What is a good ratio of LF/O for SSTO space plane?

[RainDreamer]

I have been sending a lot of space planes up reccently, and now I kind of want to optimize my flights. Most of the time I just keep almost the basic ratio of LiquidFuel:Oxidizers (45%/55%) because the majority of my flight would be in space. However, I realized that is not exactly a good idea, since when inside kerbin atmosphere, I wouldn't touch the oxidizers thanks to the jet engines, and it becomes extra weight that I will have to haul up to space too, and I might run out of fuel before oxidizers depending on how bad the trip to orbit is.

Now, I understand that each craft will consume fuel differently due to engine configuration and balancing issues may also affect the ratio. That said, I want to know a rough estimate of how much fuel/oxidizer that people use, so I can have a base point to guess for my own crafts.

So, what is your ratio of LF:O for your SSTO space planes?

Edited December 2, 2014 by RainDreamer

[LordFjord]

Well, you have basically answered your question already:

it depends

I usually make a test flight fully loaded, then check how much leftover oxidizer I have when in a stable orbit. Then I try to remove as much Oxy that I end up with a bit liquid fuel to spare - just in case I have to fly a bit when I miss my target at reentry.

In general: heavier vessels need more fuel per engine as lighter ones, because you will spend more time in high atmosphere.

[capi3101]

I generally assume a maximum take-off weight of 10 tonnes per engine if I use Basic Jets, 13 if I use RAPIERs and 15 if I use Turbojets. I then multiply that assumed figure by 39 - the result is the amount of Liquid Fuel I pack (in units). For Oxidizer, I multiply the assumed figure by 24. Those figures come from DocMoriarty's KSP Space Plane Construction and Operation Guide, which assumes use of RAPIER engines that switch over going somewhere around 1,750 m/s; I've found the ratio still works well for traditional Turbojet/rocket combos.

Edited December 1, 2014 by capi3101

[Empiro]

You don't need very much. I play with FAR, but I find that the radial engine attachments that include the little bit of liquid fuel works quite well.

At the end of the day, you just need to experiment. If you find yourself with more Oxidizer than Liquid Fuel, then you need more Liquid Fuel. If the other way around, then more Oxidizer.

[Brainlord Mesomorph]

I find that jets sip so little fuel, I just use a standard rocket mix, and end up with a little left over oxidizer at the end.

If I know how much that is, I'll defuel some o2 before takeoff.

[Signo]

If you are looking for the best efficiency I think you should have a look at this thread.

http://forum.kerbalspaceprogram.com/threads/101426-Slashy-s-quick-and-easy-recipe-for-an-SSTO-spaceplane-%28for-25%29?highlight=goslash27

Slashy did a lot of research on this stuff and his tips definitely work.

Edited December 1, 2014 by Signo

[GoSlash27]

http://forum.kerbalspaceprogram.com/threads/101426-Slashy-s-quick-and-easy-recipe-for-an-SSTO-spaceplane-(for-25)

The ratio I have in this is pretty generous for a standard LKO orbital rendezvous. The #1 thing that will throw off your fuel/ oxidizer is the velocity at which you begin to use oxidizer in the launch. Starting your rockets at 1700 m/sec will burn a huge amount of o2 and fuel, whereas holding off until you're over 2300 will hardly cost you anything and only a couple units of fuel.

Best,

-Slashy

Edited December 1, 2014 by GoSlash27

[numerobis]

I tend to assume I need about 6 km/s of jet fuel deltaV and that turbojets get about 10,000s Isp. Then I plug that into the rocket equation and I make orbit with a bit of jet fuel to spare.

[cantab]

If you aren't averse to mods, TAC Fuel Balancer will let you dump the excess oxidizer once you're in orbit.

[LethalDose]

The "ideal ratio" is totally idiosyncratic, based on vessel, flight profile, engines, etc etc.

My advice is "trial and error". Fly the vessel, see if you have excess Ox, and remove it for the next flight. Repeat as necessary until you've got it tweaked to your liking. It usually only takes me 2-3 flights get the mixes right. I'll also dump excess fuel at my orbital station to keep the tanks up there topped off.

[Red Iron Crown]

I only use LFO tanks, never LF only, the theory being that I can refuel the craft in orbit and use all the tankage for orbital maneuvering with the rocket engines. After a test flight or two I tweak out the amount of oxidizer I've determined empirically is unnecessary.

[numerobis]

The LF tanks are more efficient -- 1 tonne dry per 6 tonnes fuel, versus a bit worse than 1:4 ratio for LFO with no O. But the smallest one is still quite large, so you need a large plane before it's worth using the LF tanks.

[GoSlash27]

The LF tanks are more efficient -- 1 tonne dry per 6 tonnes fuel, versus a bit worse than 1:4 ratio for LFO with no O. But the smallest one is still quite large, so you need a large plane before it's worth using the LF tanks.

A few more things to add:

1) Never use the Rapiers. A turbojet with a couple tiny rockets is definitely preferable. Not just because the turbojet has better thrust to weight and is cheaper (even accounting for a couple small rockets), but mainly because it will continue to accelerate to 2,400 m/sec. The extra 200 m/sec DV you can get from the turbojet/ rocket combo accounts for a huge amount of fuel and O2 you have to bring with the rapiers.

2) Always use the XM-G50 intakes. They have the best airflow to drag at the top end.

3) Always use the Porkjet strakes. They have the best lift to drag at the top end. #2 option is the Porkjet Structural D wing and #3 is the classic swept wing.

Best,

-Slashy

Edited December 2, 2014 by GoSlash27

[numerobis]

2) Always use the XM-G50 intakes. They have the best airflow to drag at the top end.

The ram air intake is quite a bit better. The air scoop is easier to place.

The delta-deluxe is the best lift:drag (about 4x as much lift as the strake) -- but it's a control surface. The strake is the best among the wings.

Edited December 2, 2014 by numerobis

[RainDreamer]

Thanks for all the help. I will try out the ratios that are given and find a good one for my craft. And thanks slashy for the flight profile tip, I usually just switch to rocket instantly once air ran out and raced to orbit instead of coasting a bit more with jet.

Also, I just realized that I can make things easier with orbital refueling stations full of oxidizers and just fly up with minimal oxidizers needed to rendezvous, which should help with the amount of weight I have to haul. Though...that require infrastructure and crafts that can haul those refueling stations up there first.

[capi3101]

The delta-deluxe is the best lift:drag (about 4x as much lift as the strake) -- but it's a control surface.

That said, if you've got a small enough plane, a delta-deluxe makes for a pretty descent main wing body...

(Not mine - this design is Cirocco's; he uses it to conduct Kerbal rescue missions for contracts.)

I'm trying to understand what makes the Strake so Sierra Hotel...I'm assuming it has something to do with the ratios of either lift:drag or lift:mass. By my calculations and by the information on the wiki, the classic Swept Wing blows it out of the water on both scores (and of course the delta-deluxe winglet is king in the lift:drag department).

[Signo]

The ram air intake is quite a bit better. The air scoop is easier to place.

The delta-deluxe is the best lift:drag (about 4x as much lift as the strake) -- but it's a control surface. The strake is the best among the wings.

About the wings lift/drag ratio, as far as I am aware about the current version, there are some of them with an incorrect drag calculation. So at the moment the "old" swept wings, the wing strakes and the structural "type D" (I think) can give you a little extra edge.

[GoSlash27]

That said, if you've got a small enough plane, a delta-deluxe makes for a pretty descent main wing body...

http://i.imgur.com/4M2M4Vb.jpg?1

(Not mine - this design is Cirocco's; he uses it to conduct Kerbal rescue missions for contracts.)

I'm trying to understand what makes the Strake so Sierra Hotel...I'm assuming it has something to do with the ratios of either lift:drag or lift:mass. By my calculations and by the information on the wiki, the classic Swept Wing blows it out of the water on both scores (and of course the delta-deluxe winglet is king in the lift:drag department).

It has to do with how the game calculates drag. You have to multiply the sine of the angle of attack by the drag coefficient and mass.

Treating the angle of attack as equal for comparison purposes, the lowest drag/ lift is achieved by the structural D, strake, and swept in that order. Best lift to weight is achieved by the same 3, but in reverse order. The overall best wing is the strake.

As for the Delta deluxe, yeah, it's superior to all wings... but only because it's a control surface and all control surfaces exhibit infiniglide properties. This is why using control surfaces as wings is banned in so many challenges. As an aside, the small control surface is superior to the delta deluxe as a lifting surface.

Best,

-Slashy

Edited December 2, 2014 by GoSlash27

[numerobis]

As an aside, the small control surface is superior to the delta deluxe as a lifting surface.

The small control surface appears not to exist anymore in 0.25, so the delta-deluxe is the best one that's left.

Placed at a 60 degree angle, it provides 30 kN of lift when you're going straight down the runway at 50 m/s, while incurring only 0.12 kN of drag -- a lift:drag ratio of 238:1. At 2km/s you still get a lift:drag ratio about 6, and it takes until 12 km/s for lift:drag to fall to identity (lift grows linearly, drag grows quadratically, so lift:drag falls linearly with speed).

You get similar lift:drag numbers on the strake at a 30-degree AoA, but the absolute numbers are smaller: just 8 kN of lift on the runway.

[RainDreamer]

So, not related to the original question, but I don't want to make another thread when there are already experts here: how do you exactly determine the performance of an air intake? So far I just get whatever with the highest intake surface with the most intake atm resource and slap it on. Is there anything else that is needed to be considered?

[numerobis]

So, not related to the original question, but I don't want to make another thread when there are already experts here: how do you exactly determine the performance of an air intake? So far I just get whatever with the highest intake surface with the most intake atm resource and slap it on. Is there anything else that is needed to be considered?

Here! Basically, intake area is what mostly matters for how much air you get (more is better), mass is what mostly matters for how much drag you suffer (less is better), conclusion: ram air intakes are best, radial air scoop second-best and easier to place. Exception: structural intake is best for normal-plane speeds (less than about 600 m/s).

[GoSlash27]

So, not related to the original question, but I don't want to make another thread when there are already experts here: how do you exactly determine the performance of an air intake? So far I just get whatever with the highest intake surface with the most intake atm resource and slap it on. Is there anything else that is needed to be considered?

Pretty much what Numerobis said, except that the mass needs to be considered twice. Not just because it, along with the cross- sectional area and air mass, are used to calculate drag, but also because it is mass that the wings must lift (creating more drag from the higher angle of attack) and the engines must accelerate (incurring a DV penalty).

This is why empirical results tend to differ from the theoretical predicted results. XM-G50s usually out-perform all the other intakes on spaceplanes in actual practice. Not by much, but it's there.

But more to the point, whatever intakes you use should be in the back on spaceplanes and preferably a little high for high speed stability. This is difficult to achieve with other intake types, whereas it's easy for the XM-G50.

This is why I recommend that if you want to build an efficient spaceplane, the #1 choice should be the XM-G50 intake, the fewest possible needed to get over 2,250 m/sec orbital velocity. This usually works out to 4 intakes per engine.

Going over that number *can* yield additional kinetic energy by getting closer to 2400 m/sec and higher altitude, but you end up burning more fuel in drag getting them up there, so it's a net loss.

Best,

-Slashy

Edited December 2, 2014 by GoSlash27