Running out of oxidizer on SSTO?

[Hund]

Trying to build an SSTO and my oxidizer keeps running out faster than the liquid fuel so I never can get into orbit. Shouldnt they run out at the same time? I switched over to rocket lv-45 around 20k from 2 basic jet engines. I did a forum search and found someone with a similar problem and people seemed to think the fuel/oxidizer should run out at the same time and it was a bug or corrupted file? But there was no fix listed in the post. And its only showing up in SPH builds not in rockets built in VAB.

Any ides?

also whats the point of closing the air intakes when switching over to rocket mode from jet power? Drag?

thanks

[cantab]

The jets use liquid fuel alone, the rockets use liquid fuel and oxidizer and at a much more ferocious rate. If you have a mixture of fuel-only tanks and liquid-fuel+oxidizer ones you might run out of oxidizer first. Same goes if you're using LFO tanks but reduced the oxidizer supply by right-clicking them.

The solution is either to give the rockets less to do by gaining more speed with the jets or add more oxidizer, by increasing the oxidizer supply in the tanks you have or swapping some fuel-only tanks for LFO ones.

Yes, shutting intakes lowers drag.

[Red Iron Crown]

As cantab says, you likely have too much liquid fuel or not enough oxidizer. Very, very little liquid fuel is needed for the jet phase of the flight, so much so that I generally don't use liquid-fuel-only tanks on planes meant to go to orbit. It's easier to just tweak out a bit of oxidizer from regular liquid fuel + oxidizer tanks.

A pic of the SSTO in question would be helpful in giving more specific advice.

[Hund]

The build was using mk2 parts. I used the round jet fuel tanks for the two basic jet engines which is all I have unlocked at the moment. They didn't have fuel lines to the mk2 LFO parts so the rocket engine wasnt using any of the jet fuel. I used the mk2 LF+O part and the part that narrows at the back to attach the rocket to which also has LF+O in it. I'll have to post a pic a bit later. I did play around with adding fuel tanks of just oxidizer but still run out just before or at 70k.

[Red Iron Crown]

I can almost guarantee that the rocket was drawing the jet fuel. All LFO tanks have LF and O in the proper ratio, if the rocket was only drawing from those it would run out of LF and O simultaneously.

[Hund]

I will recheck again. Maybe its because the jet fuel is added to the total amount bar in the upper right menu so it reads as having leftover LF when the oxidizer runs out so I think its not running out at the same time?

Thanks for the help

[Red Iron Crown]

That is likely. The resource panel shows the whole amount, sounds like you have some LF left in the jet tanks when you run out. Does the fuel bar next to the rocket engine in the staging list run out at the right time?

[Anquietas314]

I can almost guarantee that the rocket was drawing the jet fuel. All LFO tanks have LF and O in the proper ratio, if the rocket was only drawing from those it would run out of LF and O simultaneously.

It sounds like OP was watching the resources tab rather than the actual tanks; oxidizer will of course run out sooner if you have spare liquid fuel in the tanks for the jet engines - there's liquid fuel with no oxidizer for the rockets to burn it with

OP: You probably shouldn't try to build an SSTO using basic jets. They're simply not good enough to get you going fast and high enough that it makes a significant difference to efficiency compared to rockets, especially when you take the added mass and drag of the wings, intakes, etc into account. In stock the turbojets are the best for getting you out of the atmosphere cheaply, while the RAPIERs are easier to work with since you only need one kind of engine.

[numerobis]

Nothing wrong with a basic-jet SSTO if that's all you have unlocked (that's what I use to launch my first missions to Mun and Minmus).

[Anquietas314]

Nothing wrong with a basic-jet SSTO if that's all you have unlocked (that's what I use to launch my first missions to Mun and Minmus).

Maybe for getting past the first 10km, but after that they're mostly just dead weight if you don't jettison them (if you do, it's not really an SSTO is it...). Seeing as most of the useful work happens after that 10km, they're just not worth it imho.

[Hund]

Bloody hell

Did more testing

thats what it was. I was looking at the resources panel which shows the jet fuel combined with rocket fuel. Too many bleary eye late night Kerbal sessions. Either way I still cant get into orbit. SSTOs sure are hard to figure out. Sorry bout the post and thanks.

Edited January 4, 2015 by Hund

[Pecan]

Bloody hell

Did more testing

thats what it was. I was looking at the resources panel which shows the jet fuel combined with rocket fuel. Too many bleary eye late night Kerbal sessions. Either way I still cant get into orbit. SSTOs sure are hard to figure out. Sorry bout the post and thanks.

SSTOs are easy - a pod/core, a fuel tank and an engine is all you need. 48-7S engine for tiny ones, T30 (start tech in career/science mode) for manned, almost any engine with high enough thrust to launch after that. As long as you aren't trying to SSTO with an ant, nuclear or ion engine you should have no trouble.

[LethalDose]

SSTOs are easy

*sigh* No, they're not easy. To make them fully reusable and land-able at KSC can be tricky.

Stating "they're easy" someone who's clearly having trouble with them isn't helping, it's condescending.

We're not getting into definitions again.

I was looking at the resources panel which shows the jet fuel combined with rocket fuel. Too many bleary eye late night Kerbal sessions. Either way I still cant get into orbit. SSTOs sure are hard to figure out. Sorry bout the post and thanks.

Yeah, both rockets and jets consume "LiquidFuel". And, yes, SSTOs are hard to figure out when you're trying to do them right (recoverable and precision landing). Keep at it, every time you make a mistake, you learn how to do something better. That's probably the most important thing you can learn from the game.

Maybe for getting past the first 10km, but after that they're mostly just dead weight if you don't jettison them (if you do, it's not really an SSTO is it...). Seeing as most of the useful work happens after that 10km, they're just not worth it imho.

100% agreement with this. IMO, trying to build an SSTO spaceplane requires at least Turbojets to make the vessel worth it, as basic jets just don't produce enough thrust at high altitude to make them worth while.

Obviously, you can make them work, I'm not debating that fact. I just don't think they're worth the effort for the meager gains.

[Pecan]

*sigh* No, [sSTOs are] not easy. To make them fully reusable and land-able at KSC can be tricky.

Stating "they're easy" someone who's clearly having trouble with them isn't helping, it's condescending.

We're not getting into definitions again.

Just because making a hard version of something is hard doesn't mean making a basic version of that thing is hard. It's a good reason NOT to start complicated ^^.

Stating "they're easy" reminds someone that they're trying to over-think or over-design things. Start small and simple and SSTOs are easy.

EVEN you agree that SSTO doesn't mean spaceplane so, no we aren't getting into definitions again, especially as you're the only one that uses your definition and insists on complicating things, making it hard.

[Claw]

SSTOs (or any particular design for that matter) can be easy for some and difficult for others. It's best to focus on the problem though and to help the topic of the thread. Examples of SSTOs, be it space plane, rocket shaped, or anything else, do a lot more to aid understanding the problem.

At least more so than worrying about whether or not the topic is complex. Please don't derail.

Thanks,

~Claw

[Hund]

Easy? Maybe in sandbox with all the parts available, but trying to get into orbit with the basic jet engine which cuts out around 24k it sure aint. I figure I need to unlock the turbos after many unsuccessful tries. I have gotten to orbit once and actually landed at KSC on the first try in .25 but that was with the turbos.

Its a much bigger balancing act.

[locustgate]

Easy? Maybe in sandbox with all the parts available, but trying to get into orbit with the basic jet engine which cuts out around 24k it sure aint. I figure I need to unlock the turbos after many unsuccessful tries. I have gotten to orbit once and actually landed at KSC on the first try in .25 but that was with the turbos.

Its a much bigger balancing act.

All my engines cut out around 24k, with 8 intakes.

[Pecan]

Easy? Maybe in sandbox with all the parts available, but trying to get into orbit with the basic jet engine which cuts out around 24k it sure aint. I figure I need to unlock the turbos after many unsuccessful tries. I have gotten to orbit once and actually landed at KSC on the first try in .25 but that was with the turbos.

Its a much bigger balancing act.

Yes mate, easy -

Career mode, tech-0: command pod, parachute on top, 10 fuel tanks and a T30 engine underneath. It's a Single Stage To Orbit.

Sandbox mode: QBE, FL-T100, 48-7S. SSTO.

Big bunch: core, several jumbo stacks, KR-2Ls. SSTO.

NOW we've got over "it's hard" part - what is it that you want to DO with a SSTO?

Re-usability? Design for >4,900m/s launch deltaV and 2-4 drogue chutes for assisted powered landing within 1 or 2 km.

Precision landing? Use MJ.

Horizontal landing? Add wings and landing gear. Add power and fuel for carrying them around all the rest of the time.

Jets? Well - USE them by staying in the atmosphere for as long as possible but, yes, using jets properly is more complicated. That doesn't mean SSTOs are hard though, it means the way you maximise jet efficiency is hard.

- - - Updated - - -

All my engines cut out around 24k, with 8 intakes.

8 intakes per engine is plenty, even spamming (it's what I do too ^^). THE trick to using jets is that as they run out of air they don't just become a bit worse/lower thrust they will 'suddenly' flame-out. Flame-out at full-throttle, that is, but still work at 70%, 50%, 20%, almost no, throttle...

When (preferably just before) the jets flame-out reduce the throttle a little and they will need a little less air. Although they're giving a little less thrust they are still working for you. Repeat as required. In a lot of cases you'll find jet-powered launch vehicles ascending from 30 - 60km with the jets eventually just 'ticking over' at practically no throttle and 0.1KN thrust for a lot of the way. Base speed (~30km) will get you an Ap outside the atmosphere, thrust after that can bring your Pe to the edge of it (best I've managed is 58km Pe).

Edited January 5, 2015 by Pecan

[Anquietas314]

THE trick to using jets is that as they run out of air they don't just become a bit worse/lower thrust they will 'suddenly' flame-out.

Actually, jets do gradually become worse as they run out of air. The flame-out happens after that reaches a critical point (10% of required intake air for turbojets).

[numerobis]

The basic jet lends itself to a very different ascent profile than what you use with rockets or turbojets.

The ascent is that I burn vertically on full throttle until the jet flames out, around 36 km. That gets me an apoapsis around 40km or so, with 10-15s to apoapsis. At that point I treat the situation as if I were on the surface of an airless body: I burn my rockets at full throttle mostly horizontally, but with just enough pitch to keep the apoapsis slowly rising.

I use a basic jet, four intakes, and about 25-30 units of liquid fuel, along with a pair of LV-909 and FL-T400 for circularization, and a pair of radial parachutes for the return. I can handle a payload of about 1.5t; more than that and I can get suborbital but the payload will have to do 100-200 m/s of circularization. And my recoverable first stage will land quite far from KSC.

[Pecan]

Actually, jets do gradually become worse as they run out of air. The flame-out happens after that reaches a critical point (10% of required intake air for turbojets).

(Sidenote: as originally stated, they don't just become a bit worse/lower thrust, they will 'suddenly' flame-out. True, they also become worse on their way towards that, but that isn't the essential point).

- - - Updated - - -

The basic jet lends itself to a very different ascent profile than what you use with rockets or turbojets.

The ascent is that I burn vertically on full throttle until the jet flames out, around 36 km. That gets me an apoapsis around 40km or so, with 10-15s to apoapsis. At that point I treat the situation as if I were on the surface of an airless body: I burn my rockets at full throttle mostly horizontally, but with just enough pitch to keep the apoapsis slowly rising.

I use a basic jet, four intakes, and about 25-30 units of liquid fuel, along with a pair of LV-909 and FL-T400 for circularization, and a pair of radial parachutes for the return. I can handle a payload of about 1.5t; more than that and I can get suborbital but the payload will have to do 100-200 m/s of circularization. And my recoverable first stage will land quite far from KSC.

Is that a new-ish finding? I've never tried much of anything with basic jets but is 1.5t/engine consistent? Your research really is great :-)

[numerobis]

No, I've been doing basic jet SSTOs for a long time. It's not exactly recently that I figured out that you should just go straight up with them, given that they lose thrust as you gain speed.