StarShip's RCS Thruster system

[darkdave3000]

TL;DR Summary

I noticed the internet has contradictory data on the position of the top RCS. One source place them just above the top fins and another just below it.

Source https://www.physicsforums.com/forums/astronomy-and-astrophysics.71/post-thread

Can everyone help me solve this mystery?



Starship has 8 sets of RCS thrusters, 4 at the top and 4 at the bottom near the base. The 4 at the top some youtube videos show them above the top fins:



Nitrogen RCS test:





But some other videos show them below the fins:





Are they both right because the Nitrogen ones are ment to be above the fins while the hot methane and oxygen gas piston version below the fins?



Or is the second one wrong and both types are supposed to be above the top fins?

Also can I assume that the COG is between wherever the top and bottom RCS are? Wont they shift?
Lastly does the top and bottom RCS need to work together or can the top fire without the bottom and vice versa to rotate the ship around the COG?



Check attached photo.



Merry Christmas!

David

 

[mfb]

They use cold gas thrusters now but plan to switch to hot gas thrusters in the future - using the same fuel as the engines to make refueling simpler (especially on Mars). It's possible the location has changed in the past and/or will change again in the future.
For SN9 and SN11 and SN15 they were above the fins. Rotating the ship works with a single set of thrusters but then you change your orbit slightly in addition.

 

[DaveC426913]

Summary::

Source https://www.physicsforums.com/forums/astronomy-and-astrophysics.71/post-thread

 

[berkeman]

View attachment 293294

I think it is a PF forum glitch that does that. I've seen it several times, but have not been able to figure out what causes it...

 

[mfb]

Just a copy&paste error if you take the URL from the wrong tab when starting a thread (i.e. from the tab where you start the thread).

 

[DaveC426913]

Just a copy&paste error if you take the URL from the wrong tab when starting a thread (i.e. from the tab where you start the thread).

Yeah. The OP would do better with this:

 

[darkdave3000]

They use cold gas thrusters now but plan to switch to hot gas thrusters in the future - using the same fuel as the engines to make refueling simpler (especially on Mars). It's possible the location has changed in the past and/or will change again in the future.
For SN9 and SN11 and SN15 they were above the fins. Rotating the ship works with a single set of thrusters but then you change your orbit slightly in addition.

How many seconds do the RCS thrusters stay on for both Apollo and Starship? Are they preprogrammed to do a certain number of seconds bursts per button click? Or just how long the button remain depressed?

Also do they use joysticks or buttons for RCS? I am thinking buttons make more sense for preprogrammed duration of bursts. Joystics might imply variable thrust/force output which I am guessing neither Apollo nor Starship uses?David

 

[mfb]

No one presses buttons on Starship for now, it flies autonomously.

Dragon flies autonomously by default but it can be controlled manually. This simulator has been widely praised for being very realistic (excluding the Easter-egg behind your starting position). The astronauts have a touch screen.

For Apollo I have found this documentation, which suggests that the astronauts had joysticks but the actual on/off commands to the thrusters still came from the computer interpreting the joystick motion.

 

[darkdave3000]

No one presses buttons on Starship for now, it flies autonomously.

Dragon flies autonomously by default but it can be controlled manually. This simulator has been widely praised for being very realistic (excluding the Easter-egg behind your starting position). The astronauts have a touch screen.

For Apollo I have found this documentation, which suggests that the astronauts had joysticks but the actual on/off commands to the thrusters still came from the computer interpreting the joystick motion.

Question remains: do either system use continuous or discrete thrusts and are they fixed or variable force?

 

[Benjies]

Question remains: do either system use continuous or discrete thrusts and are they fixed or variable force?

The thrusters are very likely variable force. A strong selling point of the pressure-fed thruster is deep throttle. If you are an engineer tasked with controlling either ascent or descent of this vehicle, you are only complicating the task by using a fixed-thrust RCS. In staged cycle engines throttle requires regulation of a preburner, but in this case, all that needs to be done is vary the mass flow of propellant. The RCS is incredibly simple, in theory, and easy to control.

Whether or not the thrusts are continuous or discrete I cannot comment on- I'll leave that up to those with more knowledge in dynamics than me. I'd make a venture of a guess and say that they are continuous to allow the vehicle to unendingly correct (in a worst-case scenario maneuver, see Astra's crazy launchpad correction) if necessary. Discrete thrusts to attempt to correct trajectory seems less versatile than continuous thrust for trajectory correction. Imagine being forced to hit a pool ball with precisely the minimum required force to reach a pocket, as opposed to providing the player the freedom to accelerate the ball until I am perfectly confident the ball will reach the pocket. Perhaps an over-simplification, but an illustration of my thought process. Again, please consider the opinion of someone with more dynamics experience than me.