Would the real Titan SRB specs please stand up...

[Usonian]

I am modeling a TitanIIIE (with Velcro rockets) to launch a Voyager addon I have in progress. I am running into a snag on the 5 segment solid rocket boosters (UA1205). As always, the numbers do not agree among various on-line sources, but they are close enough:

Total Mass: 226,533 kg - 226,800 kg
Empty Mass: 33,798 kg

Fuel Mass (Mf): 192,435 kg (maximum possible - simply the difference of the total and empty mass)
Thrust (Fmax) : 5.34 kN - 5.849 kN
Burn Time (Tb): 110 - 115 seconds
Isp: 263 sec. or 2,580 m/s (assuming I correctly converted from seconds to meters/second).

If you apply these figures to the formula used by Orbiter:
Tb = Mf * Isp / Fmax
You come up with a burn time of 85 seconds. I usually see these things checking out a whole lot closer.

I am suspicious of the Empty Mass figure. I can not find a NASA or manufacturer's source for empty mass or fuel mass. (Go figure - the booster was last manufactured 20 years ago.) The 33,800kg figure comes from astronautix.com -- not always a reliable source.

What do the rocket scientists think? Is the 110 second burn time wrong? Or is the empty mass wrong? Is it reasonable to have a solid rocket with with only 85% of its total weight in fuel? (These boosters did have the nitogen side tanks - or was that N2O2? - for thrust vectoring, so maybe that's the difference.) Or is it the 263 second Isp, a figure that also comes from astronuatix?

WTF?

 

[tblaxland]

The thrust is the maximum rated thrust but a typical SRM will only achieve that for a very brief time. Lower thrust -> increase burn time. Thrust profiles for the shuttle SRBs are available online, that should give you an idea how much lower the average thrust is.

 

[sputnik]

The UA1207 thrust profile is very non-linear. The maximum thrust value is only achieved for a short time, then the burn area decreases a bit and so does the thrust.

CVEL Titans models this by setting thrust level to max, then implementing a throttling-back profile hard-coded into the .dll.

Velcro Rockets models this by choosing an average thrust and having it stay constant throughout.

 

[tblaxland]

Would the thrust profile of the STS SRB's be a reasonable approximation to the UA1207 profile?

 

[sputnik]

Looks vaguely familiar, yes.

Probably an inevitable consequence of any core-burning propellant starting from an initial star shaped surface, I suppose.

 

[Usonian]

Excellent answers -- thanks for the replies.

Sputnik, I have a Velcro question that I will post in you addon thread.

Thanks