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7:24 pm
January 5, 2010
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Luke Maurits
| | Adelaide, Australia | |
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Post edited 1:27 am – January 6, 2010 by Luke Maurits
A while ago I suggested that our original mission plan (now called mission plan alpha), involving the low energy transfer of the lander in advance of the manned CM, might not make as much sense as we had thought. The drastically lower mass of the lander compared to the CM seemed to me to mean that using the same orbital bus to carry both into lunar orbit would be quite wasteful. Thus was born mission plan bravo, which has the lander and CM in a single stack.
This analysis was based on the assumption that the orbital bus carrying the lander would have to push around 500 kg, the estimated total mass of our lander. This was kind of stupid, because of course that bus has to push around the lander's fuel as well. The mass of the lander plus fuel will be about 1200 kg if we use cryogenic LOX/H2. However, on a low energy transfer, the lander will spend about 5 months in space. Storing cryogenic fuels for that long is difficult (though maybe not impossible, I don't know enough about it), so in mission plan alpha we would probably use hypergolic fuels which are typically much heavier than cryogenic ones, for the same amount of delta-v. Which means that the lander would be even heavier than 1200kg. The total mass of the lander plus fuel may not be that different to the total mass of the CM and extension module. It may even be a little heavier! But that would be okay because the low energy transfer requires about 20% less delta-v.
The bottom line is that mission plan alpha may actually make sense afterall, from a fuel standpoint.
That said, I still think plan bravo is perhaps more attractive overall (one launch instead of two, one engine instead of 3, no toxic hypergolic fuels, no difficult maneuvering of CM to within spacewalking range of lander), but we should probably crunch the numbers on both options for a while to see which plan is truly best.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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8:30 pm
January 5, 2010
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Plan bravo is attractive to me because only using one booster to put our payload into orbit would be so much cheaper than using two slightly smaller rockets. Plus we eliminate one rendezvous and there is no need to worry about antenna time to follow the lander for five months.
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Main Workgroups: Propulsion & Spacecraft Engineering
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7:48 pm
January 6, 2010
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Luke Maurits
| | Adelaide, Australia | |
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These are very good points, I'd forgotten about the communication issues involved with plan alpha. I think we should adopt bravo as our "official" current plan, although I hesitate to do so on the opinion of just two members. I'd like to hear brmj's thoughts on this, but he is obviously busy IRL at the moment (I think he's even missed out on the very existence of plan bravo).
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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8:16 pm
January 6, 2010
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Yeah, I think that two is too few to make a "decision" like this. The big problem is that without accurate knowledge of vehicle masses, engine efficiency, and fuel costs it is nearly impossible to make an informed decision what is cost effective.
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Main Workgroups: Propulsion & Spacecraft Engineering
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3:05 am
January 10, 2010
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brmj
| | Rochester, New York, United States | |
| Member | posts 402 | |
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Post edited 9:06 am – January 10, 2010 by brmj
I'm just now getting re-involved with the project. Sorry everyone. I kind of drifted away over break due to a big project, though I did do some work towards Open Luna's space suit computer, so it's not entirely wasted.
Anyway, I've looked over the forums a bit, and my initial sense is that the fuel consumption numbers you promoted bravo based on are possibly bogus. Wikipedia says that the Japanese space probe Hiten had only 10% of the fuel it would have needed for a regular Hohmann transfer but was able to do a low energy transfer to the moon. If they are correct, and there is no weird set of circumstances that made it possible which we couldn't repeat, then we were too quick to discard alpha. If Wikipedia was wrong, however, then I would be willing to support bravo. It has some definite simplicity, launch scale and duplication reduction advantages, and if alpha isn't as beneficial from a fuel standpoint as Wikipedia has led me to believe, I think we ought to go with bravo.
In any case, it would be nice to have some real numbers and know what the hell we were doing, but I guess we'll just have to make do.
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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4:20 am
January 10, 2010
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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Good to see you back again, glad to know you're still around!
The easiest way for you to catch up on what's gone on while you were away would be to read this thread. In principle a lot of what is in there should have gone in more specific parts of the forum, but the advantage to it all being there is that you can follow a long unbroken chain of thought.
This thread would also be a good one to focus on early in your return.
If a low energy transfer from LEO to lunar orbit really is possible with 10% of the fuel of a Hohmann transfer then Alpha was absolutely written off on bad grounds and may be worth reconsidering. However, this would be a huge surprise to me. All the papers I found on low energy transfers gave requirements considerably larger than that: the quoted savings over a Hohmann transfer were always around 20%. I suspect maybe the Wikipedia article means it had 10% of the required fuel for a lunar capture burn, not an entire Hohmann transfer. This arrangement would leave it with just enough fuel for a low energy transfer like the one I described, where the 1km/s lunar capture burn is replaced by a patch point burn on the order of 10m/s (i.e. 10% of lunar capture) and the TLI remains unchanged.
Note that even if Alpha ended up requiring less delta-v once in space, it seems quite unlikely that it would be more efficient from the point of view of launches into LEO, requiring two instead of one. Also, a 5 month trip in space would make cryogenic fuel storage very difficult, forcing us to consider hypergolics which are (i) nasty and (ii) have a much lower specific impulse, increasing the mass of fuel required and hence launch cost. It would also require a larger power source and more powerful comms gear to send telemetry data from Earth-Moon L2 to Earth over a long time period. Basically, Bravo feels much easier overall, and our fuel requirement calculations for it show that we could get the entire mission stack weighing around 8000 kg, which would be easily launchable by a Falcon 9.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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8:55 am
January 10, 2010
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brmj
| | Rochester, New York, United States | |
| Member | posts 402 | |
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Okay, I did a little more research on the Hiten mission, and it looks like it my have started with a kind of wonkey orbit designed to allow it's sub-probe, which failed, to establish lunar orbit, hence the lower fuel requirements.
Plan Bravo it is, I guess.
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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