Advantages and Disadvantages of Takeoff Assist Systems

[MedwedianPresident]

Everybody has once been confronted with the following problem when building aeroplanes and spaceplanes in KSP: you have got a large plane which can fly or glide nicely, but just doesn't want to get off the runway or crashes just after leaving the ground.

Takeoff problems are the nightmare of every spaceplane pilot and often lead to the complete failure of a winged craft which has good general qualities and flies well. For example, there are many planes which fly well but have problems with takeoff because the lift is too small or the thrust is too low, resulting in the runway being too short.

In this topic, we will discuss the various possibilities of Takeoff Assist Systems (TAS) and their uses. Takeoff Assist Systems are any systems that are used to help the plane to smoothly and evenly traverse the planned runway distance, to shorten the runway distance needed to safely leave the ground, to allow the plane to safely and smoothly and quickly leave the ground and to prevent failure during the first moments of flight and while gaining altitude.

List of TAS systems:

1.) Takeoff assist engines or boosters

Sometimes, the plane is too weak to get off the ground with the runway length avaliable from the stock runway or from the modded runway which the craft is supposed to take off from. This may be caused by the plane having not enough lift or too small wings or by the plane having not enough thrust. In this case, engines or boosters (liquid, solid or jet) are engaged for the duration of the takeoff, which support the plane with additional thrust that is unneeded in cruise flight but may be vital while getting off the ground, accelerating and gaining altitude. The engines can either be reusable and be refueled after landing to prepare the next takeoff, or be jettisoned after leaving the ground to remove unneeded weight and drag.

WIP

[Levelord]

What about VTOL engines?

[DunaRocketeer]

I've though about take-off assist, but I usually solve these sorts of problems with canards and/or making the nose gear a little taller than the rear landing gear. Also move the main landing gear forward (provided it's not in front of the COM) also helps rotate the plane on the runway.

[juanml82]

At that point, I feel it's best to put the aircraft in the VAB, attach some SRBs to the wings (with structural pylons, of course) and launch from the lauchpad at a 40° angle. Horizontal RATO never worked well for me.

[magnemoe]

Well real world RATO tend to be mounted close to center of mass and point a bit downward. If you put them a bit forward of center it should give extra lift to nose.

An fun KSP tricks is to put them on decoplers behind rapiers, main issue is avoiding them hitting runway during liftoff

main benefit is making takeoff more fun

[cantab]

While KSP's runway is on the short side, the more common problem is people building planes that are unable to rotate because the main gear are too far back, and therefore unable to generate enough lift because the wings don't reach enough of an angle of attack. I've had a similar issue with tailed delta aircraft which need a high angle-of-attack at takeoff and landing, since they require tall landing gear to attain that angle of attack without the tail being underground.

But if you do need a short takeoff, another good option is STOVL operation with lift engines. That can significantly reduce takeoff speeds, while requiring fewer lift engines (or alternatively carrying more fuel or payload) than pure VTOL operation.

[Brainlord Mesomorph]

The Nazis used rocket assisted take off.https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=video&cd=1&cad=rja&uact=8&ved=0CB0QtwIwAGoVChMIudW724z3yAIVAzwmCh2KaAOY&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DbHCkWlG4pio&usg=AFQjCNEfnUvPSOiMbn8ZcpHbn54NPd_Ftg

If its good enough for them, its good enough for me. (LOL)

[A_name]

Does dropping off the lip of the runway count as a TAS?

Sometimes, depending on how severe the problem is, adding one or more vernor engines on the bottom of the nose is enough to get the nose to pitch up for a take off.

Other than that, a couple SRB's on decouplers work well, as does a VTOL engine located under the COM.

Edit: Thought of another one, giving your main wing a bit of incidence so it has a better AoA while rolling off the runway.

[magnemoe]

The Nazis used rocket assisted take off.https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=video&cd=1&cad=rja&uact=8&ved=0CB0QtwIwAGoVChMIudW724z3yAIVAzwmCh2KaAOY&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DbHCkWlG4pio&usg=AFQjCNEfnUvPSOiMbn8ZcpHbn54NPd_Ftg

If its good enough for them, its good enough for me. (LOL)

So did the US, its also used today mostly with cargo planes like C-130 on snow or rough runways.

I saw it used in an airshow.

[Enorats]

A year or so ago I was playing a career mode game where I decided to put everything into planes to start with. I built myself a nice little fighter with a science kit and then found that I had no landing gear whatsoever and that it was out if reach entirely.. and I couldn't build a rocket that'd let me unlock any science either. So... I put a radial decoupler on the bottom of my jet, stuck two structural blocks on either end of it and put four sepetrons on each. Placed it so the CoT would be balanced around the CoM.. hit space bar and the plane would launch 100 feet into the air as the jet engine warmed up, tilt the nose down slightly due to forward thrust, drop the jumpers and away she went. Used radial parachutes balanced around empty CoM to land. Worked better than any landing gear I've ever used. Zero fatalities and no damaged craft.

[Van Disaster]

Just go off the end of the runway. Or take off in rocket mode. Or both! if you think taking off in rocket mode is going to hurt, use some small drop tanks for the extra rocket fuel. One other thing I did was put 2/3ds of my jet engines on droptanks & dumped them along with the tanks when I was in the rocket phase; you don't need many jets to recover an empty spaceplane.

One thing you really don't want to do is use lift engines at the front; you're going to use them for a few seconds of flight and then you'll have to haul all that dead weight to space.

RATO does look spectacular though even if it is a bit pointless

[Hodo]

If you need a RATO to get anything off the ground on the fully upgraded runway at KSC, then you have designed it poorly.

[Van Disaster]

If you need a RATO to get anything off the ground on the fully upgraded runway at KSC, then you have designed it poorly.

Or it just has low TWR & can't accelerate in time even if it's designed perfectly otherwise. Takeoff TWR of 0.29 doesn't get you any reasonable acceleration.

[GoSlash27]

Or it just has low TWR & can't accelerate in time even if it's designed perfectly otherwise. Takeoff TWR of 0.29 doesn't get you any reasonable acceleration.

Van Disaster,

Not so. I've built several extremely underpowered spaceplanes that could take off just fine without assistance. I agree with Hodo on this one.

Best,

-Slashy

[BlueCanary]

Here's a very quickly and poorly thrown together 0.27 TWR plane lumbering into the air at 70m/s in about half the runway, without even big wings or flaps to help it. It can be done, you just have to get the balance right and be really careful not to stall. Another thing that helps to hold the plane on the brakes while the engines spool up so you can accelerate away faster.

(Ok, so there would be a screenshot here, but Imgur is wierd, so now theres not. It would just be a picture of a pretty generic twin wheesley airliner taking off. Nothing special.)

So I also agree with Hodo, I can't see any time you're really going to need RATO in KSP, although it is insanely cool. Maybe if you need to get a really big overloaded plane out of a short field (landing in hills or something), especially in high altitude, it might be useful though.

[Van Disaster]

Van Disaster,

Not so. I've built several extremely underpowered spaceplanes that could take off just fine without assistance. I agree with Hodo on this one.

OK, if they're small. If they're large ( talking 400+ tonnes ) then the wing area from the gigantic wings you'd need to get the takeoff wing loading down enough is going to just murder it higher up. I'd rather just let it roll out on a longer runway & get to a reasonable takeoff speed rather than try and do what would be a short field takeoff.

Also when it comes to personal stuff, FAR, as ever.

Edited November 5, 2015 by Van Disaster

[GoSlash27]

OK, if they're small. If they're large ( talking 400+ tonnes ) then the wing area from the gigantic wings you'd need to get the takeoff wing loading down enough is going to just murder it higher up. I'd rather just let it roll out on a longer runway & get to a reasonable takeoff speed rather than try and do what would be a short field takeoff.

Also when it comes to personal stuff, FAR, as ever.

Van Disaster,

The wing loading and incidence is independent of overall spaceplane mass. I don't know if that holds true for FAR ( I run strictly stock), but to my way of thinking anything that's too underpowered to get off the runway is going to be too underpowered to break Mach 1.

Best,

-Slashy

[Van Disaster]

Van Disaster,

The wing loading and incidence is independent of overall spaceplane mass. I don't know if that holds true for FAR ( I run strictly stock), but to my way of thinking anything that's too underpowered to get off the runway is going to be too underpowered to break Mach 1.

Right, well that's most definitely not true under FAR, you can happily chug up to 20km/5.0m at miniscule TWR. I should note ( because I just looked it up for someone ) that the Skylon's planned rotation speed is 155m/s at a TWR of something like 0.6, so those are not bad ballparks to aim at for cargo spaceplanes I guess. We don't exactly have many examples to look at yet...

[cantab]

70 m/s is a nice slow takeoff speed though. If you can get it, great, but it's not uncommon for otherwise well-designed spaceplanes to need to reach 100 m/s or so for takeoff. Such speeds, while faster than typical airliners, actually aren't excessive - Concorde usually took off around 220 knots or 110 m/s for example. Landing can be slower because by then the plane is much lighter. I don't believe Concorde could have taken off fully-fuelled from a 2500 m runway, hence the interest in either reducing takeoff speed or increasing initial acceleration in KSP.

[Brofessional]

My general rule is that if the plane can't take off on it's own, it needs to be redesigned.

[Van Disaster]

The thing is, and I've had this out before, everyone keeps going "plane". This isn't an aeroplane, it's a spacecraft that takes off horizontally - we don't have *any* real world examples. We don't even have real world ground launched hypersonic craft. The nearest we've got is Skylon at the moment ( or some less developed design studies in the past ), and I don't think there's an aircraft in existance with a designed rotation velocity of 155m/s... that's just over 300 knots, which is about 330mph.

OK, Kerbin is a lot smaller than Earth, but lets get the point over that these are not terrestrial aircraft. Stop expecting them to fly like spacegoing 747s.

[BlueCanary]

IIRC early jets (when jet engines didn't have much power for takeoff but could get high speeds once in the air just because they were jets) used to use RATO a lot, so actually thinking about it there is a lotmore sense in using RATO on low powered but eventually fast-planes than I first thought.

Completely irrelevant, but my Grandad was flying Sea Furies off an aircraft carrier in the Korean War and they were using RATO bottles to get them in the air a lot. One day, he was just waiting in the plane to takeoff and the RATO packs just randomly ignited right under his seat and blew holes in the wings and stuff. There's photos somewhere of this conspicuously shredded aeroplane where you can see how close he was to effectively getting a rocket shot up his butt. It's pretty Kerbal now I think about it.

[sjwt]

Ok guys, this is a discussion ABOUT takeoff assist system, its not for arguing against them or attacking other players design philosophies.

Keep it clean, keep it on topic, keep it productive.

-Mod Team

[Hodo]

The thing is, and I've had this out before, everyone keeps going "plane". This isn't an aeroplane, it's a spacecraft that takes off horizontally - we don't have *any* real world examples. We don't even have real world ground launched hypersonic craft. The nearest we've got is Skylon at the moment ( or some less developed design studies in the past ), and I don't think there's an aircraft in existance with a designed rotation velocity of 155m/s... that's just over 300 knots, which is about 330mph.

OK, Kerbin is a lot smaller than Earth, but lets get the point over that these are not terrestrial aircraft. Stop expecting them to fly like spacegoing 747s.

All I do is SSTO space planes, and if you need a RATO to get off the runway then the craft does not generate enough lift, and thus you are wasting a great deal of energy on take off. Thus would be better off launching vertically.

Like this craft, which has more than your .27TWR... but is still low by my standards when loaded.

It has a take off TWR of .76 empty and .45 fully loaded. It still manages to take off at 105m/s.

If your craft is over 300 tons and you are having such low TWR you have other issues, like it will never accelerate fast enough to maintain level flight let alone climb.

Edited November 5, 2015 by Hodo

[wizzlebippi]

Airliners routinely operate off runways in the 7-10k ft range (KSP's is 2.5km or ~8200ft) with max takeoff weight TWR's of ~0.3. I know this doesn't quite map 1:1 with KSP, but it's a point of reference. Here's my process:

When unable to get off the runway, try holding full nose up control to see if your craft can rotate (try not to blow it up). If not, try moving your landing gear closer to your CG (should be slightly behind the CG for tricycle gear) or adding more pitch control surfaces. If so, try taller landing gear to give your craft more AoA to achieve more lift or more wing to produce more lift with the AoA you can achieve. I rarely end up adding thrust for takeoff reasons alone.