Delta V budget for Spaceplanes

[Whitecold]

I'm quite unsuccessfully trying to build a spaceplane, and plan on employing the new rapier engine. The problem is what kind of delta v do you approximately allocate to each phase? Also what kind of flight plan should I use? I mostly tried going up at a low angle, then accelerating still on air, but when I tried to pull up, it just generated a lot of drag and slowed me down again.

[Murph]

Personally, I'd avoid the RAPIER in its current form, and use turbojets + aerospikes. The RAPIER is underpowered in air breathing mode, and less fuel efficient in both modes.

As for ascent profile, it's different for every plane, pretty much. From runway to around 20,000m (maybe even 25,000m), you only really want to be flying at terminal velocity  you're aiming to get altitude and out of the thick air, not for orbital speed at that point of the ascent. At 20–25,000m, possibly level out to build speed (depends on the craft), keeping the prograde vector on or just above the horizon. As you're gaining speed, you're continuing to climb slowly. Once you reach close to max altitude and max speed on jets, it's time to pull up a little (e.g. to 30º nose above horizon), activate rocket power, shutdown jet power, close jet intakes, and shoot for the stars. It's not about absolutely maxing out altitude and speed on jets, just getting as much as you reasonably can out of them for the fuel you are burning.

If you've not tried it already, try the wiki tutorial for the Aeris 4A. Once you have mastered that tutorial, you should be in a better position to evaluate what's going on with your own plane, and to figure out a good ascent profile for it.

[Crown]

Regarding the flight profile, I would go with the way the Aeris 4A is flow. I thought Harvester wrote a tutorial on how to fly it the correct way, but I can't find it.

Basically it was a very high angle, about 45°, right after lift-off. And then a more shallow angle to get some speed. Somewhere you had to level out (no height gain) to speed up to about 1200 m/sec and then pitch up, enable the rocket engine to leave the atmosphere and obtain a stable orbit. Surely this isn't a useful explanation, that's the way I would fly a SSTO.

Does anyone know where this tutorial is?

edit: Thanks Murph. That's a good place.

I know how to calculate the delta-v of rockets, but I usually to this for vacuum flight. So I can't help at this part.

Edited December 21, 2013 by Crown
Tutorial location was mentioned

[Dispatcher]

The above is good advice. Figuring dV for spaceplanes (or any planes) is like herding cats. This is due to the fact that so much depends on your design (especially regarding lift/ control surfaces), which does not enter into the calculation of dV very much (except for the mass of the wings). Besides design/ lift, your piloting style and skills are very significant factors. I've seen several and I have my own. I've seen each make it to orbit. But that's your real question. How do I make it to stable orbit? Besides the above advice, I'd say: practice.

[arq]

dV is not the metric for spaceplanes, everything is interdependent. It's all a careful tuning act that you will perform over half a dozen (or more ascents). Got into space with a bit of extra jetfuel? Bring less next time. By bringing less, you will get up faster and use less (sometimes much less), so this may need to be repeated. You may also realize you need fewer or more engines and that changes stuff even more dramatically.

[numerobis]

I tend to successfully fly by assuming about 5-6 km/s for the deltaV, and assume 10ks Isp on average (for the mass of LiquidFuel).

On Laythe, 4 km/s is more than enough to launch from anywhere, coast up high to your exit point, then push apoapsis near escape.

Edited December 22, 2013 by numerobis

[Hodo]

You only need about 2.5k d/V for a good space plane SSTO design with the RAPIER. 1900m/s d/V for FAR designed space planes.

[numerobis]

Hodo: how do your numbers work? 1900 m/s doesn't seem possible even with zero drag: that equates to under 2100 m/s orbital speed.

[Hodo]

Hodo: how do your numbers work? 1900 m/s doesn't seem possible even with zero drag: that equates to under 2100 m/s orbital speed.

That is about what my lowest fuel SSTO has on it, 1900m/s d/v, and it can still achieve a 100km x 100km orbit.

I am using KER to estimate my d/v and sometimes it doesn't calculate right if I have air breathing engines AND rockets on the same craft. It kind of averages them I guess.

[Whitecold]

Thanks for all your tips, I'll be trying them out with some more lift and thrust. Also how many intakes do you usually employ?

[Cashen]

My SSTO has two jet engines and two rocket engines, and four air intakes. It can get to orbit with between 500 and 800 m/s delta-V remaining. Spaceplaces, you can't really do much math with them. For me it was very much a trial and error process; building something, flying it, and correcting deficiencies. One of the things I learned to do was to try to keep the thrust level of my rocket engines similar to my jets, otherwise if the jets are overpowered you'll actually lose speed at first when switching to rockets. I also picked engines from AEIS Aerospace rather than aerospikes, because the particular rocket engines I used have a crazy good TWR. Weight is extremely important for spaceplanes, you want as little as possible.

As for the ascent, I fly up at 45 degrees until about 15km, then 30 degrees until 20km, then 15-20 degrees up until I get to the point of shock heating effects and low air intake. Once I think the engines are about to die and I'm going over 1300 m/s, I turn on the rockets and leave the jets on for a bit longer (the boost in speed gives them extra life for a few more seconds), pitch up to 30 degrees again and wait for my apoapsis to climb to 80km, then coast and circularize. My particular design has very good lift characteristics, and will take off at just 60 m/s of speed.

Here's what it looks like:

[numerobis]

Thanks for all your tips, I'll be trying them out with some more lift and thrust. Also how many intakes do you usually employ?

With 8 or more ram intakes per turbojet you can fairly easily reach orbital speeds, so your rocket stage has little to do. With fewer than four ram intakes per turbojet, you'll need the rockets to do a lot of the work. In between, some skill is required. I'm not as experienced with the rapier but the numbers should be similar. You need 2.5 radial intakes to get as much air as one ram intake.

[numerobis]

That is about what my lowest fuel SSTO has on it, 1900m/s d/v, and it can still achieve a 100km x 100km orbit.

I am using KER to estimate my d/v and sometimes it doesn't calculate right if I have air breathing engines AND rockets on the same craft. It kind of averages them I guess.

KER is giving you garbage numbers. MechJeb is also unable to calculate properly: it'll typically tell you that your jets will provide about 1 or 2 m/s dV. Actually, a plane ascent tends to be slightly less efficient than a rocket ascent, with the extra cost in momentum more than made up by not needing to carry the intake air mass.

[tavert]

With 8 or more ram intakes per turbojet you can fairly easily reach orbital speeds, so your rocket stage has little to do. With fewer than four ram intakes per turbojet, you'll need the rockets to do a lot of the work. In between, some skill is required. I'm not as experienced with the rapier but the numbers should be similar. You need 2.5 radial intakes to get as much air as one ram intake.

These are pre-0.23 numbers, right? Have you experimented much with 0.23 yet? From my limited time in 0.23 and various other people's observations, it seems the tweaks to the way intake air works now mean you don't need as many intakes to get the same performance.

[numerobis]

Not much yet, indeed. Too busy wrapping up the year, and then wrapping up the presents.

[Moar Boosters]

KER is giving you garbage numbers. MechJeb is also unable to calculate properly: it'll typically tell you that your jets will provide about 1 or 2 m/s dV. Actually, a plane ascent tends to be slightly less efficient than a rocket ascent, with the extra cost in momentum more than made up by not needing to carry the intake air mass.

Yeah. Mechjeb gets confused for all sorts of reasons. It doesn't like it when you have multiple engine types using the same fuel tanks, activated by action groups and not staging.

It also doesn't know your ascent profile, and jets provide a wide variety of thrust and ISP levels depending on your altitude and speed.

[numerobis]

More critically, MJ doesn't know how intakes work (that is, the fuel flow simulator doesn't; other parts do).

[Hodo]

KER is giving you garbage numbers. MechJeb is also unable to calculate properly: it'll typically tell you that your jets will provide about 1 or 2 m/s dV. Actually, a plane ascent tends to be slightly less efficient than a rocket ascent, with the extra cost in momentum more than made up by not needing to carry the intake air mass.

I figured as much, but even then, it takes less d/V for me to get a craft into orbit flying like a jet then it does as a rocket. I know because I have built a rocket that used the same engines had the same TWR and had the same amount of fuel, but no wings and it was a pure rocket launch vehicle. The SSTO space plane version which weighed slightly more because of the wings, had the same d/v and TWR. It made it to an orbit of 75km x 75km with just about 700d/v left. The wingless version made it with just under 200d/v left.

Again those numbers are using KER, but the craft engines were identical along with the fuel and cockpit, the only difference was the presence of wings on one and no wings and launched vertically on the other.

[tavert]

Again those numbers are using KER, but the craft engines were identical along with the fuel and cockpit, the only difference was the presence of wings on one and no wings and launched vertically on the other.

I'd put that down to difference in trajectory. Have MechJeb fly them with the same trajectory settings, then let us know.

Wings can counter some gravity drag, and let you take off with less-than-1 initial TWR. But really you want to get out of the lower atmosphere as quickly as you can whether you take off horizontally or vertically, a vertical launch just starts you off pointing the right direction to do that.

What numerobis is talking about is whether you're going totally horizontally in the 20-30 km altitude range to build up speed with a jet (higher if you've got lots of intakes), or if you're continuing to ascend on a typical gravity turn trajectory with a rocket. At those altitudes drag and lift are fairly minor, so wings are mostly just dead weight.

Edited December 23, 2013 by tavert

[Hodo]

I'd put that down to difference in trajectory. Have MechJeb fly them with the same trajectory settings, then let us know.

Wings can counter some gravity drag, and let you take off with less-than-1 initial TWR. But really you want to get out of the lower atmosphere as quickly as you can whether you take off horizontally or vertically, a vertical launch just starts you off pointing the right direction to do that.

What numerobis is talking about is whether you're going totally horizontally in the 20-30 km altitude range to build up speed with a jet (higher if you've got lots of intakes), or if you're continuing to ascend on a typical gravity turn trajectory with a rocket. At those altitudes drag and lift are fairly minor, so wings are mostly just dead weight.

That maybe, as for using mechjeb, sorry I refuse to use that program for several reasons. Mostly because I am a, I can do it better myself, kind of guy.

[tavert]

That maybe, as for using mechjeb, sorry I refuse to use that program for several reasons. Mostly because I am a, I can do it better myself, kind of guy.

More power to you, but one thing you can never do better than a computer is repeating the exact same experiment under controlled conditions.

[thereaverofdarkness]

The RAPIER is actually great for getting into orbit. It really gets going in the upper atmosphere when it's just a smidge from switching to oxidizer. Keep it almost level and climb slowly, and watch your speed pick up. Soon enough you'll find your apoapsis above the atmosphere before you've even switched to oxidizer. Not to mention these things save engine weight. People who say the RAPIER sucks have spent too much time looking at the stats and not enough time flying them.