COOOOOOOOOOOOOOOOOOL!!!

September 19, 2005 12:03 PM

NASA has released details today of their new "exploration architecture" which they hope will take them to the moon and mars. (really awesome movie). Help me understand a design decision.

Here's a shot of the earth-moon transit stack. According to the above movie, the part on the left (with two engines and a great honkin tank) burns all the way to the moon, and is then ejected. The engine on the right (bearing a remarkable likeness to the Apollo service module) brings the spacecraft back.

As I understand it, The Apollo service module performed both duties, (it separated from the Saturn third stage while still in earth orbit).

So why the giant engine this time, and why for just one way?

posted by Popular Ethics to science & nature (20 comments total)

Seems like they want more power, etc.
posted by delmoi at 12:18 PM on September 19, 2005

The new capsule is three times the size of the Apollo capsule. That might be (part of) the reason.
posted by winston at 12:26 PM on September 19, 2005

By far the toughest thing it has to do is escape Earth's gravity. It only has to do that one way. The moon's gravity is only 1/6 that of the Earth's.
posted by nthdegx at 12:28 PM on September 19, 2005

Yup. "The Crew Exploration Vehicle (CEV) capsule will weigh about 50% more than the Apollo capsule." "...the crew of four astronauts is double the number that landed on the Moon first time around. The maximum time the first astronauts will spend on the Moon's surface will also increase to seven days from Apollo’s three."

Of course, as you add more mass, you need more fuel to take it places, then you need more fuel to take the extra fuel places, etc. Not linear.
posted by trevyn at 12:29 PM on September 19, 2005

Note: I am not a rocket scientist, and my knowledge of the new design stems from your linked article and this one. Basically, the Apollo system was designed solely with moon-landing in mind - the service module had just enough thrust to put the CEV on an arc to be captured by the moon's orbit. Because the moon is more-or-less planar to the earth, the Apollo system could only land astronauts on the equatorial zone of the moon - they didn't have much extra fuel to change their orbit. Not only is this new system much more scaleable (there's no way the Apollo design could have accomodated for the distance to Mars), the large engine allows NASA to orbit and land pretty much anywhere they want. This is also in preparation for Mars, where it would have been much harder to utilize Mars' gravity well.
posted by muddgirl at 12:30 PM on September 19, 2005

nthdegx, trevyn, etc are missing the point of PE's question - he's wondering why they have to use a huge engine at all for that stage of the trip, instead of just pointing them in the right direction and giving them a bit of a push, like they do in the Apollo missions.

I just realized it would shorten the trip considerably if you were accelerating and deccelerating the entire trip.
posted by muddgirl at 12:32 PM on September 19, 2005

The service model didn't get us out of Earth orbit. The Saturn V third stage did. It was only on the way to the moon that the command module seperated from stage 3, grabbed the LM, and then left the third stage behind. The command module then used its engine to get in and out of lunar orbit.
posted by bondcliff at 12:39 PM on September 19, 2005

By "command module" I mean the CM/SM stack, of course.
posted by bondcliff at 12:41 PM on September 19, 2005

Saturn V at Wikipedia
posted by bondcliff at 12:42 PM on September 19, 2005

OK, I've got it. The apollo article I linked to lied to me. The Saturn V third stage was used for "trans-lunar injection" (not discarded in orbit as I thought. In fact, one of them reappeared recently). I didn't appreciate how much extra energy is needed to escape Earths gravity.

As for being so much bigger than Apollo, I think you've all got it right: The CEV is bigger, and can go further, so requires more fuel. I'm marking trevyn and muddgirl only because their answers are more verbose :)

Thanks everyone.
posted by Popular Ethics at 12:45 PM on September 19, 2005

and why for just one way?

That's the portion of the question I chose to answer, muddgirl. Hardly "missing the point".
posted by nthdegx at 12:50 PM on September 19, 2005

I don't believe the "Earth departure" stage of the new system burns all the way to the moon. According to the fact sheet:

The Earth departure stage ignites suborbitally and delivers the lander to low Earth orbit. After the crew spacecraft docks with this system, the Earth departure stage performs a trans-lunar injection burn, which starts the vehicle's journey to the moon. When the burn is completed, the Earth departure stage is discarded.

I think the "Earth departure stage" of the new system is roughly equivalent to the Apollo program's S-IVB third stage. In the Apollo program, the Saturn V's third stage (S-IVB) delivered the lunar and command service modules to low Earth orbit and then restarted to perform a trans-lunar injection burn, accelerating the craft to 40,000 km/hr and placing Apollo on a lunar trajectory. A couple of hours later the command service module separated from the third stage, turned around, docked with lunar module, and the combined lunar/command/service craft then separated from the third stage and basically "coasted" to the moon.

With regards to the movie, I think you overlooked the "fade to black" sequence in the movie after the the Earth departure stage burn and separation and before arrival in lunar orbit. I think that fade was intended to indicate the (long and visually uninteresting) trip of the new crew vehicle and lander between earth orbit and lunar orbit.
posted by RichardP at 1:01 PM on September 19, 2005

Hah, after busying myself in another task, I should have re-previewed before making my post.
posted by RichardP at 1:03 PM on September 19, 2005

Now that the original question seems to have been answered, I'd say that the better question is why does NASA want to spend over $100 Billion to send manned flights back to the moon. We've been to the moon. There isn't anything there.

I'm a huge fan of space exploration, but a bigger waste of money, I cannot imagine. Does this gain us anything at all?
posted by willnot at 1:49 PM on September 19, 2005

There isn't anything there.

Well, there's that monolith buried under Tycho...
posted by bshort at 1:57 PM on September 19, 2005

The lunar mission seems to be, in part, a proof-of-concept of both the spacecraft design and the life-support systems in preparation for a Mars mission.

And they probably expect many more spin-offs.
posted by Zed_Lopez at 2:06 PM on September 19, 2005

willnot - I believe one goal is to set up a semi-permanent moon colony for research and commercial purposes.
posted by muddgirl at 2:27 PM on September 19, 2005

A few notes about this design, not sure if it answers the question, but...

I recently read an article about a private firm proposing a similar approach. The firm was made up of ex astronauts and nasa folks. I suspect that this proposal is what we are now seeing.

There are several immediate benefits.

1) it uses a tried and tested rocket system (the boosters and tank from the current shuttle)

2) by placing the crew modual ontop of the rocket, we eleminate the nasty foam problem causing reentry issues.

3) because it is fairly modualistic, it's easy/cheap to send up an assortment of different types of missions. A cargo vessel, A crew vessel, a lunar lander, etc. The current shuttle is fairly limited in comparison.
posted by mmdei at 4:06 PM on September 19, 2005

Just for kicks, and to have something to show for the 30 minutes I just spent reading up on this, here's a comparison of the amount of energy required to reach various goals. In rocketry and orbital dynamics, the amount of energy is expressed in "delta-V". I won't explain what it means, but in short it's the amount of energy needed.

Suborbital flight, like Alan Shepard in 1961 or SpaceShipOne in 2004: 1.4 km/s
Low earth orbit like the Space Shuttle or ISS or Hubble: 9.7 km/s
Geosynchronous orbit like most communication satellites, including DirecTV and Dish: 13.8 km/s
Lunar landing: 15.2 km/s
Mars landing: 13.5 km/s *
Return from Mars surface to Earth surface: 6.4 km/s

* note that it takes less energy to land on Mars than to land on the moon because you can use the Mars atmosphere for braking; note also that says nothing about getting back off Mars or how quick the "cheapest" trip to Mars might be (years?).

Please note, again, these are "delta-V" values, not speeds. Read this excellent explanation of delta-V for more; that's where I got most of this data from.

Metafilter uses [strong] instead of [b] for bolding. That is so HOT.
posted by intermod at 7:59 PM on September 19, 2005

Metafilter can use both strong and b!
posted by delmoi at 8:06 AM on September 21, 2005