I Apollo 11: Torque & Thrust of LM RCS Systems

[darkdave3000]

TL;DR Summary

Is angular acceleration by torque reduced if the object in question is also accelerating linearly by rocket thrust? For example, if the "Eagle" from Apollo 11 was firing it's main rockets, will it impede rotation by RCS?

I'm writing a simulation software for the Apollo 11 missions, I'm not allowed to mention my website because of forum rules but you can see it in my profile.

I am trying to now implement the RCS systems into the LM. The LM is the lander such as the one called "The Eagle" in Apollo 11. What I want to know is if firing the main rockets will weaken the torque caused my RCS rotating the LM. Or if RCS torque will be completely unaffected and the spacecraft will rotate at the same angular acceleration and resulting angular velocity even with the main rockets at full thrust.

Also can anyone tell me the distance from center of gravity of the LM the RCS thrusters are so I can calculate the torque? I have the numbers for Newtons for thrust from each RCS rocket but I don't have the distance from COG of it to calculate the resulting torque from said thrust. I assume rotations are caused by 4 RCS thrusters at any time? So I have to multiply the force x 4? For example, if the LM rotats left then the two on the left both toward the viewer and away with fire the top thrusters and the 2 on the right will fire two similarly mentioned RCS thrusters but fire the ones poinging down?

 

[jbriggs444]

Summary:: Is angular acceleration by torque reduced if the object in question is also accelerating linearly by rocket thrust? For example, if the "Eagle" from Apollo 11 was firing it's main rockets, will it impede rotation by RCS?

Ideally, "no". Practically, perhaps "yes".

As long as the main engine thrust is aligned with the center of mass of the object, it will have zero torque about that center of mass. Accordingly, main engine thrust will not affect the angular momentum of the craft about the craft's center of mass and, consequently, will not affect its pitch or yaw rate.

That said, I have no practical experience with how well main engine thrust is actually aligned with craft center of mass. But I do know that thrust vectoring is a thing.

 

[darkdave3000]

Back in those days there was no thrust vectoring correct? May I assume thrust vectoring means the main rocket can cause the ship to change directions by swiveling the nozzle?

 

[jbriggs444]

Back in those days there was no thrust vectoring correct? May I assume thrust vectoring means the main rocket can cause the ship to change directions by swiveling the nozzle?

What I picked up from Wikipedia when I tried to sharpen my understanding to properly answer your question was that vanes in the exhaust stream were one of the first ways to achieve a useful effect. I'd always assumed the main purpose is to keep the craft stable during launch.

Back in the day (ancient chinese rockets) you would likely have to fall back on fin stabilization or spin stabilization.

 

[Ibix]

I thought the main engine of the lander was gimballed. I recall reading that Alan Bean (I think) described it as "a sporty ride".

 

[jbriggs444]

The main engine of the lander was gimballed, as I recall. I think Alan Bean described it as "a sporty ride".

With a better search term to Google with, it is easy to confirm that recollection.

used hypergolic propellants and a gimballed pressure-fed ablative cooled engine that was capable of being throttled.

 

[darkdave3000]

Does anyone happen to know the distance from COG to RCS on the Apollo 11 Lunar Lander?

 

[Lnewqban]

Does anyone happen to know the distance from COG to RCS on the Apollo 11 Lunar Lander?

I don't, but you may find enough information here as to make a rough estimate:
https://nssdc.gsfc.nasa.gov/nmc/ spacecraft /display.action?id=1969-059C

:)

 

[cjl]

Back in those days there was no thrust vectoring correct? May I assume thrust vectoring means the main rocket can cause the ship to change directions by swiveling the nozzle?

That is what thrust vectoring is, and it was indeed a thing on the Apollo rockets.