General Question Apollo LM Ascent Stage Hardware Question

[wllmpeek]

This is a general question about Apollo hardware not necessarily about Orbiter.
Looking at diagrams of the Lunar Module it appears to me the Landing Radar was located on the underside of the decent stage. So,,, once the ascent stage lifted off, how did the astronauts measure their altitude above the lunar surface? From what I can see, the ascent stage only has a rendezvous radar not a ground radar. Did they just rely on accelerometers to predict these measurements? Are there any Apollo era geeks out there in the know????

 

[indy91]

Yes, they did rely on accelerometer measurements during the ascent. The current position and velocity was calculated by the Average-G Routine. During the ascent altitude didn't matter too much, the insertion altitude was simply 60,000 feet above the landing site radius. They just launched and from that point on the computer didn't care about altitude.

During the landing altitude is of course important, so they improved the onboard knowledge of the current position (at least the vertical component) with the landing radar while also mostly relying on the accelerometers, especially in the first part of the descent when the landing radar couldn't work yet.

 

[Urwumpe]

Also, it was cheaper to install a landing radar, than to teach the computer to store terrain maps.

 

[Hlynkacg]

Primary guidance was via the accelerometers, but this could be augmented the CSM providing its own track. IE the CSM knows its own position + velocity and could get a relative position + velocity of the ascent module via radar and transponder.

 

[Urwumpe]

Primary guidance was via the accelerometers, bit this could be augmented the CSM providing its own track. IE the CSM knows its own position + velocity an could get a relative position of the ascent modlue via radar and transponder.

This would likely only amplify the errors. The landing radar is the best way since it does not add additional errors - the IMU of the CSM + radar tracking on the other hand would reduce the accuracy of the LM, since it is offset in the gravity field of the moon, has IMU errors and radar errors as well. And then there is also the problem that the IMU is usually powered down except when it is really needed, because it consumes a lot of electric power.

 

[GLS]

Also, it was cheaper to install a landing radar, than to teach the computer to store terrain maps.

I think they did something like that from Apollo 15 on, to correct the radar data as they flew over mountains.

 

[indy91]

During powered flight the accelerometers were the primary guidance. During coasting flight the computer periodically did a state vector integration to update the state vector.

Now, additionally to that the LM had the rendezvous radar and the CSM had a VHF ranging system and the optics subsystem to update their state vector or the state vector of the other vehicle. This was important for the lunar orbit rendezvous, because after insertion the LM did not have a perfect knowledge about its state. So before a nominal ascent the LM got the most recent CSM state vector, which was mostly calculated by MSFN tracking. This state vector stayed quite accurate during the rendezvous because nominally only the LM did the rendezvous maneuvers, so it comes down to orbital mechanics for the CSM. So the LM was alternating between updating the LM state vector with the rendezvous radar and calculating and executing the rendezvous maneuvers.

The IMU of the CSM was powered up at all times, except during Apollo 7 (for testing purposes) and Apollo 13 (for obvious power saving reasons). The LM was not powered up until a few hours before landing and from Apollo 12 on they powered down LGC and IMU on the lunar surface. At all other times (which isn't much in the grand scheme of a complete lunar mission) the IMU of the LM was running, too.

The landing radar has nothing to do with any of this. :lol:

I think they did something like that from Apollo 15 on, to correct the radar data as they flew over mountains.

Ah, right, I forgot about this. It wasn't quite a "terrain map", but at least five line segments so that the sensed altitude could be converted to the correct lunar radius for updating the state vector when flying over those mountains. It would be nice to have Luminary 1E (The AGC version flown on Apollo 15-17 LM) for NASSP in Orbiter 2015 at some point to use this.

 

[Hlynkacg]

This would likely only amplify the errors. The landing radar is the best way since it does not add additional errors...

The landing radar is not present during ascent and rendezvous (being mounted on the descent stage) and wouldn't provide much in the way of meaningful data even if it were. Altitude AGL and ground track aren't nearly as vital to establishing a stable orbit as knowing your velocity and acceleration vectors.

Likewise seeing as the whole point of the exercise is to rendezvous, knowing your orbital velocity and inclination relative to your target is of far greater immediate use than knowing your absolute velocity and inclination in the equatorial or ecliptic frame.

 

[Urwumpe]

For the rendezvous, a coarse initial orbit is already good enough - it did use rendezvous radar later, when the coarse data was no longer accurate enough.

 

[wllmpeek]

Hmm... Seems to me, I'd only feel safe near any planetary body with a ground radar attached to a computer running short arc extrapolations at all times to give me piece of mind that I'm not going to run into the darn thing...

 

[MikeB]

... I'd only feel safe near any planetary body with a ground radar attached to a computer running short arc extrapolations at all times to give me piece of mind that I'm not going to run into the darn thing...

You have to trust the force (of gravity).