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6:00 am
November 30, 2009
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Luke Maurits
| | Adelaide, Australia | |
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I'm surprised this didn't ocur to me sooner during all my orbit simulations: the different masses of the Earth and moon allow for a significant asymmetry in "to" and "from" trip time.
To get from the Earth to the moon one has to travel at least as fast as Earth's escape velocity (about 11 km/s). Travelling at this speed or just a little bit faster, you'll get to the moon in about 3 days.
However, to get from the moon back to Earth, you only need to go faster than the moon's escape velocity, a mere 2.4 km/s. While we could give ourselves a large delta-v when leaving the moon and travel home at about 11 km/s over about 3 days, we could also leave the moon at, say, 3 km/s, and coast home over closer to 12 days.
Taking a slow trip home could save us big time when it comes to propellant mass. Every extra day in space adds some mass in extra food, water and air, but it saves us mass in propellant and oxidiser by allowing a smaller trans Earth injection burn. I am willing to bet that even travelling at 2.41 km/s and taking as long to get home as practically possible, this would still result in an overall mass reduction.
Of course, there are other factors to consider. The longer our astronaut is in space the more likely they are to start suffering psychological problems, and the more of a chance we have for rare equipment failure to mess things up. We probably don't want to draw the return trip out too long, but there's probably a lot to be said for making a relatively leisurely trip home.
This may also have implications for reentry since we would be hitting the atmosphere at different speeds depending on which approach we took. Definitely worth thinking about.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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7:30 am
November 30, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Post edited 1:32 pm – November 30, 2009 by Rocket-To-The-Moon
I think that I am in favor of a slow return for several reasons. As you mentioned it will reduce the required propellant mass which ultimately makes everything else scale down. Also the reduced reentry velocity means that our heat shield can be thinner and lighter. I don't really think that psychological effects would manifest themselves that quickly in an otherwise healthy person. They will be in 24 hour contact with the ground so it isn't like they are totally isolated.
We would have to do the calculations to figure out what is the most efficient speed from a total vehicle mass standpoint. But I'm willing to bet that the extra food and water is only a fraction of the mass of the saved propellant.
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Main Workgroups: Propulsion & Spacecraft Engineering
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3:13 pm
November 30, 2009
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perpindicular
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i don't really think a slow return is really goign to affect the size of our heat shield significantly. If anything, I would be up for returning faster, we have data to analyze. the faster we analyze that data, the faster we can go back. anythinig more than 5 days for a return seems to me to be excessive. The longer the craft is out there, the more time it has to fuck up
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4:45 pm
November 30, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Post edited 10:50 pm – November 30, 2009 by Rocket-To-The-Moon
perpindicular said:I don't really think a slow return is really going to affect the size of our heat shield significantly. If anything, I would be up for returning faster, we have data to analyze. the faster we analyze that data, the faster we can go back. Anything more than 5 days for a return seems to me to be excessive.
The energy that the spacecraft's heat shield has to dissipate is proportional to the square of its velocity. If we can halve the atmospheric relative reentry speed then we reduce the amount of energy that has to be dissipated by a factor of four.
According to Wolfram|Alpha, a 1000kg reentry vehicle has 58 gigajoules of kinetic energy while traveling at 10770m/s (the speed of Apollo 4's reentry). Reducing the reentry velocity to 6000m/s results in a kinetic energy of 18 gigajoules. Backing all the way to Luke's figure of 2.4km/s would come down to 2.88 gigajoules (but the craft will accelerate as it nears earth…right).
We could also dump all of the remaining RCS/Bus fuel to slow the capsule down even further.
From this analysis I see reduced propellant mass + reduced heat shield mass = lighter craft = smaller booster = less money
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Main Workgroups: Propulsion & Spacecraft Engineering
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5:22 pm
November 30, 2009
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perpindicular
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cant argue against numbers but i would rather have a thicker shield than a thinner one. I know we're trying to save on money, but I'd rather the astronaut be safe and alive than having some sort of risk with it being thinner.
if anything we could compromise by traveling 2/3's of the way at 6km/s then reducing speed to half of that and less on the approach, through retrorockets/etc and have a (relatively) slow descent/landing.
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7:13 pm
November 30, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Post edited 1:15 am – December 1, 2009 by Rocket-To-The-Moon
Luke Maurits said:To get from the moon back to Earth, you only need to go faster than the moon's escape velocity, a mere 2.4 km/s. While we could give ourselves a large delta-v when leaving the moon and travel home at about 11 km/s over about 3 days, we could also leave the moon at, say, 3 km/s, and coast home over closer to 12 days.
Is this true? Can we really coast at 2.4km/s? Won't the Earth continuously accelerate the CM as it moves toward the Earth?
According to Wikipedia the Apollo vehicles needed to accelerate by 1076m/s for TEI. I can't readily find how fast the Apollo CM orbited the Moon, but my guess is that the Delta-V of 1076m/s brought the vehicle just up to escape velocity. Then it continuously accelerated toward Earth (due to gravity alone) and finally reached its peak velocity of ~10,000m/s.
From my understanding the only way to slow down on the return trip is to perform a retro burn which would consume a lot of fuel.
Am I missing something?
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Main Workgroups: Propulsion & Spacecraft Engineering
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7:25 pm
November 30, 2009
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brmj
| | Rochester, New York, United States | |
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Rocket-To-The-Moon said:
Is this true? Can we really coast at 2.4km/s? Won't the Earth continuously accelerate the CM as it moves toward the Earth?
According to Wikipedia the Apollo vehicles needed to accelerate by 1076m/s for TEI. I can't readily find how fast the Apollo CM orbited the Moon, but my guess is that the Delta-V of 1076m/s brought the vehicle just up to escape velocity. Then it continuously accelerated toward Earth (due to gravity alone) and finally reached its peak velocity of ~10,000m/s.
From my understanding the only way to slow down on the return trip is to perform a retro burn which would consume a lot of fuel.
Am I missing something?
No, I think your analysis is acurate. Something was bothering me about this thread, and now I feel like an idiot. Thanks.
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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10:30 pm
November 30, 2009
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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brmj said:
now I feel like an idiot.
How do you think I feel? :p
For circular orbits of the moon at an altitude of 100, 150, 200 and 250 km, the required orbital velocities are 1633.8,
1612.1, 1591.1 and 1571.0 m/s. A delta-v of 1076 would indeed bring these sorts of velocities up to just over esape velocity, so obviously acceleration by the Earth is enough to significantly increase the final approach velocity.
My bad, good catch by Rocket.
As for:
If anything, I would be up for returning faster, we have data to analyze. the faster we analyze that data, the faster we can go back.
I think the limiting factor in how long it would take us to go back would be fund raising and construction time. While data analysis is going to be interesting and useful, it's in no way urgent or essential to a return trip (the very fact that the first trip has worked shows this) and hurrying back faster than we need to just to drop off the data makes little sense to me.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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11:20 pm
November 30, 2009
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perpindicular
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Luke Maurits said:
As for:
If anything, I would be up for returning faster, we have data to analyze. the faster we analyze that data, the faster we can go back.
I think the limiting factor in how long it would take us to go back would be fund raising and construction time. While data analysis is going to be interesting and useful, it's in no way urgent or essential to a return trip (the very fact that the first trip has worked shows this) and hurrying back faster than we need to just to drop off the data makes little sense to me.
the thing is, we will want to know what went wrong, or why something went right or why something unexpected happened. These things will help us improve the next launch and be prepared for other events. While i was wrong in saying we should rush to bring back the data, i'm sure there will be a parade waiting for the returning astronaut when he (or she) comes home.
and if we are the first people to get someone back on the moon, i don't think finding funding will be very hard
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