According to Propep 85% peroxide has the following “frozen” ISPs at our altitude.
- 250 PSI :125
- 162.5 PSI 118
- 75 PSI : 103
Our realized ISP was about 83.
If we had gotten 90% of the lowest number: or 92.7 we would have hovered for 99 Seconds.
Checking Cpropep against the FMC peroxide manual we get 130.32 (propep) compared to their 129.0 comparing like to like. (250 PSI 90%, 1ATM Pe)
So one possibility is that we switch to a solid catalyst. This sheds some weight, but lowers chamber pressure for the same tank pressure…. crunchingg the ISPs second by second as the chamber pressure goes from 220 down to 72 assuming 80% of theoretical performance we get 100 seconds. Assuming 100% we get 124
If we bump from 85% to 90% HTP and switch from N2 to He for our presurant (Our back up reserve boosters) still assuming 80% of theoretical we get 109 seconds.
One of the problems is that if you set the expansion ration so you are good at your lowest presure you are kind of stuck with that ISP for the whole flight. So you plan on running underexpanded for 50% or so of the flight. I’ve not found a good way to model performance underexpanded.