Spaceplane, Lift Force and AoA

[Green Baron]

Hi there,

normally i'm more into rockets than spaceplanes but every now and then i try one. So i realised that flying at high velocity (2000m/s) at 30km alt. and at a high angle of attack (let's say 20 degrees above prograde) the lifting force on parts seems to heavily exceed the drag force: the apoapsis climbs rapidly, engines cut off (1.0.4). Which is nice to have.

I never realised that effect when flying rockets, they allways (well, mostly :-)) point prograde ...

k

[Frank_G]

Hm... so far a very shallow AoA has proven to be the most efficient way to get into orbit with a spaceplane for me. I leave the engines running throughout the whole maneuver, meaning from the runway into full circular orbit while keeping the spaceplane nose as close to the prograde marker as possible. This is essential to build up speed when going with a low TWR of about .60 to .70. A spaceplane with more TWR is usually a fuel leech. Using this technique i get cargo of up to 35% of the launch mass into 85 km orbit... meaning a 100 ton of launch weight from which 35 tons are cargo. I have never done that with a more aggressive spaceplane (not with one, that has actual use and is not designed for a challenge run - there i have seen 50%...)

The trick is, to circularize very early, say at an altitude of 55.000m. Then you need VERY low thrust (below 10%) to increase your orbit. On multi-engine setups it saves fuel when you can switch off engine pairs during the insertion burn.

[Edit] However, i am curious. What craft:payload ratio do you get into orbit and what is the TWR of your craft?

Edited August 1, 2015 by Frank_G

[Green Baron]

So far i didn't lift anything into space with a spaceplane (i mean in 1.x), and this thing barely manages to lift itself into orbit.

But, interesting, when you say you circularize in the upper atmosphere, maybe you could try to lift your apoapsis from 55 to 75 just with a change of angle of attack and use the engine to then lift your periapsis out of the atmosphere .... will play more with that :-)

(and then 1.1 resets everything ...)

k

- - - Updated - - -

Ok, i changed the tanks and now have plenty of delty-v for orbit and back.

But to the lifting force effect: it's definitly strong enough to lift the apoapsis from 60 to 68 km just by coasting at 2000m/s with an AoA of 20-30 degrees between 40 km (engine cut off) and 60km altitude.

Am not quite sure about the consequences to the ascent profile though ...

k

[Frank_G]

I usually target an apoapsis of 85 km, as i park my orbital stations and construction sites there. At the moment i am trying to build a miniaturized version of my Delta II spaceplane which lifts 41.25 tons to 85 km orbit. This here is the prototype of the miniaturized version. Currently it lifts everything you put in it beyond a 100 km orbit, but i have nearly nothing left for orbital rendezvous...

The ascent profile is like this:

The spaceplane takes off at 130 m/s, nose pointing at 25° and then climb to 15.000 m. Then the nose is pushed down slowly, so the plane levels at 21.000 m to go beyond 1.600 m/s. Then it switches to the closed engine cycle and the spaceplane gets pulled back up to 10° and throttle is rapidly reduced immediately. I hunt the apoapsis at roughly 40 seconds distance until it runs off, after periapsis appears... then i continue the slow burn until i am in orbit.

Your configuration isnt much different from mine concerning engine power and fuel mass. Try exchanging a long LFO tank for a short LFO and an LF tank if you like. This will give you the chance to build up more speed in jet mode in the upper atmosphere. This is pretty essential (at least for my type of ascent profile).

[Pecan]

...

The ascent profile is like this:

The spaceplane takes off at 130 m/s, nose pointing at 25° and then climb to 15.000 m. Then the nose is pushed down slowly, so the plane levels at 21.000 m to go beyond 1.600 m/s. Then it switches to the closed engine cycle and the spaceplane gets pulled back up to 10° and throttle is rapidly reduced immediately. I hunt the apoapsis at roughly 40 seconds distance until it runs off, after periapsis appears... then i continue the slow burn until i am in orbit.

...

Interesting. I'm only just starting to mess around with spaceplanes in 1.x and I'm surprised this approach works so well for you ... I shall try it and report later. In 0.90 and earlier it was certainly worth doing a flat, low-thrust, burn to circularise in the atmosphere, but that was making the most of jets' fuel-efficiency. In 1.x, once on rockets (or closed-cycle rapier) I would have thought you'd want to gain altitude as soon as possible to get out of the draggy air, then circularise. With rapiers, especially, TWR isn't an issue as they've got to have the high-drag jet punch anyway, so must have enough rocket power. The main thing is not to climb so steeply that you lose the horizontal velocity you've previously fought to gain of course. Again, I shall check what results I get with a flatter rocket-stage climb.

(to be slightly baffled by the failure of the SPH engineer, KER and MJ to recognise that my wing-mounted fuel pods do, actually, feed the engine so I have plenty of dv, thank you very much computer ^^).

========================

ETA, for below: yes, I understand the low TWR, but rapiers (as I see in your ship pic) have a higher TWR as rockets than jets anyway, so I would have thought you'd have the punch for the climb. I'm hoping it works for me - always keen to get better results :-)

Edited August 1, 2015 by Pecan

[Frank_G]

A shallow ascent is essential for my type of spaceplane design, as i usually go with a very low TWR (below .70) which makes them very fuel efficient. However, the lack of punch requires a shallow angle to build up speed. There is just no other chance to get into orbit with those planes, but actually it delivers every time, with each new design.

Can´t wait to see, what your experiments will bring forth.

[juanml82]

Don't low TWR means you're expending too much time in the very draggy transonic speeds? Once I reach match 0.85 or something, I always prefer to punch through the sound barrier as quickly as possible, but maybe I'm doing things wrong

[Frank_G]

Going above 300 m/s in high pressure atmosphere is not fuel efficient. I start the main acceleration, once the plane reaches an altitude of 8.500 m. Getting through the sound barrier can be tricky at low TWR though... with some designs, you first climb to 12.000 m, then enter a shallow dive to punch through the barrier. After reaching 350 m/s acceleration is no problem anymore. This way, you use less fuel and carry less weight, because you need fewer engines (and lesser fuel mass). However, i am talking about heavy cargo spaceplanes here. It might be easier for a light and tiny craft to just go by raw power.

- - - Updated - - -

The main rule is... To fight air pressure, go as slow as possible - To fight gravity, go as fast as possible. So as long as your plane is in a somewhat horizontal position, going slow is more efficient. Once the air gets thinner, the acceleration kicks in automatically.

[match]

Hi there,

normally i'm more into rockets than spaceplanes but every now and then i try one. So i realised that flying at high velocity (2000m/s) at 30km alt. and at a high angle of attack (let's say 20 degrees above prograde) the lifting force on parts seems to heavily exceed the drag force: the apoapsis climbs rapidly, engines cut off (1.0.4). Which is nice to have.

I never realised that effect when flying rockets, they allways (well, mostly :-)) point prograde ...

k

At 30 km altitude, the drag forces are indeed less, but they are not absent. The gains in apoapsis you are receiving are not free. I would bet that you are paying for it with angular momentum. That is, you are not also increasing your periapsis at the same time. You may get to space today, but your circularization burn will cost you more dV.

On a side note, 30 km is about the altitude I flatten out during my descent. Many folks talk about lowering your periapsis to 30-40 km and then killing your velocity with airbarkes. I use a burn that places the ground intersection of the orbit at KSC. I spend little time in the higher ranges of the upper atmosphere and keep my AoA around 20*. By the time I hit the thicker air around 30 km, my orbit appears to be taking me directly into the mountains west of KSC. And I would hit them without the lift at that point (probably burn up first, but the end si the same).

Angular velocity is exchanged for vertical velocity as drag and lift equalize, and the orbital path begins to "flatten out", raising the periapsis and pushing the orbital ground intersection past KSC and into the sea. My horizontal velocity remains about the same during this, but my (negative) vertical velocity is drastically reduced. I believe you are experiencing the same effect, except in reverse.

[Green Baron]

Yes, i did the change the tanks and now have plenty of delty-v.

Haven't obeserved the pe yet, but circularization "cost" (for circ. between 75 and 80km) was between 100 and 200 dV during my flights, so maybe the cost for the effect is low.

It's a pity that ker seemingly can't compute delta-v for the rapiers ...

In my flights i started with a high climb rate to 15km and than lowered gradually to a height of about 27km and 1000-1200m/s before switching to closed cycle, so i'm far away from what you guys achieve with open cycle flight.

k

[Frank_G]

Nice to hear, that the tank layout gives better results .

From what i read, you are simply going too high before leveling out. The R.A.P.I.E.R.s rapidly loose thrust at altitudes above 18.000 m. Try to go in more shallow and build up speed in the denser atmosphere between 16.000 and 20.000 m of altitude. You should easily break 1.500 m/s. And it is no problem to go in a shallow dive if you are above 20.000 m... trade 500 m altitude for 100 m/s velocity and you saved a lot of precious delta V, when switching to rocket mode.

And yes... too bad with KER. But i think, upcoming versions will handle that (or SQUAD will integrate the pilot skill related delta V layout they talked about and KER becomes somewhat obsolete in that point).

[Pecan]

Took a while to make a spaceplane that is stable and easy to fly, my main purpose being to present a beginner's vehicle. It lands at ~40m/s, 5-degrees pitch so it probably has way too much wing and drag from them. Single-rapier design and I'm also using automatic switching to keep it simple, so flight profile can definitely be improved.

However - tests so far show a 10-degree climb on closed-cycle, through the 30s km altitude, until Ap reaches 75km, then prograde cruise saves about 100m/s dv compared to the 20-degree climb I was holding before, having had to use that to force the rapier to switch mode. Simply put, the extra dv it takes to fight atmospheric drag is more than compensated by the reduction in parasitic drag (with this design).

@ Frank_G - this probably wasn't what you meant; I haven't yet tried your technique of only raising Ap to 55km (ish) then circularising and continuing with low-thrust.

[Green Baron]

After a few tries i'm adapted to the 1.0.4 aerodramatics now and realised that the lifting effect is not that strong or not even there with larger, heavier constructions (like mk3). Btw., that quickly slabbed together mk2-Spaceplane flies really nicely ...

But i think my spaceplane-phase for this release is allready over. Thanks so far :-)

k