Convair's 1962 Lunar Excursion Module proposal

[Sus_pilot]

Great movie. Some of the best air-to-air footage ever. (Paul Mantz' work.)

[Jason Thompson]

I've often wondered why Grumman chose to go with separate descent and ascent engines.  It always seemed to be a lot of extra mass that you could shed just by leaving a hole in the bottom of the descent stage as in this design.  I have read Kelly's Moon Lander but I don't recall his addressing that specific design choice.

One possibility I can think of (and again, I'm not an engineer) is that it might have been a more 'comfortable' proposition to have a plane of separation between the two stages rather than having the engine poking through a hole in the descent stage, as in the Convair design. In the event of an abort you'd have to be sure of a clean withdrawal of the engine through the hole.

ka9q mentioned redundancy, and it's possible that the convair design may have morphed to having separate ascent and descent engines had it been carried forward. They already have the 'standby thrust chambers' there, so it doesn't seem like a huge step to put the big engine on the descent stage instead and save weight on the ascent stage. That would also remove the need to sever fuel lines between the two stages on staging.

[Jason Thompson]

http://www.flickr.com/photos/sdasmarchives/7142968041/

Would someone please reassure me that I'm not the only person who mentally sees those RCS booms flapping in this image?

[LunarOrbit]

It does look a lot like a hummingbird, or the Twitter logo.

[Daggerstab]

As I said, Angry Birds: Convair Edition.

[BazBear]

Reading the post and then clicking on the link, it looked a lot less ridiculous than I was expecting

I agree. Whenever I think Convair, the first thing that pops in my head are Delta Darts/Daggers and Hustlers.

B-36's. 

Lovely beast!  They had gear-reduction between the engine & props to keep the blade tips sub-sonic, and this gave them a very distinctive sound.  Time to watch Strategic Air Command, starring Jimmy Stewart, again!

Good flick, but I still hate the looks of that bird! IIRC the distinctive sound was more due to the fact that it used a pusher configuration and the props "chopped" the pressurized airflow off the wing.

[Daggerstab]

Thread necromancy alert!

Here's a 1962 Convair promotional film about their LM proposal:

I was looking at something else on YouTube when I noticed it in the suggested videos and it reminded me of this thread and my old efforts verifying Notchev's alleged contributions to Apollo. (He's still on that Wikipedia page...)

[raven]

I've often wondered why Grumman chose to go with separate descent and ascent engines.  It always seemed to be a lot of extra mass that you could shed just by leaving a hole in the bottom of the descent stage as in this design.  I have read Kelly's Moon Lander but I don't recall his addressing that specific design choice.

One possibility I can think of (and again, I'm not an engineer) is that it might have been a more 'comfortable' proposition to have a plane of separation between the two stages rather than having the engine poking through a hole in the descent stage, as in the Convair design. In the event of an abort you'd have to be sure of a clean withdrawal of the engine through the hole.

ka9q mentioned redundancy, and it's possible that the convair design may have morphed to having separate ascent and descent engines had it been carried forward. They already have the 'standby thrust chambers' there, so it doesn't seem like a huge step to put the big engine on the descent stage instead and save weight on the ascent stage. That would also remove the need to sever fuel lines between the two stages on staging.

The Soviet LK also used a single rocket for both descent and ascent. One reason to use two is it means you don't have as much dead weight, just like any multistage system. You need a larger engine and bigger fuel tanks to get from the orbit to surface, and using just one means dragging the tanks and larger, more complex engine back up. A second reason might be safety. While it's true a single system is inherently more reliable than two, if something goes wrong during descent, you can light the ascent  engine and get your ass back to orbit. In a one engine setup, you either need a backup engine (which the LK had, which means more dead weight) or your ass is grass. These are my thoughts as a non-engineer, so I could be completely off base here. Any and all criticism and education is welcome.