SSTO Delta-V...

[jessyager]

Hi, I am in an SSTO phase atm. One of the biggest issues I have is balancing how much fuel/thrust I need. I have some of the Delta-V charts that tell how much Delta-V required to travel to various other locations. Is the Delta-V for an SSTO any different than a standard rocket launch?

[Taki117]

dV is dV, doesn't matter if it's all in one stage or in many stages. The only difference between and SSTO and a standard rocket is that an SSTO uses a much more efficient engine while in the atmosphere to build up much more horizontal velocity faster than vertical velocity. Rockets build horizontal and vertical velocity at the same time. This is probably an oversimplification, but you get the idea.

[UmbralRaptor]

Anerobic SSTOs are pretty normal, just with a somewhat worse payload fraction because you can't drop unneeded engine/tankage. Aerobic SSTOs have absurdly efficient but altitude (and speed) limited engines, and need to take a different path, maximizing horizontal speed at surprisingly low altitudes. Expended ÃŽâ€V may be in the range of 7-10 km/s, but this doesn't matter, since the rocket portion may need as little as 20-40 m/s to circularize.

[Wanderfound]

Once you're in orbit, ÃŽâ€V is ÃŽâ€V. Don't even bother looking at atmospheric ÃŽâ€V figures, though; there are too many variables involved in a spaceplane flight for it to be a meaningful figure. Depending on how it's piloted, the exact same plane can reach orbit (or fail to reach orbit) with vastly different levels of fuel remaining.

[Jovus]

That said: I find that, with a standard ascent path (getting to at least 25km going at least 1500m/s) you generally want to shoot for somewhere a bit over 500m/s with a TWR of ~0.6 to make orbit. Those are minimum figures, and as noted by posters wiser than I above, very rough.

[Alshain]

Once you're in orbit, ÃŽâ€V is ÃŽâ€V. Don't even bother looking at atmospheric ÃŽâ€V figures, though; there are too many variables involved in a spaceplane flight for it to be a meaningful figure. Depending on how it's piloted, the exact same plane can reach orbit (or fail to reach orbit) with vastly different levels of fuel remaining.

I got the impression he was talking about the other kind of SSTO. Could be wrong, but I think he means a rocket not a plane.

[UmbralRaptor]

I got the impression he was talking about the other kind of SSTO. Could be wrong, but I think he means a rocket not a plane.

Ooh, in that case I can be directly useful!

(I'd like to link to my blog posts, but they're sort of outdated with the engine changes in 0.23.5, 0.24, etc)

[jessyager]

I actually meant a Single Stage To Orbit winged aircraft. I already assumed once in orbit all numbers would be the same I however was looking for any ideal figures that would give me a rough idea f how much lift/fuel/thrust ratio's that would be helpful in planning for getting out of the atmosphere. Thank you for all of your previous replies they are all very helpful. If anyone has any other advice/ resources please let me know .

[Laie]

When running on jets, how much oomph you get per unit of fuel depends on airspeed and altitude. It's impossible to determine the delta-V without knowing your flight plan.

For the same plane, Mechjeb tells me that it has 30000m/s and KER says 1900m/s. For both numbers there is exactly one combination (or at best two) of speed and altitudes where that number would be right.

In a nutshell: when using jets, you should ignore all delta-V figures. The best way to determine your fuel requirements is by trial and error: Just put in some fuel and see how far you can go.

Edited November 5, 2014 by Laie

[silks]

Rough ratios would be

1 turbojet per 20t launch mass

4 intakes per engine

1 lift unit per ton

Not sure how much fuel the turbojet would consume but you can calculate or test that and take enough with you for the ~13 minute ascent.

Tilting the wings up by one notch in the SPH helps you fly at a positive angle of attack while having the engines and intakes aligned with your flight path.

Maybe you can find something here

http://forum.kerbalspaceprogram.com/threads/32913-How-many-lift-units-do-you-need-per-ton

[Red Iron Crown]

I echo the sentiment that delta-V calculations for air-breathing engines are not particularly useful. The dV calculators like MechJeb and Kerbal Engineer do the best they can, but the performance of those engines varies so much with altitude and speed conditions that it's hard to put that information to good use.

It is better to derive some rules of thumb, like those silks posted, by playing around with planes and seeing how they perform for you. Note that your rules might be different from those others use, depending on ascent profile and piloting skill. For example, I use more engines than silks does as I prefer a >1 TWR on takeoff to make the early ascent easier. Use the rules of others as a starting point and see what you like.

[numerobis]

For a turbojet ascent with 1 ram intake per tonne of mass (or one radial per 600kg), I plan on 6km/s delta-v and I use a turbojet Isp of 10ks. That would allow me to orbit at 40km. Dropping the number of intakes per tonne means you have to be a bit smarter about your ascent, or use more rocket fuel, but is still quite doable.

The best lifting surface these days is the delta-deluxe. One delta-deluxe tilted 60 degrees (tilt it 12 times in the SPH) provides you nearly 30 kN of lift when you're going down the runway at 50 m/s, and continues to improve as you pitch up to a max angle of attack of 30 degrees (so the wing is at 90 degrees angle of attack). The lift is substantially the same until about 10km on a normal climb trajectory, then starts to fall off exactly when your engines start to really pick up speed. That argues for 1 unit of control-surface-tilted-60-degrees-lift for every 4 tonnes. Too much, and you can never land.

[capi3101]

I actually meant a Single Stage To Orbit winged aircraft. I already assumed once in orbit all numbers would be the same I however was looking for any ideal figures that would give me a rough idea f how much lift/fuel/thrust ratio's that would be helpful in planning for getting out of the atmosphere. Thank you for all of your previous replies they are all very helpful. If anyone has any other advice/ resources please let me know .

I generally point spaceplane newbs to two places - Keptin's Basic Aircraft Design Explained - Simply with Pictures post (a great place to begin) and to DocMoriarty's KSP Space Plane Construction and Operation Guide (a great place for specifics and ideas on what exactly you can do with planes). DocMoriarty's guide focuses on use of the RAPIER engine. It's slightly out of date (by slightly I mean it was designed for 0.24.2 and he hasn't yet updated it for the changes in 0.25), but so far the only thing I've seen that's major is that the new Wing Connectors are functionally equivalent to Delta Wings.

Only bit of general advice I'll give when you're designing a spaceplane is to assume your payload fraction will be 25%; use that figure to guesstimate the final mass of your craft, and start designing from there.

[Redshift OTF]

One way to look at it is if you have an SSTO plane that can get to an orbital speed of 1600 m/s you'll need an extra 800 DeltaV from the rockets to get your speed to 2400 m/s, (for an apoapsis of 100km), and depending on how shallow your ascent path is a bit extra to circularise your orbit at apoapsis.

[jessyager]

Thank you very much... I figured that it was going to be a pretty subjective answer but thought I would try all the same .

[GoSlash27]

jessyager,

If I understand your question correctly, there's no ballpark answer to it.

Winged SSTOs have a lot of atmospheric losses. How much depends on your acceleration.

Best,

-Slashy

[rkman]

I actually meant a Single Stage To Orbit winged aircraft.

With a winged craft you'd typically be spending more time in atmosphere than you would with a rocket, so you'd incur more drag losses, which increases the d-v requirement. I'd guess that 5000m/s (instead of 4500) would be sufficient.

[LordFjord]

Why I do to roughly estimate the dV of an SSTO is to pair all air-breather engines to a separate stage than the LF/O engines. This way KER can read out the values for each "stage". I don't use staging anyway for SSTOs, so its a comfortable way to abuse it for the separated dV readouts.

Just keep in mind that depending on the SSTO you will need a few 100 dV to push it into orbit, so subtract it in your mind from the rocket stage's readout. The remaining dV is all yours for cruising around in space.