On 3/3/2018 12:44 PM, Henry Spencer wrote:
On Sat, 3 Mar 2018, Nels Anderson wrote:If I were doing it today - and the cost and complexity of gimballed solids is such that I'd seriously consider it - I'd use an aqueous HAN solution as the fluid. Only about half the free oxygen of NTO, but at least an order of magnitude less hassle on the pad. And yes, run the fluid at constant rate calibrated to run out right after the motor burns out, with steering done by a proportional four-way diverter. There's no excuse for that causing leakage on the pad; the diverter valve almost by definition can't be leak-tight but it does mean you only need one leak-tight valve upstream of the diverter.
The mass overhead can be substantial, and minimizing that requires
using reactive liquids (e.g. the N2O4 injection used in the Titan III
SRBs) which add handling hazards...
Aside from it already being available at the pad in the case of the
heavy Titans, was there particular reason for choosing N2O4?
To get maximum effectiveness -- greatest possible exhaust deflection for a given injected flow rate -- you want something that reacts with the exhaust to generate a large volume of hot gas. Since the SRB exhaust stream was fuel-rich, this meant injecting an oxidizer, and N2O4 was the obvious choice.
(If you look at a photo of a Titan IIIC on the pad, you'll see a slim cylinder nestled in the groove between the core and one SRB -- that's the N2O4 tank for that SRB.)
A subtlety of LITVC is that you want the injectant tank pretty much empty at the end of the burn, since unused injectant is dead weight that hurts payload. This is a little tricky if you're not quite sure how much you'll need for vectoring! The simplest way would be to dump some occasionally during the burn, getting rid of injectant that you're pretty sure you no longer need. The Titan III designers decided to be clever instead: the ring of injection valves is actually dribbling injectant into the exhaust continuously, with vectoring done by sending more flow to one side and less to the opposite side. Arguably more elegant, but it complicates the valve system and may have contributed to the leak problems.
Henry
