Pure Oxidiser fuel tanks for SSTOs

[Specula]

I've been wanting these for a while since I began my foray into designing SSTOs, but now with the R.A.P.I.E.R engine especially, I crave it. The idea of a fuel tank, the same width as the small jet fuel MkI tanks, that carries nothing but oxidizer. Especially since jet engines and the Rapier engine are just going to use straight liquid fuel to get up out of the atmosphere, there's always going to be a fuel/oxidizer imbalance and I could really go for some extra pure oxidiser tanks to keep them separate.

Just my two cents.

[Murph]

Here's the thing that confuses me with this. What situation does a pure oxidiser tank cover that isn't covered by a combination of fuel only and fuel+oxy tanks? The current parts allow extra fuel-only space, and you do need fuel-only. There isn't a situation which needs more oxy than fuel.

[WhiteWeasel]

Here's the thing that confuses me with this. What situation does a pure oxidiser tank cover that isn't covered by a combination of fuel only and fuel+oxy tanks? The current parts allow extra fuel-only space, and you do need fuel-only. There isn't a situation which needs more oxy than fuel.

It's easier to convert jet fuel to rocket fuel with pure oxidiser tanks. Mo more need to weigh down your ssto with another tank of LFO when just needing the O.

[Murph]

It's easier to convert jet fuel to rocket fuel with pure oxidiser tanks. Mo more need to weigh down your ssto with another tank of LFO when just needing the O.

So, why not just carry a suitable number of fuel-only tanks and a suitable number of fuel+oxy tanks? I still don't see any need for oxy-only  just carry less fuel-only instead. I've followed the whole fuel tank tweaking thread, it makes no sense to me in there either.

[Supernovy]

I would personally use a rocket tank and tweak the oxidiser level down. From what I can tell, a 0-oxidiser rocket fuel tank actually has a better liquid fuel mass ratio than the 1.25 metre jet tank.

[Specula]

You can tell WhiteWeasel is the only other person in this thread that's done anything with SSTOs and knows what's up.

And Supernovy, I'm talking about getting MORE Oxidiser on board, not LESS.

[WhiteWeasel]

Thanks, Specula . Tweakables kinda fix this now, but once you get into orbit you won't need liquid only. And if you plan on landing, you hardly need any if at all assuming a good glide ratio. Still just as we have dedicated LFO tanks for jets, even if they are small in lines or radial mounts, it would be appreciated to have O only tanks.

[Murph]

You can tell WhiteWeasel is the only other person in this thread that's done anything with SSTOs and knows what's up.

And Supernovy, I'm talking about getting MORE Oxidiser on board, not LESS.

I can assure you that you're entirely wrong in that conclusion. I've spent a significant amount of time successfully building and flying spaceplane SSTOs from 10t / 400kN to 250t / 4000kN (numbers vaguely approx for thrust, I can't be bothered checking the specifics right now). Not once have I encountered a situation where I wished for an oxy-only tank.

To me, you need the correct number of fuel-only tanks to cover the air-breathing phases of your flight, and the correct number of fuel+oxy tanks to cover the rocket phases of your flight. There's very much a need for fuel-only tanks, which is met by current parts (although we might, perhaps, need more choice of fuel-only tanks). There is very much a need for more fuel than oxy. There is never a need for more oxy than fuel (talking entire craft here, not per tank).

Since the fuel is the same for both phases, there's a little bit of flex in the middle towards extra fuel for atmospheric coming from the rocket tanks. Tweakables now let you tune that even further.

[Specula]

I can assure you that you're entirely wrong in that conclusion. I've spent a significant amount of time successfully building and flying spaceplane SSTOs from 10t / 400kN to 250t / 4000kN (numbers vaguely approx for thrust, I can't be bothered checking the specifics right now). Not once have I encountered a situation where I wished for an oxy-only tank.

To me, you need the correct number of fuel-only tanks to cover the air-breathing phases of your flight, and the correct number of fuel+oxy tanks to cover the rocket phases of your flight. There's very much a need for fuel-only tanks, which is met by current parts (although we might, perhaps, need more choice of fuel-only tanks). There is very much a need for more fuel than oxy. There is never a need for more oxy than fuel (talking entire craft here, not per tank).

Since the fuel is the same for both phases, there's a little bit of flex in the middle towards extra fuel for atmospheric coming from the rocket tanks. Tweakables now let you tune that even further.

Cool. So how many of those were done with a Rapier engine where you have to run fuel and then fuel/oxidizer through the same set of engines for both phases of the flight?

I'm betting the answer is: None.

[FenrirWolf]

I'm hard-pressed to imagine what the RAPIER would do to necessitate oxi-only tanks. You're still only going to be consuming fuel during the majority of atmospheric flight and not even touching the oxi until the final burns to orbit.

[allmappedout]

The thing to remember, which supernovy alluded to, is that with tweakables, you can choose a bigger tank, and scale down the oxy fuel you need to provide the required liquid + (liq/oxy) amount to get to orbit. The fact is that, more than anything, less is more on an SSTO, so adding oxy will mean you have to add more fuel/wings/engines. It's probably better to just take off maybe 15% of the oxy in a normal tank and use the liquid fuel available. Odds are you will get more dV out of your craft doing that than adding oxy (which is very heavy!).

Slightly off-topic actually - has anyone noticed that with tweakables your liquid fuel/oxy seems to snap to set numbers rather than be a sliding scale? I'm not sure whether I love or hate this as a feature yet I think I like it, it makes sense, but I feel like there's scope to have a pairing button which reduces oxy and liquid fuel by the same ratio amount, which you can then untick to top up a little bit of liquid fuel. Maybe i'm just being picky!!

[Themohawkninja]

Slightly off-topic actually - has anyone noticed that with tweakables your liquid fuel/oxy seems to snap to set numbers rather than be a sliding scale? I'm not sure whether I love or hate this as a feature yet I think I like it, it makes sense, but I feel like there's scope to have a pairing button which reduces oxy and liquid fuel by the same ratio amount, which you can then untick to top up a little bit of liquid fuel. Maybe i'm just being picky!!

It's only annoying when you want to limit the thrust of an SRB to 50, but the slider only allows 49.5 and 50.5.

[Murph]

Cool. So how many of those were done with a Rapier engine where you have to run fuel and then fuel/oxidizer through the same set of engines for both phases of the flight?

I'm betting the answer is: None.

Wrong again. Many of them were done using the B9 SABREs.

[WhiteWeasel]

Not once have I encountered a situation where I wished for an oxy-only tank.

Oh gee, look at all that extra jet fuel I'm probably not going to use again/very little of. I wish there was some magical liquid containing device that could allow me to transform that dead weight jet fuel into something useful for space operations without having to dock.

[Murph]

http://i.imgur.com/cOm29UX.jpg

Oh gee, look at all that extra jet fuel I'm probably not going to use again/very little of. I wish there was some magical liquid containing device that could allow me to transform that dead weight jet fuel into something useful for space operations without having to dock.

Seriously? You want to compromise your design by jamming an entire oxy-only tank into it for the sake of using a tiny amount of spare jet fuel? Why not just start out with less jet fuel, gain more efficiency from not carrying the weight into orbit in the first place?

I certainly don't see any need for oxy-only tanks from that picture.

[WhiteWeasel]

Seriously? You want to compromise your design by jamming an entire oxy-only tank into it for the sake of using a tiny amount of spare jet fuel? Why not just start out with less jet fuel, gain more efficiency from not carrying the weight into orbit in the first place?

I certainly don't see any need for oxy-only tanks from that picture.

How bout this one? Even a small radial mount would be too much? Also jet tanks don't come in a wide variety of sizes compared to LFO tanks.

Edited December 19, 2013 by WhiteWeasel

[Murph]

http://i.imgur.com/A2I1GBp.jpg

How bout this one? Even a small radial mount would be too much?

Nope, entirely unconvinced by that as well. Either swap the jet fuel and FL-T400 around (carry 1 jet fuel and 2 FL-T400s, instead of 2 jet fuel and 1 FL-T400), or use tweakables to carry less jet fuel, or enjoy having some fuel for the atmospheric flight back to the runway.

[WhiteWeasel]

I do not want less liquid fuel. Or I would have used smaller tanks. I'd want the ability to convert it into useable rocket fuel when I reach orbit.

[Murph]

Also jet tanks don't come in a wide variety of sizes compared to LFO tanks.

Now that is something I can see some justification for, as I said in one of my earlier posts. Half and double-size jet fuel tanks might well be needed. Tweakables do partly solve that, but I'm not sure that's sufficient.

[Murph]

I do not want less liquid fuel. Or I would have used smaller tanks. I'd want the ability to convert it into useable rocket fuel when I reach orbit.

Change the tanks in the second picture as I suggested (to 1 jet, 2 FL-T400), and you achieve that. In the first picture, the net dV gain would likely be negligible after the loss from carrying extra oxy into orbit in the first place.

[RadHazard]

Why not a set of small, radial attach LF-only and O-only tanks? That way, if you need just a bit more liquid fuel than you can get from a single jet fuel tank, you can use the tiny LF tanks. If you need most of the liquid fuel a jet fuel tank gives you but not all of it, you can strap on one or two Oxi tanks to turn the rest into rocket fuel. Sure, you can use tweakables to adjust how full the tanks are, but there's no sense in carrying around the dead weight of an entire tank when you only need half the fuel it gives you.

[ss8913]

you know how easy it is to make these parts, right? Just copy the part files for one of the existing tanks, edit them in vi/notepad/whatever osx uses, change the resource { } block to only contain oxidizer, and there you go. takes you about 30 seconds to make an oxidizer tank. I made a whole set of empty fuel tank parts too, so I can use them as space station modules and then fill them later.. much easier to launch them into orbit whilst empty. I don't find anything 'cheating' about doing this, since an empty tank is a very realistic object, as would be an oxidizer tank. If you want to use the radial monopropellant tanks, just change them to contain oxidizer instead of MP... they'll use the same visual model, but will be functionally different.

[WhiteWeasel]

you know how easy it is to make these parts, right? Just copy the part files for one of the existing tanks, edit them in vi/notepad/whatever osx uses, change the resource { } block to only contain oxidizer, and there you go. takes you about 30 seconds to make an oxidizer tank. I made a whole set of empty fuel tank parts too, so I can use them as space station modules and then fill them later.. much easier to launch them into orbit whilst empty. I don't find anything 'cheating' about doing this, since an empty tank is a very realistic object, as would be an oxidizer tank. If you want to use the radial monopropellant tanks, just change them to contain oxidizer instead of MP... they'll use the same visual model, but will be functionally different.

Not everyone knows how to muck about with cfg. files.