Why are rockets so easy and planes so hard?!

[Corona688]

My point?  You're comparing the most complex rockets to the simplest airplanes, and expanding the scope from engines to the entirety of aerospace technology for one of them but not the other.  That's not fair.

You can get to space in a rocket the size of a telephone pole.  It just won't stay there very long.

Even if you limit it to powered models, it still doesn't work out.  Functioning model rockets are far simpler and easier than functioning model airplanes.

Edited August 25, 2016 by Corona688

[pincushionman]

The reason is twofold, really. First, the REALLY hard stuff in rocket design - the nozzle design, pumps and plumbing, durability and reliability, multiple restarts, fuel boil-off, those kinds of details, are abstracted away or not present at all. The small scale and lack of life-support requirements doesn't hurt either.

Second, planes benefit less from these abstractions than do rockets. But while even the most advanced rockets are more or less cylindrical tubes with an emphasis on drag, airplanes are highly shape-dependent and are slaves to all aspects of aerodynamics. And the simplified flight model of KSP is, at best, adequate for flight. FAR helps, but there are still limitations.

Parts are heavy. The small scale and overpowered rockets means rockets suffer less. The LEGO wings don't actually aggregate into large wings so we don't get drag occlusion or aspect ratio or wing sweep advantages. We can't play with the airfoil shapes. We don't have the freedom to put fuel in places that would let us easily balance. We don't have a good way to evaluate drag. Wheels are still borked. And the joints. Aren't. Stiff. Enough.

What it really boils down to, though, is the tools we are given are great for dealing with the problems that the game presents us in rockets, but are only pretty okay for dealing with aircraft design problems. Which is okay, to be quite honest.

[Bill Phil]

Well, it's actually like reality (somewhat). We've had rockets for centuries. Not orbital ones, but we could've gotten away with one in the 30s if we were okay with insanely inefficient designs. Airplanes have only been around in their self propelled form since 1903. Although we've had kites for a long time, as well.

[Diche Bach]

1 hour ago, pincushionman said:

The reason is twofold, really. First, the REALLY hard stuff in rocket design - the nozzle design, pumps and plumbing, durability and reliability, multiple restarts, fuel boil-off, those kinds of details, are abstracted away or not present at all. The small scale and lack of life-support requirements doesn't hurt either.

Second, planes benefit less from these abstractions than do rockets. But while even the most advanced rockets are more or less cylindrical tubes with an emphasis on drag, airplanes are highly shape-dependent and are slaves to all aspects of aerodynamics. And the simplified flight model of KSP is, at best, adequate for flight. FAR helps, but there are still limitations.

Parts are heavy. The small scale and overpowered rockets means rockets suffer less. The LEGO wings don't actually aggregate into large wings so we don't get drag occlusion or aspect ratio or wing sweep advantages. We can't play with the airfoil shapes. We don't have the freedom to put fuel in places that would let us easily balance. We don't have a good way to evaluate drag. Wheels are still borked. And the joints. Aren't. Stiff. Enough.

What it really boils down to, though, is the tools we are given are great for dealing with the problems that the game presents us in rockets, but are only pretty okay for dealing with aircraft design problems. Which is okay, to be quite honest.

This is exactly the answer to question right here.

I don't think the difference in game has anything to do with "properly reflecting" the real world difference in how challenging it is to fly (or design) an aircraft vs a rocket ship. The game was initially designed as a rocketship game and it works great for that. It works adequately for aircraft too. Indeed, when combined with the fact that one can design one's own aircraft, and that there are plenty of ways to work around the problems inherent to the game design, one might even say that: planes are not harder, they just have a more advanced learning curve.

I think the mistake Squad has made so far is to fail to provide _really_ good stock models for people to play with. I flew a couple of those, and glanced at the rest and frankly they all stink. Perhaps they were not so bad several iterations ago when they were first added to the game, but assigning one or two of their best aircraft design staff members to: (a) design three to seven (one total beginner plane, that uses the lowest tier techs; one intermediate, one advanced, additional at each level and/or a spaceplane). (b) do a Youtube or three in which they walk the viewer through how these models are designed and why, basically use them as an example of how to effectively design a really good basic plane.

[wumpus]

12 hours ago, pincushionman said:

The reason is twofold, really. First, the REALLY hard stuff in rocket design - the nozzle design, pumps and plumbing, durability and reliability, multiple restarts, fuel boil-off, those kinds of details, are abstracted away or not present at all. The small scale and lack of life-support requirements doesn't hurt either.

Second, planes benefit less from these abstractions than do rockets. But while even the most advanced rockets are more or less cylindrical tubes with an emphasis on drag, airplanes are highly shape-dependent and are slaves to all aspects of aerodynamics. And the simplified flight model of KSP is, at best, adequate for flight. FAR helps, but there are still limitations.

Parts are heavy. The small scale and overpowered rockets means rockets suffer less. The LEGO wings don't actually aggregate into large wings so we don't get drag occlusion or aspect ratio or wing sweep advantages. We can't play with the airfoil shapes. We don't have the freedom to put fuel in places that would let us easily balance. We don't have a good way to evaluate drag. Wheels are still borked. And the joints. Aren't. Stiff. Enough.

What it really boils down to, though, is the tools we are given are great for dealing with the problems that the game presents us in rockets, but are only pretty okay for dealing with aircraft design problems. Which is okay, to be quite honest.

You seem to get about 30 years of "free" airplane science with KSP.  The hard bits of getting into the air:

• You need an engine with a strong power to weight ratio (nothing like a rocket, but more car-like).  KSP starts with jet engines.
• You need a strong airframe.  Check out aircraft 1903-1920:  Lots of braces and framing on wings.  Eventually aluminum and real engineering took care of that, and is handed to you with easy LEGO style parts.  Sure the parts might not be stiff enough, but that is a huge job for aircraft designers.
• You need some clue about aerodynamics.  This was hard in 1903 and still not easy.  It is also typically counterintuitive so new players often don't even know the basics about CoL behind CoM.
• You need to fly the thing.  Ok, flying is easy (for more tolerant values of "flying level") but you still need to land (although at level 0 and maybe 1 you might want that perfectly flat grassy plain next to the runway).  Also there are more controls to worry about and they aren't great with a keyboard, while a rocket should ideally be flown with just a spacebar (real rockets don't throttle, and any adjustments to your gravity turn are going to be inefficient).

But yes, once you abstract away things like turbopumps and light materials (important for aircraft as well), you reduce rocket science down to the bare bones.  From the sound of it, spacex might be going to a *cheaper* material with the carbon fiber, I remember something about lithium being in their aluminum alloy which can't be cheap.

[eddiew]

Rockets are hard too, if you're trying to land them on the runway 

[Diche Bach]

Observed something last time I had the game running that I think is indicative of why the game makes getting the hang of "flying" a rocket relatively easier than flying a plane.

Jeb and Bob were in a rover I built: totally basic frame of girders, two command seats on the top, one quadcore (from universal storage) with an alkaline fuel cell [generates electricity] and three Science Jr. (Uni Storages compact "hex" version of the materials science lab) towrad the front. Tier 1 ladder on one side (with small girder and additional ladder to allow the kerbal to make it over the lip of the climb), four of the Tier 1 rover wheels, two Mk1 command pots attached to the back with cheapo sepratrons. Even with the pods still attached, a nice balanced design that suffers little to no tendency to roll or topple in any particular direction.

Launched it, Jeb EVAed and managed to hop from the Mk 1 hatch ladder and get into his command chair . . . Bob EVAed but stumbled and fell on the ground, his head very close to the right rear wheel (maybe even clipping into it a bit even).

When Bob got up he tipped the whole damn thing over!

In sum, the physics are a bit whacked out here and there, and I think this is why airplanes have to be "juussst right" or they act like boomerangs/bolos/catapults.

The physics probably reflect what is necessary to get rockets with their respective mass, dimensions, thrust, etc. to behave in ways that can pass (more-or-less) as "normal," but the unfortunate side effect is that, many of these same algorithms applied to aircraft (or kerbals and rovers) are hair-trigger and produce ridiculous effects like a dude stands up and tips over an entire car with the force of raising his head.

[Corona688]

The physics of kerbals on EVA are quite definitely whacko.

[panzer1b]

I have to say ive always had an easier time designing SSTOs then rockets, then again, the first thing i EVER sent into orbit in the game was a sci-fi aircraft thing that used jets and then a rocket once in space.  Ofc that was back in 0.21 or something old so it was like impossible not to get a jet into space when you gave it 20 intakes per jet engine .  Now rockets are easier (place fairing, adaptor if needed, largest fuel tank, kr-2l engine, and 2 boosters which can be anything from jumbo-64+engine to massive tank+quad engine), but before nasa parts getting anything that didnt look like your stereotypical cylindrical craft into orbit was extremely painful and hard.  Then again, given the primary space vessels i make in KSP are heavily armed and armored capital ships, yeah, my experience with launchers wasnt exactly a easy one until fairings and those nasa engines came into play (now its only hard to launch a ship that wont fit into a fairing (carriers are still a pain in the arss to get into orbit due to excessive size, wings everywhere that ruin aerodynamics, and well heavy).

Currently SSTOs are still a joke to make (pod, fuel tank, rapier, wings, wheels, any drag inducing radial parts like weapons inside the bomb bay), but its a real challenge to get over 7000dV out of one after reaching LKO.  Anything that goes fast enough down low (most efficient on fuel) will fry itself every time, and anything that doesnt fry needs way too much fuel to pull off those ranges.

[Xyphos]

On 8/24/2016 at 1:29 PM, Sharpy said:

One very serious problem with airbreather spaceplanes is a little talked about thing called Ram Rise.

 

Ram Rise = 0.2 x SAT x M^2.

This is how much the "perceived" (measured by a thermometer on the airplane surface) air temperature rises above ambient (static) temperature. SAT is that static temperature measured in Kelvins. M is the Mach number.

So if the air is at -40C that's 233 Kelvin. So, 23.3 degree increment at Mach 1, a neat -17C. At Mach 3, that's already 209K, or +164C. Skin on the plane can still withstand it, but starts heating rapidly.

At Mach 10, we have 21 times the ambient temperature or 2290C. Steel melts at 2500. At Mach 11 we're melting tungsten.

And we need to reach Mach 23.

24,600K. We're at half the temperature of Sun surface.

This REALLY makes air-breathing spaceplanes difficult.

 

that's assuming that we need to do the whole Mach 25 while still in the atmosphere, and we surely don't do that in KSP...
a space plane only has to go as fast as possible before switching to rocket mode. if we had an engine and ablative materials that can handle Mach 9 in the upper atmosphere, the other 14 can be done as a rocket.
after all, once a spaceplane leaves the atmosphere, it becomes a rocket.

[Laie]

On 8/24/2016 at 1:07 PM, Laie said:

It's been said several times already, but to put it most simple: think of the difference between throwing a rock dart and making a paper plane that flies well.

(sorry, minor fix)

On 8/24/2016 at 1:13 PM, -DDD- said:

If you can launch rock that reach 7800 m/s above 200 km, congraturations

Category error. If your device can't even cover ten meters, going any further (like, space) is out of the question.

 

Edit to add: What makes rockets more problematic than planes, IRL, are their respective failure modes.

A plane may not take off in the first place or only make a short hop. Hardware recovered, lesson learned, tweaking possible. Even if the engine fails in mid-flight you have a decent chance to recover the whole thing with only minor damage, and can do a proper inspection to find out what went wrong.

Rockets require a new item at every launch even when they work, and the remains may not tell you much in either case.

Edited September 1, 2016 by Laie

[Sharpy]

1 hour ago, Xyphos said:

that's assuming that we need to do the whole Mach 25 while still in the atmosphere, and we surely don't do that in KSP...
a space plane only has to go as fast as possible before switching to rocket mode. if we had an engine and ablative materials that can handle Mach 9 in the upper atmosphere, the other 14 can be done as a rocket.
after all, once a spaceplane leaves the atmosphere, it becomes a rocket.

Yes, but the whole idea is to do as much on efficient air-breathing engines, and as little as possible on rockets, which by definition must carry a lot of (reaction) mass. And ram rise means that merely developing better, faster, stronger air-breathers is insufficient. Some people like to picture spaceplanes as "gain orbital speed in the atmosphere, then perform a token circularization burn with RCS." This is why that approach is impossible.

[wumpus]

3 hours ago, Sharpy said:

Yes, but the whole idea is to do as much on efficient air-breathing engines, and as little as possible on rockets, which by definition must carry a lot of (reaction) mass. And ram rise means that merely developing better, faster, stronger air-breathers is insufficient. Some people like to picture spaceplanes as "gain orbital speed in the atmosphere, then perform a token circularization burn with RCS." This is why that approach is impossible.

This belief is only held by people who haven't looked into rockets at all or haven't really noticed the differences between Kerbin (and KSP jet engines for that matter) and Earth.

According to here: http://forum.nasaspaceflight.com/index.php?topic=34464.0 the Falcon 9 separates stages 1 and 2 at mach 6 for recoverable stages and mach 10 for non-attempted landings (no idea what the "coming in hot" separations were).  Stage 2 is ~10% of the mass of stage 1, so presumably "only" getting to mach 10 is quite valuable.

NASA has built an airbreather that hit mach 9.6 (and I'm guessing it melted down instead of running out of thrust, but don't really know.  They didn't try to recover, so it could have melted *after* running out of thrust), so these are hardly "impossible" values.  ISPs (effective, presumably ignoring the mass of the air) were well over 1000, and even higher around the mach 6 range Skylon would top out at.

 

[The_Rocketeer]

I'd like to flag a relevant issue that I don't think has been directly approached.

A rocket takes off facing straight up, so at the moment of highest resistance the resisting forces (gravity and drag) are basically aligned and directly opposed to the force of thrust. This remains the case for as long as the pilot likes, and can be left completely alone until the rocket is in vacuum, at which point there's only the force of gravity to worry about, and due to the fundamental physics of attaining orbit (gravity turns etc) is no longer really a resistance force, just a limiting factor (you have x time to raise periapsis above 70km). Going up fast enough that you don't fall down again is mostly a case of going in a straight/radial line a lot and a perpendicular/horizontal line a little, and the more up you go the less across you need to go for the same result. Even a total noob (given enough time) should be able to figure out how to go way way up and then have a load of time to figure out they need to go across to miss the planet on the way down. Inefficient, but easy.

A plane is fundamentally more complicated. First of all drag and gravity are at 90 degrees to each other. Second, rather than just having an engine that pushes them in a straight line, they also have wings, which take some of the forward thrust of the engine and turn it into perpendicular 'thrust' (lift), and extra drag. So now the vehicle isn't just opposing two resistances that are aligned, it's opposing 2 perpendicular forces using two perpendicular forces, one of which is a variable product of the other. Confused? Me too.

Add to that the difference in effect of shifting CoM (due to fuel consumption) - in a rocket, CoM just moves towards the engine end, never out of alignment with the direction of thrust, and remains relatively stable. However in a plane, unless it is well designed, the CoM may be moving out of alignment in relation to either or both of the direction of thrust and the direction of lift, meaning that over time the aircraft just gets less and less stable. Understanding how to avoid this is part of the design process, but this is not simple and seems unintuitive until it has been learned and understood, usually by failure.

TL:DR, the two things are as different as learning to jump and learning to swim.

[Sharpy]

On 25.08.2016 at 2:59 AM, Corona688 said:

My point?  You're comparing the most complex rockets to the simplest airplanes, and expanding the scope from engines to the entirety of aerospace technology for one of them but not the other.  That's not fair.

 

No, I'm comparing most complex rockets to most complex airplanes. Neither supersonic stealth fighters, nor most massive airliners face this kind of complexity. They do have their own problems - but the amount of problems they have that are not shared by rockets is quiiiiite small.

Unless I'm mistaken. Care to provide a list longer than mine - or with entries more critical - of problems in aviation, which rockets never have to worry about? Use as complex and as obscure ones as you like.

In all seriousness, I can think of very few. High maneuverability, landing on a grass landing strip horizontally, good performance at low airspeeds, durability against gunfire, stealth. None of them makes airplanes as a whole inherently more difficult - these are niche issues, and none of them can compare to the slew of problems I listed. Well, maybe with exception of stealth. But hey, it's a quirk far more niche than say, "manned spaceflight".

[The_Rocketeer]

4 hours ago, Sharpy said:

the amount of problems they have that are not shared by rockets is quiiiiite small.

Small, but significant.

Fundamentally a rocket uses very simple physical theory and therefore can be a very simple vehicle - push this way to accelerate in that direction, be smooth all round so that you don't have to push so hard, and try not to tip over. The process of getting from a to b is just a question of pointing the right way and burning for long enough.

Meanwhile for planes, all of that applies, but you now have to add that you're not just pushing your way through a medium, you're actually using that medium to resist gravity as you go, by bleeding off some of your 'getting there' burn as a 'not crashing' force - lift. This is exactly why burn times and fuel calculations don't work accurately inside an atmosphere in KSP - the formula to work it out just has too many additional variables. Rockets get around that by deliberately spending as little time as possible in that state, whereas for aircraft it's just a fact that you have to accept and work around. Nonetheless, the demands of any given plane will be different because their shape and drag profile are different - they're not just cylinders full of go-juice, and whereas a single engine will work fine for a rocket, a single wing is NOT going to work for a plane.

It's not that a rocket doesn't have to beat the same physics as a plane (after all they all exist in the same physical universe and start at very similar starting points), it's that they are fundamentally easier to beat in rocket than in a plane.

[Sharpy]

4 minutes ago, The_Rocketeer said:

Fundamentally a rocket uses very simple physical theory and therefore can be a very simple vehicle - push this way to accelerate in that direction, be smooth all round so that you don't have to push so hard, and try not to tip over. The process of getting from a to b is just a question of pointing the right way and burning for long enough.

Meanwhile for planes, all of that applies, but you now have to add that you're not just pushing your way through a medium, you're actually using that medium to resist gravity as you go, by bleeding off some of your 'getting there' burn as a 'not crashing' force - lift.

This is all true, nice and correct if you reduce the purpose of both to flight.

As opposed to - say, spaceflight, astronavigation, orbital dynamics, life support - versus commercial aerial transport, aerobation, or wartime operations. With that kind of simplification, yes, rockets are easier. But that's a very unrealistic simplification, because outside of hobby area, that's not what these things are made for.

[Red Iron Crown]

1 minute ago, Sharpy said:

As opposed to - say, spaceflight, astronavigation, orbital dynamics, life support - versus commercial aerial transport, aerobation, or wartime operations. With that kind of simplification, yes, rockets are easier. But that's a very unrealistic simplification, because outside of hobby area, that's not what these things are made for.

Take any scenario for which both types of vehicle are appropriate and the rocket will almost always be simpler. Apples to apples, and all that.

[The_Rocketeer]

1 minute ago, Sharpy said:

that's a very unrealistic simplification, because outside of hobby area, that's not what these things are made for.

Except that we're talking about starter-level planes and rockets in KSP, not RL applications. Both can be taken into very niche specialised scenarios in which the necessary understanding and functionality becomes very finely sophisticated, but we're not talking about this, we're talking about their difference in the difficulty of understanding, designing and using them from the get-go.

[Sharpy]

22 minutes ago, The_Rocketeer said:

Except that we're talking about starter-level planes and rockets in KSP, not RL applications. Both can be taken into very niche specialised scenarios in which the necessary understanding and functionality becomes very finely sophisticated, but we're not talking about this, we're talking about their difference in the difficulty of understanding, designing and using them from the get-go.

The subject is: "Why are rockets in KSP easier than planes?" and the answer being challenged is "Because rockets IRL are easier than planes."

They are not. They can be, if we use them for nothing but making rockets fly. But we are using them for spaceflight, and that makes them very complicated, and much more complicated than any airplanes.

[The_Rocketeer]

30 minutes ago, Sharpy said:

The subject is: "Why are rockets in KSP easier than planes?"

Agreed.

 

 

30 minutes ago, Sharpy said:

the answer being challenged is "Because rockets IRL are easier than planes."

They are not. They can be, if we use them for nothing but making rockets fly. But we are using them for spaceflight, and that makes them very complicated, and much more complicated than any airplanes.

Er, no.

We can demonstrate this with a simple benchmark test. Name a rocket that has achieved LEO. Now name a plane that has achieved LEO.

To compare different vehicles in this way you have to measure them by the same yardstick. Otherwise you're saying 'it's easier to make a plane to do plane-y things than it is to make a rocket to do rocket-y things'... which now I think about it also isn't true in KSP, or in RL, anyway.

Edited September 2, 2016 by The_Rocketeer

[Corona688]

Further, as previously mentioned, a sounding rocket -- nothing but a giant solid in 1 or 2 stages -- goes to space needing very little in the way of sophisticated controls.

[Sharpy]

33 minutes ago, The_Rocketeer said:

We can demonstrate this with a simple benchmark test. Name a rocket that has achieved LEO. Now name a plane that has achieved LEO.

To compare different vehicles in this way you have to measure them by the same yardstick. Otherwise you're saying 'it's easier to make a plane to do plane-y things than it is to make a rocket to do rocket-y things'... which now I think about it also isn't true in KSP, or in RL, anyway.

Name a bicycle that has achieved LEO. There, we have a proof that bicycles are harder to make IRL than rockets.

The correct benchmark here is the amount of expertise necessary to achieve the goals - the number and complexity of problems that need to be solved. A sounding rocket can achieve orbital altitude, but not orbit. The simplest orbital rockets measure up in complexity with pretty advanced airplanes. And I can guarantee there is no airplane beating Apollo 11 in complexity. Not F-22, not Dreamliner, not An-225. And the benchmark MUST be the top of the line, not an arbitrarily picked apple or orange from somewhere midway-towards-the-bottom. What do you want to compare the sounding rockets to? RC jet model? Cessna? Glider?

 

edit:

...moreover: Bicycles ARE harder to make in KSP than rockets too!

Edited September 2, 2016 by Sharpy

[The_Rocketeer]

Just now, Sharpy said:

Name a bicycle that has achieved LEO. There, we have a proof that bicycles are harder to make IRL than rockets.

...

the benchmark MUST be the top of the line, not an arbitrarily picked apple or orange from somewhere midway-towards-the-bottom

You basically made my point back at me - flying/LEO capable bicycles clearly ARE harder than flying/LEO capable rockets.

I think you're defeating your own argument here. If an airplane capable of a moon landing is so complicated and difficult that we can't even imagine let alone build it, then it's obviously harder to do that than use a rocket, ergo, rocket is easier.

I get what you're saying, space rockets are complicated and hard, but planes of comparable performance are orders of magnitude harder in every single respect. Even if you pick a role for which a rocket is totally unsuited, e.g. sustained flight, it's STILL easier to make a rocket that can do this than a plane because it's a much simpler mechanical beast - push up to not go down. Planes are more complicated precisely because they DON'T push up to not go down.

[Corona688]

4 minutes ago, Sharpy said:

The correct benchmark here is the amount of expertise necessary to achieve the goals - the number and complexity of problems that need to be solved. A sounding rocket can achieve orbital altitude, but not orbit.

Do I hear a goalpost being moved?  I do believe I hear a golapost being moved.

Insisting on the most complicated just because that's the only way your argument makes sense is entirely arbitrary.