Zoom climbs vs Creeping up

[AeroGav]

I'm just trying to get my head around the theory behind all this. My regular spaceplane "creeps up", basically goes on a top speed run at 18,000-20,000m , then i milk the nose up trying to stay airbreathing as long as possible getting all the horizontal speed i can. 2 RAPIERS and a Nuke on a 35 ton launch weight.

The "other woman" is much more minimalist, couple of "whiplash" bolted to a light 13 ton airframe. What I find myself doing is getting into a zoom climb after punching through the sound barrier around 10,000m, accelerating to 900m/s by 20km despite climbing at a 60 deg angle, loosing power rapidly thereafter but still coasting to 40k.

I'm trying to work out if there's anything special about the zoom climb vs creeper method... is my theorycrafting all hogwash, if not, is it only applicable to very high thrust-weight designs?

1. At 14km and 800 m/s your engines have an enormous surplus of power over drag.

2. Used to accelerate horizontally, drag rises exponentially and you're going past the max power speed of your engine - the faster you go , the less thrust you have.

3. So, use the surplus thrust to build momentum in the vertical direction instead.

Is there anything special about upward momentum? Is it better to be climbing at 45 deg at 800 m/s when your whiplash shut down at 24km, or 1150 m/s but barely climbing at all?

I'm expecting the answer to be "no", 800m/s in a 45 degree climb is the same kinetic energy as 800 m/s horizontal. Although once I go anaerobic, I normally pitch up 10 deg AoA to try get out of the draggy air sooner, generating the lift to create the g forces for this pull up manouvre is something you'd bank at lower altitudes when your airbreathing engines still have lots of thrust, if you were using the zoomer method.

Incidentally, my sandbox mk 2 gets to 27km and 1600 m/s before switching to closed cycle. She gets to orbit with only the oxidiser that comes with a pair of short mk 2 - mk 1 adapters (440 oxy?). For a pure LEO truck, it'd make sense to carry more oxidiser and leave off the LV-N, but the main mission of this one is single stage to a Minmus refuelling station, whereupon it can swallow 3000 units of liquid fuel and go basically anywhere.

[Skorpychan]

Vertical momentum means a higher AP, which means more efficient raising of the PE to circularise. Otherwise, the higher you are, the better the LV-N works because ISP.

[KerikBalm]

60 degrees is way too high... try something more like 35... you may fight more drag, but your engines can accelerate you for longer... Its not so good having an Ap above the atmosphere if you still need 2,000 m/s to get to orbit.

Basically, the highest velocity you can get before they cut out means you got the most use out of them.

However, I know I can get going a bit faster if I am going level at ~25 km.... but then below 30km, the drag is still pretty high, so in my ascent, I try to have the Ap reach above 30km before my velocity starts to decrease.

You'll probably find that if you do a shallower climb of about 35 km, you'll still reach an apoapsis of ~40km.

I get >30 km perapsis with the final stage of my airbreathing acceleration at a 20 degree or less climb

[AeroGav]

Vertical momentum means a higher AP, which means more efficient raising of the PE to circularise. Otherwise, the higher you are, the better the LV-N works because ISP.

Since my primary spaceplane takes so long to get up, i have time to click on stuff. The LVN is already producing near-vacuum ISP at 22km, when i generally start it. Together with the dregs of airbreathing that gets us to 1600ms and 27k at flameout. 440 oxidizer only gets me another 8km and maybe +150 m/s. But the drag is low enough at that altitude, for the LVN to do the rest.

[Alshain]

Vertical momentum means a higher AP, which means more efficient raising of the PE to circularise. Otherwise, the higher you are, the better the LV-N works because ISP.

Incorrect. Vertical Momentum is negated by gravity while horizontal momentum is not. Barely climbing at all but increasing speed means increasing your Pe while Ap still increases (slightly), and it is momentum that gravity does not consume. Ideally, if you can do it without gaining time on your Ap, you should be at a 10 degree climb. The end result will be a MUCH higher Pe by the time your Ap is at target and MUCH shorter circularization burn (visually depicted by a wide arc around the planet rather than a short vertical one), a much more fuel efficient method. This concept does not change between planes (after engaging LFO) and rockets, a flat ascent is more fuel efficient, the more flat you can make it based on your crafts thrust capabilities, the better.

Edited October 30, 2015 by Alshain

[Van Disaster]

Gravity drag vs aerodynamic drag, just like a rocket profile. You will want to get all the *speed* you can out of your airbreathing engines, which is why I usually get to 15km or so as fast as possible and then flatten off. Once you've switched to rockets *then* is the time to pitch up, because you don't want to be wasting rocket fuel on aero drag in thick atmosphere.

[JTpopcorn]

I've successfully done both, however I find going for speed more fun. While not necessarily as efficient, hitting space in near rocket times is satisfying.

What you need to do a speed approach

-High TWR (generally)

-Low drag hull shape. Long with short wings and maybe winglets at the front

-pull up to 30< wait till rapiers get thrust boost after 3-400 m/s

-pull up to 60< and hold until overheat, then lower throttle but dont reduce angle

-at 30km push nose down and hope its works.

-JT

[Clipperride]

Its a complex mix to work out! You also have to consider the wings which, whilst adding to drag, give you lift and therefore altitude. A rocket gets all its vertical speed directly from the engines, whilst wings allow your engines to increase horizontal speed whilst also adding a vertical element.

It might be helpful to open up a sandbox game and build a space plane with excess fuel and several engine mounts. That way you can see how much fuel you burn getting to a certain altitude or orbit, switch engine configuration and rebalance the craft (as much as you can) by tweaking the fuel load. That way you can see what works for your preferred ascent profile.

Unless you want to build a SSTO, you could also add a SRB assisted take off, which can get air breathing engines up to speed quickly.

[LTrotsky]

My problem is I have so much thrust in my SSTO attempts that the craft burns up with a less than 30d ascent. I removed two engines, and still can't get into orbit.

[ForScience6686]

I was losing panels and rcs, so I would climb as fast as possible above 13km before really going for speed. My ssto tend to use nukes, so I need as much vertical speed as possible before nukes to compensate for the low twr.

[Vegvisir]

Incorrect. Vertical Momentum is negated by gravity while horizontal momentum is not. Barely climbing at all but increasing speed means increasing your Pe while Ap still increases (slightly), and it is momentum that gravity does not consume. Ideally, if you can do it without gaining time on your Ap, you should be at a 10 degree climb. The end result will be a MUCH higher Pe by the time your Ap is at target and MUCH shorter circularization burn (visually depicted by a wide arc around the planet rather than a short vertical one), a much more fuel efficient method. This concept does not change between planes (after engaging LFO) and rockets, a flat ascent is more fuel efficient, the more flat you can make it based on your crafts thrust capabilities, the better.

Most of the "how do I even spaceplane? / what launch profile?" type threads have been answered thus. Use air breathing engines to their maximum potential. It's cheaper (or so I think).

Look at it this way: If you're "zoom" climbing; you're pretty much using a rocket. May as well launch a rocket. Put a plane-like thing on the top of engines and boosters if you want to fly around in atmos after coming back; I've done that early Career before unlocking the Turbojet. It works well enough if your craft is stable enough for the gravity turn.

I'm not 100% sure, but even with Turbojets, rapidly climbing to 10km or so and then levelling off to build speed; doesn't that still lead to lost thrust overall? I guess it depends on your craft, its weight and so on.