General Question MainThrust and ISP values used on the SC3 Atlantis

[4throck]

Ok, this is n00b question but I'm confused with the MainThrust and ISP values used on the SC3 Atlantis.

On the config we have:
Main_Thrust=53400
ISP=5000

But on Astronautix, Wikipedia, etc, the values are:
Main Thrust = 1859 kN
ISP = 453s
Effective exhaust velocity = 4440 m/s
[ame="http://en.wikipedia.org/wiki/Space_Shuttle_Main_Engine"]http://en.wikipedia.org/wiki/Space_Shuttle_Main_Engine[/ame]

I understand that Orbiter ISP will be s*9.8066, (or exhaust) so ISP=5000 is approximate.

But for thrust I don't understand. It should be Main_Thrust = 1859000*3 = 5577000.
And in this case shouldn't ISP be tripled?

Sorry for such a basic question, but I've searched the Forum, the SC3 and Orbiter docs, etc and didn't find an answer with any real world examples. :facepalm:

 

[DaveS]

There was a time when the technical specs of the default Atlantis on which the SC3 Atlantis is based, was not terribly accurate. This lead to the desire to create a new more accurate version which was named "Atlantis Mod".

 

[4throck]

Ok, and what values are used on that version? Or the current one?

The Orbiter variables are the same, regardless of SC3, LUA, dll or vessel config.

 

[N_Molson]

And in this case shouldn't ISP be tripled?

Why ? Regardless if you have 1 or 10 same engines, this doesn't affect the Isp

Also I think that 53400 N is for the OMS pods, and 1859 kN (1859000) is for 1 SSME. I guess that there is 1 engine defined for the 2 OMS pods and 3 engines for the 3 SSME in the mod.

 

[DaveS]

Ok, and what values are used on that version? Or the current one?

The Orbiter variables are the same, regardless of SC3, LUA, dll or vessel config.

The last version of Atlantis Mod (1.5) can be found here: [ame="http://www.orbithangar.com/searchid.php?ID=1804"]Atlantis Mod 15a[/ame]

Source code is included.

 

[4throck]

Thanks for the info, but my point is more about what values to use in a simple implementation.

So back to my original question.

If I consider the 3 SSME as one engine then Main_Thrust = 1859000*3 = 5577000 right :yes: ?

That's all I need to know. Sorry for the insistence, but If I don't ask, even if the question is dumb, I'll learn nothing. So I prefer to learn (even if I found out that I'm really just being dumb... )

 

[DaveS]

Thanks for the info, but my point is more about what values to use in a simple implementation.

So back to my original question.

If I consider the 3 SSME as one engine then Main_Thrust = 1859000*3 = 5577000 right :yes: ?

That's all I need to know. Sorry for the insistence, but If I don't ask, even if the question is dumb, I'll learn nothing. So I prefer to learn (even if I found out that I'm really just being dumb... )

The best source for technical data on the Shuttle is the Shuttle Crew Operations Manual (SCOM) available for download here: http://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf

 

[Cras]

Simple, dont use the default Atlantis, use Shuttle Fleet instead

 

[4throck]

It checks out. The indicated thrust is 2283049N at 109% and that's for the 3 engines combined.
That's close enough to the 2278kN on Wikipedia.

Perhaps I wasn't clear on my original question, but my problem was not the Atlantis performance, was with the strange values I generally see in configs! Atlantis was just an example. I have no clue where the Main_Thrust=53400 comes from.

All this started because I tried to correct the Gemini Agena Docking Target. Again, I find no relation between the actual values and the ones on the add-on. So I checked Atlantis, just to find the same thing... So I posted about it

 

[francisdrake]

To clarify the shuttle Main_Thrust: Thats the OMS thrust.
2 OMS x 26 700 N each = 53 400 N total.

All thrust values are in Newton [N], all ISP values in [m/s].

 

[4throck]

Thanks, that's exactly what I needed to know! :thumbup:

I was interpreting main_thrust literally, for the shuttle's main engines! Of course that makes no sense, but it happens when one is only looking at numbers....