air-breathers - balanced LOX for rocket?

[draeath]

Spaceplane flyers - I have a question for you! Once you're out of the atmosphere, how do you arrange to have enough LOX for the remaining fuel for rocket engines (or SABRE in rocket mode?)

My thought at this point is that I should be using the 'fuselage' fuel tanks (or dumping LOX on the pad or whatever) such that I can carry -just- fuel for that, and have regular tanks disabled so that I have a balanced stock once out of the air, and getting as much speed in-atmosphere from those fuel-only tanks as possible before nudging out.

Presuming there was some of that fuel left - what would you do with it? I'm thinking dumping the fuel would be best, as without the LOX that fuel is just dead weight.

Of course I'm thinking way ahead of myself. I can't even get a plane off the runway, yet. (FAR)

Edited May 5, 2014 by draeath

[O-Doc]

Remember you'll need enough fuel to land.

Me? I simply run a battery of tests which may be boring to most but, I find quite fun. So, I use !science! to figure out how much or each fuel to take up. Having said that, my space planes are multi-role and I achieve this by designing around an engine core and adding range extension drop-tanks as needed on the mission. I test a number of drop-tank setups to get the balance right. If, on a mission, I have way too much jet fuel at the apogee of my suborbital then I'll do a transfer into a drop-tank and discard it prior to my orbital insertion burn.

[DeMatt]

I test-fly -> I tweak -> I test-fly -> I tweak. Don't think there's a simple algorithmic way of solving for optimum fuel/oxidizer ratios, given all the variables in play.

Do know that you can use the SPH tweakables to strip out unnecessary supplies. Got no RCS? Take the monopropellant out of the cockpit. Want less oxidizer, but not enough less that swapping an FL-T400 for a Mk1 Jet Fuel would work? Tweak down the oxidizer in the rocket fuel tank.

[Random Tank]

I will never use jet fuel tanks on a SSTO; I always use rocket tanks and take the oxidiser out (because I can then refill it with oxidiser in orbit so all the fuel can be used in the rockets). I usually take half the oxidiser out of a FL-T400, as that will usually leave enough surplus fuel to get the SSTO to 30km when I start the rocket engines... But most of my SSTOs are quite small.

[Grumman]

Mine so far have a single Mk1 fuselage and three FL-T400 tanks, for a total of 690 units of fuel and 660 units of oxidiser.

You don't have to worry about disabling fuel tanks - the game is quite capable of drawing rocket fuel from one tank and oxidiser from the other.

[Hodo]

I am the same as Random Tank, never just bring liquid fuel tanks, they are really just wasted mass once you get into orbit.

Here is one of my older designs, that I used a few months ago, to rescue a SSTO I stranded on the Mun.

Javascript is disabled. View full album

Take note at the Dry CoM (red ball) and the wet CoM, in relation to the CoL. This is what will make or break an SSTO spaceplane. If its CoM shifts to far back and ends up behind the CoL, it will be unable to be controlled once it hits atmosphere. To far forward and it will become to nose heavy to do anything but lawn dart. This is expecially true in this case of my SVO-19 and all of my SVO line of craft, because they are also VTOLs and their CoM can't shift to far off of the verticle CoT.

Javascript is disabled. View full album

This was my SP-314A it was a failed design. It looks like it should have done alright, but if you note the dry CoM is almost sitting on top of the CoL, which makes it VERY "twitchy", the amount of drag at the front of the aircraft doesn't help either. It actually pushest the nose up when coming in at any re-entry angle higher than 10deg off of flight path. Once that happens the aircraft goes into an unrecoverable spin and often pulls itself apart.

[cantab]

worth bearing in mind is the fuselages have poor tankage ratios (full:dry mass ratio). It's worst for the Mk1, but all the fuselages are better in tankage by regular tanks drained of oxidizer. The fuselages do have much higher impact tolerance though.

[Taki117]

Over engineering for the win! Since I haven't unlocked SABREs yet I'm still stuck using the jet/rocket combo. But My trick is to take the most powerful, most efficient engine I have and sticking it on the back. Also, with Tack Fuel Balancer I move fuel around to make sure they have access to all the fuel they need.

Here's an Album with the Cargo Variant delivering a communication satellite to orbit.

Javascript is disabled. View full album

[Tank Buddy]

I usually remove x amount (I just eye it) of oxidizer from a fuel tank (or more depending on the arrangement of engine(s)). After the first flight I adjust accordingly.

[draeath]

I will never use jet fuel tanks on a SSTO; I always use rocket tanks and take the oxidiser out (because I can then refill it with oxidiser in orbit so all the fuel can be used in the rockets). I usually take half the oxidiser out of a FL-T400, as that will usually leave enough surplus fuel to get the SSTO to 30km when I start the rocket engines... But most of my SSTOs are quite small.

Hmm, this is not a bad idea at all - I do want to build an orbital fuel dump after all... and as others have identified it seems you get a better wet:dry ratio that way too.

Over engineering for the win! Since I haven't unlocked SABREs yet I'm still stuck using the jet/rocket combo. But My trick is to take the most powerful, most efficient engine I have and sticking it on the back. Also, with Tack Fuel Balancer I move fuel around to make sure they have access to all the fuel they need.

Here's an Album with the Cargo Variant delivering a communication satellite to orbit.

(album snipped)

That is simply beautiful!