Excess Oxidizer - LV-N vs RAPIERS

[zolotiyeruki]

This has probably been asked and answered before, but...

I'm sure a lot of us have launched a craft with a combination of LV-N's and some sort of LF/Ox motors (usually RAPIERs for me).  I often end up with excess Oxidizer that I won't need for the rest of the mission, but which I cannot vent/jettison (without a mod).  That leaves me with a conundrum:

Option 1: Use the RAPIERs until all the Oxidizer is gone, taking a dV hit early, but then benefit from the lower dry mass when the LV-Ns run
Option 2: Leave the oxidizer in the tanks, and only burn the LV-N's, due to their higher Isp

So I put together a spreadsheet, outlining the dV from each scenario.  The results are interesting, although not surprising.  The short version:  if the mass of oxidizer is more than about 24% of your vessel's total mass, use the RAPIERs until they're out of fuel.  If you have less oxidizer than that, stick to the LV-N. 

As a rule of thumb, though, you might as well just stick with using the RAPIERs until they're dry.  In the least-efficient scenario (about 15% craft mass LOX), you'll give up about 75m/s total, and your TWR will be a bit higher for the rest of your mission.

 

[AHHans]

Nice analysis!

I personally use option 2a: Leave the oxidizer in the tanks in case I need the high thrust at sometime during the mission. Well, actually, most of the time it is option 3: refuel with only Lf. (Or 3a: try to refuel with only LF, but forget that there is a convert-o-tron set to LfOx somewhere on my refueling station...)

[Pecan]

Tweak the tanks in the VAB and remove any excess before you even launch.

[Laie]

13 hours ago, zolotiyeruki said:

The short version:  if the mass of oxidizer is more than about 24% of your vessel's total mass,

Hmmm. Sorry if I'm a spoilsport, but I'd like to question your premises: Why do you even have any Oxidizer left in LKO? To me that means you brought too much to begin with. Going LF-only can be a hassle, granted, and a burst of raw closed-cycle Rapier power makes going to space so much easier. So I can see the point in carrying some Ox at launch time. But IMO, it should be spent before you even leave the atmosphere.

[AHHans]

15 minutes ago, Laie said:

Hmmm. Sorry if I'm a spoilsport, but I'd like to question your premises: Why do you even have any Oxidizer left in LKO?

Safety margin and speed? So that you don't need to re-do everything if you mess up your ascent profile. "Not having to think" is the number one reason why most people do things. There is a huge difference between <T> <Z> <SPACE> and really thinking about the ascent profile.

[Laie]

3 minutes ago, AHHans said:

Safety margin and speed? "Not having to think"?

I'm totally willing to admit both as valid reasons. But if it gets so far that one has to draw up spreadsheets in order to figure out how to best utilize the leftovers, I dare say it was too much of a safety margin.