edit: 07/27/2011 07:28 pm by Robotbeat »
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Offline sergeante

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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.

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.

Now, I'm pretty sure you're thinking that it's more expensive to do checkout and repair on a unique orbit than at an EML point. Well, only if you happen to have an inventory of ready-to go repair parts at your checkout station, and only if that inventory includes the right part (out of hundreds of thousands of possible parts), and then only if your on-the-spot human resources know how to do the repair/replacement (or can be taught how to with email). In the much more likely case that the spare and trained personnel have to be sent up to do the job, putting them in LEO is going to be cheaper and at least a couple of days quicker.

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I'm not convinced that in-space infrastructure is always the answer, but your dismissal seems to be premature.

I would say that many people's acceptance seems to me to be highly premature. We're crawling right now. Even a beginning EML/lunar infrastructure is well into walking.

Offline Bill White

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Also 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.

I agree. Therefore we need to begin with rudimentary EML architectures.

I also believe we need to look plug-n-play cryogenic tanks rather than fuel transfer for similar reasons. To fill, empty, and re-fill a liquid hydrogen tank causes that tank to experience rather significant extremes in thermal stress.

And these issues suggest at least looking at kerolox or NOFBX as the preferred fuel for a lunar lander.


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.

Docking is docking, wherever you do it.

Agreed.

However I don't believe we can afford to build a rocket big enough to go to a NEO with a single launch, from Earth.

Therefore on orbit assembly is necessary. The question becomes where is on orbit assembly best performed.
« Last Edit: 07/27/2011 07:30 pm by Bill White »
EML architectures should be seen as ratchet opportunities

Offline 93143

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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.

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.

I shouldn't have gotten involved; I don't have time for this - but...

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...
« Last Edit: 07/27/2011 07:42 pm by 93143 »

Offline Robotbeat

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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.

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.
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.
« Last Edit: 07/27/2011 07:47 pm by Robotbeat »
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Offline sergeante

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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...

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. You should also count the mass of the tankers into the propellant side of your overall mission mass ratio, to the degree they're expended after they deliver propellant.

IOW, you never get something for nothing.

The transfer vehicle already at L2 is costing you resources to refurbish after its last mission.

IOW, you never get something for nothing.

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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.

Interesting as a mission architecture, for human life safety reasons, but I'm not seeing how it requires any actual facilities at L2.

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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...

Launching all of my hardware increments into the optimum departure orbit, I can minimize my plane change cost simply by adopting the correct heading out of the launch site. That approach is as old as Mariner.
« Last Edit: 07/27/2011 11:13 pm by sergeante »

Offline Robotbeat

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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...

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.
...
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.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline sergeante

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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.

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. You just added two human life safety events to the flight plan -- folding up solar pannels going downhill, then reopening them going uphill. Or, If you started from LEO, you could launch on a free return trajectory. If the solar pannels open up and work, you start the electric propulsion and escape Earth. If they don't, you jump in your Earth return vehicle, use it's propulsion to lower your orbit's perigee for reentry, and return safely.

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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.

Or you have a bunch of tanker dry masses to add up when you finish using them at EML1/2. And smaller tanks probably have more inert mass per pound of propellant than a larger tank does.

The simple fact is that no matter what you do, you spend propellant to lift propellant, and you chuck away harware, whether you're going to EML1/2 or injecting into an interplanetary transfer trajectory.

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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.

That's why you start with NEO rendezvous and work your way up the ladder.
« Last Edit: 07/27/2011 08:03 pm by sergeante »

Offline Robotbeat

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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.

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, 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.
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Offline sergeante

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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.

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.

Offline Robotbeat

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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.

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.
Again, you're wrong because the tankers don't get put through the full delta-v. Staging your dry mass (the tankers in this case) at a lower delta-v is more efficient, thus it's something you get for "nothing." Plus, they will have a higher production rate than an EDS, thus their cost-per-dry-mass will be lower.
« Last Edit: 07/27/2011 08:12 pm by Robotbeat »
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Offline sergeante

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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.

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?
« Last Edit: 07/27/2011 08:11 pm by sergeante »

Offline Robotbeat

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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.

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?
I'm not sure that's the kind of infrastructure we need. Unmanned depots (with the OPTION of a small, pressurized safe haven shelter for Apollo 13-like scenarios) would be helpful, allowing us to accumulate cryogenic propellants over many different launches of existing (or then-current) commercial launch vehicles in a thermal environment conducive to cryogenic storage. Everything else can then be launched dry using existing (or then-current) commercial launch vehicles. No need for specialized exploration-only launch vehicles (allowing our exploration capability to survive launch vehicle cancellation), and no tying to one launch vehicle. But really, this is far off-topic.
« Last Edit: 07/27/2011 08:20 pm by Robotbeat »
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Offline sergeante

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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."

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.

Offline Mark S

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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?

Offline Robotbeat

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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."

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.
Existing launch vehicles could be used. That's a huge plus.

HLVs become even more capable for their size.

Easier to take advantage of SEP propulsion at lower cost (SEP tech wouldn't be directly coupled to your exploration architecture, but would be an improvement in efficiency... could do without).

Can take advantage of ISRU in the future (Phobos/Deimos, asteroids, atmospheric scooping of Mars or Earth, lunar propellant--which I am skeptical of, but still a possibility).

More flexibility in general, supports RLVs ESPECIALLY (since very small launch vehicles could be used for prop delivery, the high flight rates necessary to make RLVs viable are a lot easier to reach... and if you buy the propellant off the market, you can put powerful market forces to work to develop your RLV for you instead of direct government development of it), along with other alternative launch concepts (you name it, it usually works better if you're justing sending up propellant).

(And I agree with Mark S... way off-topic... there are many other threads on this board!!!)
« Last Edit: 07/27/2011 08:28 pm by Robotbeat »
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Offline sergeante

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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?

Now that would be a killer topic (in more ways than one).

I'm trying to keep it within the context of SLS-type vehicles and their alternatives (both hardware and mission architecture). But I think I've made my point, so we'll leave off right here.
« Last Edit: 07/27/2011 09:04 pm by sergeante »

Offline Bill White

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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 . . .
EML architectures should be seen as ratchet opportunities

Offline sergeante

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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 . . .

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.

Offline Bill White

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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 . . .

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.

Eh, spoilsport.  ;D
EML architectures should be seen as ratchet opportunities

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