AtlasV Centaur ISP?

[BrianJ]

Hi,
I'm wondering what ISP to use for the AtlasV "Centaur" upper stage.

Everywhere I look on the web (including the AtlasV Mission Planner's Guide .pdf) gives it as ~451s (or 4424 Ns/kg).

This seems quite high to me and I always end up with ~10% fuel left over after launching the MRO or LRO missions, even when using quite conservative values for the AtlasV 1st stage.

Jim Clem's old AtlasV add-on gives a value of ~3809Ns/kg for the Centaur which leaves very little fuel after the final burn.

Anybody have an opinion on which is the more realistic ISP value to use in Orbiter?

Thanks,
Brian

 

[Urwumpe]

4424 Ns/kg sounds like the right value for a RL-10 engine. It uses hydrogen+oxygen and is a expander cycle engine (and not a gas generator cycle)

http://www.astronautix.com/engines/rl10.htm

 

[BrianJ]

OK, thanks very much!

I wonder - would they off-load some propellant according to mission requirements? Or is the extra 10% fuel included as a "safety margin"?

 

[eveningsky339]

I'm not entirely sure, but I believe for the most part excess upper stage propellant is left as a safety margin.

 

[Urwumpe]

I wonder - would they off-load some propellant according to mission requirements? Or is the extra 10% fuel included as a "safety margin"?

Partially it is off-loaded, but another part of it is also safety, propellant utilization and "end-of-mission" reserve.

The propellant utilization part is needed because the pumps of the rocket engine should never, under no circumstances, run dry during a burn - there always has to be enough propellant left after the burn to shut the engine down intact. Running dry is about the worst that can happen to a modern pump-fed bi-propellant rocket engine, as this can cause small explosions and engine fires.

The end of mission part is the amount of fuel used for bringing the centaur stage away from the spacecraft after the spacecraft is released (also called collision avoidance maneuver). For LEO and GTO launches, this EOM phase is used for lowering the perigee of the centaur stage, so it reenters earlier.

 

[BrianJ]

Yep, that figures. Thanks for the info, guys