TRUST ON pushbutton

[Sverige]

Hello friends!

I'm unsure when the crew uses the THRUST ON pushbutton and when they decide to use the SPS THRUST DIRECT ON switch.

If I understand correctly, SPS THRUST DIRECT ON provides alternative backup mode to both CMC and SCS delta V modes? So when is there a need to use the TRUST ON pushbutton?

Another question is whether it is necessary to use the DIRECT ULLAGE pushbutton if the TRUST ON pushbutton is used? In what situation is there a need to use THRUST ON and DIRECT ULLAGE pushbuttons?

:tiphat:
Mikael

 

[indy91]

Hello friends!

I'm unsure when the crew uses the THRUST ON pushbutton and when they decide to use the SPS THRUST DIRECT ON switch.

If I understand correctly, SPS THRUST DIRECT ON provides alternative backup mode to both CMC and SCS delta V modes? So when is there a need to use the TRUST ON pushbutton?

Another question is whether it is necessary to use the DIRECT ULLAGE pushbutton if the TRUST ON pushbutton is used? In what situation is there a need to use THRUST ON and DIRECT ULLAGE pushbuttons?

:tiphat:
Mikael

This wiki article should already answer parts of your question: http://nassp.sourceforge.net/wiki/Service_Propulsion_System#Engine_thrust_on.2Foff_logic

As you said, the SPS thrust direct on switch is a backup method to have the SPS running even if CMC and SCS have failed. the Thrust On pushbutton is the main method to start the SPS under SCS control. There you only need to press it once and it will cut off the engine when the EMS DV counter has reached 0.

Direct Ullage fires the +X RCS thrusters for a propellant settlement burn, which usually would be done with the THC, for a few seconds before SPS ignition. So e.g. under SCS control you would keep the Direct Ullage button pressed until SPS ignition and then start the SPS with the Thrust On button.

 

[Sverige]

Hi Indy,

A very good and understandable article but too simple and it does not answer my question. What is the reason that there is both an SPS THRUST DIRECT ON switch and a THRUST ON pushbutton with same function and when does the crew choose one for the other? They seem to have the same function.

/Mikael

 

[indy91]

Hi Indy,

A very good and understandable article but too simple and it does not answer my question. What is the reason that there is both an SPS THRUST DIRECT ON switch and a THRUST ON pushbutton with same function and when does the crew choose one for the other? They seem to have the same function.

/Mikael

THRUST ON is part of the SCS electronics. You just push it once and the SCS keeps the engine running. SPS THRUST DIRECT ON is an override switch, it will keep the SPS running while the switch is in direct on, neither CMC or SCS can command a cutoff. If most of the electronics have failed then CMC or the SCS (Thrust On button) won't work anymore, but SPS THRUST DIRECT ON is more directly linked to the SPS and will work as long as there is any power to the SPS.

 

[Sverige]

Thank you Indy, I think I understand now. If I understood correctly, you can always start the SPS engine with SPS THRUST DIRECT ON regardless CMC or EMS / delta V setting. An EMS / delta V Setting below 0.0 will not prevent the engine from starting and will not stop it in NORMAL in SCS mode. To stop the engine, you have to set delta V THRUST NORMAL A or/and B in OFF.

/Mikael