Why are rockets so easy and planes so hard?!

[tater]

Spaceplanes are easy in KSP because they, you know, exist.

To the extent they are harder than rockets, that's a good thing about the game. They (spaceplanes) are still far easier than in real life, which is why we have yet to see any spaceplane, ever.

The air is the toughest regime for a rocket, and it by definition minimizes the time within that regime due to a rocket's flight path. Spaceplanes need to maximize their time in that regime (because the whole point is to steal propellant from the air to minimize what they must carry).

Anyway, spaceplanes should be hard.

[Corona688]

3 hours ago, -DDD- said:

Seriously, you can't compare chinese fireworks with a modern rocket...

Why not?  Because it's too simple?  That's the entire point.  gunpowder + stick = up.  Once you've got the chemistry, not hard.

Edited August 24, 2016 by Corona688

[wumpus]

4 hours ago, -DDD- said:

Seriously, you can't compare chinese fireworks with a modern rocket... It's like to say that they master aerodynamic cause they can build kites.

 

3 hours ago, Sharpy said:

Physics that is fundamental for orbital mechanics was Newton, Kepler and Lagrange. When did the first serious works on aerodynamics begin?

From what I remember from various biographies, the Wright brothers had to do their own work on aerodynamics as the available work wasn't even good enough to predict their glider performance.  Unsurprisingly, they started with kites (not sure if this was before or after realizing that they had to start from scratch).

Fans of James Burke's connection series would likely claim that that the Wright Brothers invented the wind tunnel, the airplane was a "freebie" on the tech tree right after that (although in 10 years you could simply buy an engine that would make a barn door fly).  I'm not sure when they figured out that the scale of the wind tunnel matters: the tiny tunnel they used meant the original Flyer was optimized to fly through molasses.

Counter argument: allegedly the issues with the Langley aerodrome (the more famous rival that was dunked twice into the Potomac, and later fixed and flown in 1916) were from mechanical stresses and not botched aerodynamics.  Since there was plenty of reasons to hide the modifications needed to make the thing fly, I doubt anything conclusive can be shown from this.  I wonder what source of aero knowledge was used for that flight in Brazil?

1 hour ago, tater said:

Spaceplanes are easy in KSP because they, you know, exist.

To the extent they are harder than rockets, that's a good thing about the game. They (spaceplanes) are still far easier than in real life, which is why we have yet to see any spaceplane, ever.

The air is the toughest regime for a rocket, and it by definition minimizes the time within that regime due to a rocket's flight path. Spaceplanes need to maximize their time in that regime (because the whole point is to steal propellant from the air to minimize what they must carry).

Anyway, spaceplanes should be hard.

As long as you define "spaceplane" as "using an airbreather" for a large part of your delta-v (as you did above), spaceplanes are hard.

How hard is it to make a Skylon in RO?  Are the parts roughly what they should be for a "real" SABRE?  I'd assume that a "spacejet" should be hard in RO, and impossible with existing jet engines.  This is the point I normally plug the X-43, but I'd also have to point out that:

• The engine is single use (I assume there was a reason to not attempt to recover a small expensive craft that never left the atmosphere).  Kind of defeats the point of an SSTO.
• The entire plane was critical for feeding the engine and/or providing the needed lift.  You can't just splat a scramjets engine on LEGO style (of course you can't build anything else KSP/LEGO style.  But scramjets are much worse).

Edited August 24, 2016 by wumpus
first edit, merge doublepost. Second edit, s/single engine/SSTO must have changed thoughts midstream. 3rd s/X-38/X-43

[wumpus]

deleted

Edited August 24, 2016 by wumpus
removed doubleposting and merged with above.

[Sharpy]

Wright brothers didn't start from scratch - they corresponded with Otto Lilienthal, who shared his glider design with them freely. Their work on Flyr was incremental research strongly inspired by Lilientrhal's work.

Then they fought fiercely for the hundreds of patents they filed, not sharing with anyone.

Which (as our local aviation guru told us on a lecture on Wright brothers) was actually beneficial, because a lot of their ideas were simply horrible, and by their aggressive approach to all copycats, they forced others to invent alternatives, which were almost universally better.

Rudder in front. Throttle requiring to take your hand off the pitch lever. Rail required for start. Bending wings for pitch and roll control. No role for legs. Lying on your belly position. Catapult for initial propulsion.

The engine and the propeller were great for their times. The rest... uh, not so much.

[Guest]

38 minutes ago, tater said:

Spaceplanes are easy in KSP because they, you know, exist.

Yeah this.  The fact that you can make a spaceplane at all in KSP, much less one with the absurd payload fractions the game makes possible, is indicative of just how easy the "scenario" is.

The fact that they are harder than rockets in KSP should tell you something about why we don't have spaceplanes IRL.

[tater]

29 minutes ago, wumpus said:

As long as you define "spaceplane" as "using an airbreather" for a large part of your delta-v (as you did above), spaceplanes are hard.

They are hard, or inefficient. Minus the "airbreather" part, what's the point, exactly? I consider a "spaceplane" something that functions like an aircraft, but can reach space, I guess. I don't consider Shuttle a spaceplane, for example. If it were efficient to make rocket spaceplanes (or just not far harder than rockets) we'd have seen one by now. I'm happy to change the definition I use if there is a well-established usage that I haven't paid attention to.

29 minutes ago, wumpus said:

How hard is it to make a Skylon in RO?  Are the parts roughly what they should be for a "real" SABRE?  I'd assume that a "spacejet" should be hard inRO, and impossible with existing jet engines.  This is the point I normally plug the X-38, but I'd also have to point out that:

• The engine is single use (I assume there was a reason to not attempt to recover a small expensive craft that never left the atmosphere).  Kind of defeats the point of a single engine craft.
• The entire plane was critical for feeding the engine and/or providing the needed lift.  You can't just splat a scramjets engine on LEGO style (of course you can't build anything else KSP/LEGO style.  But scramjets are much worse).

Good question, and I don't know. I do RO, but I have yet to make aircraft, I'm pretty disinterested in them in KSP, honestly.

3 minutes ago, regex said:

Yeah this.  The fact that you can make a spaceplane at all in KSP, much less one with the absurd payload fractions the game makes possible, is indicative of just how easy the "scenario" is.

The fact that they are harder than rockets in KSP should tell you something about why we don't have spaceplanes IRL.

Yeah, I always find posts about spaceplanes leaving KSC and going to Duna (or anywhere else that is not barely LKO) and back pretty funny, and a sign of how goofy KSP can be.

At best an even lightly realistic spaceplane gets you to low orbit with a small payload.

[wumpus]

2 minutes ago, Sharpy said:

Wright brothers didn't start from scratch - they corresponded with Otto Lilienthal, who shared his glider design with them freely. Their work on Flyr was incremental research strongly inspired by Lilientrhal's work.

Then they fought fiercely for the hundreds of patents they filed, not sharing with anyone.

Which (as our local aviation guru told us on a lecture on Wright brothers) was actually beneficial, because a lot of their ideas were simply horrible, and by their aggressive approach to all copycats, they forced others to invent alternatives, which were almost universally better.

Rudder in front. Throttle requiring to take your hand off the pitch lever. Rail required for start. Bending wings for pitch and roll control. No role for legs. Lying on your belly position. Catapult for initial propulsion.

The engine and the propeller were great for their times. The rest... uh, not so much.

While they benefited greatly from Lilienthal, they also found out that his work wasn't good enough to get off the ground: thus the research.

Yes, their patent fights were bad and show how bad patents in general are.  Although now anyone can file a patent regardless of whether the thing works or not, and fight it in East Texas courts if anything is remotely similar to vaguely worded patents.  Especially when their patent didn't expire until well after the "you could go down to the market and buy an engine that will make a brick fly" stage.

• Rudder in front: wildly popular in KSP, but almost certainly unstable.  Didn't the Rutan Vari-eze do this as well?  It presumably *can* be tamed, but I doubt control theory was understood at all in 1903.
• Rail required for start - how many runways were there in 1903?  I don't think a modern plane could take off anywhere in 1903 either.
• wing warping: just who else understood the importance of roll control and made sufficient provisions to turn via rolling?  This seems ideal for tiny planes and rudders and what not were only needed to emulate wing modification (consider the flaps used for takeoff)
• catapult: isn't this redundant with the rail?  I'm not sure what you would use in 1903 that would be more powerful than the multi-horsepower engine on the plane.
• belly position: do you want aerodynamic or not?  This is 1903 and you just want to get off the ground.  I'd expect plenty of record-setting current vehicles to use belly position (and in 1903 "flying at all" was record setting.

But yes, patents were bad for "I did it, now nobody else can do it" and worse with the "I thought of doing it, now nobody else can do it" we have now.

 

[Corona688]

1 minute ago, wumpus said:

 

• Rudder in front: wildly popular in KSP, but almost certainly unstable.

Not that stable in KSP, either.  SAS forgives many sins.

[Guest]

44 minutes ago, wumpus said:

How hard is it to make a Skylon in RO?  Are the parts roughly what they should be for a "real" SABRE?

You can define them if you'd like, but Skylon doesn't exist IRL and its engines have never been fired, which is likely the reason I don't recall ever seeing those engines in my installs (although I don't use B9, so there's that).

44 minutes ago, wumpus said:

I'd assume that a "spacejet" should be hard in RO, and impossible with existing jet engines.

Yes, exactly. One should also keep in mind that materials science is abstracted in RO, like KSP, so even if you manage to actually build a KSP-style "spacejet" in RO, that doesn't mean the concept is at all realizable IRL.

44 minutes ago, wumpus said:

This is the point I normally plug the X-38

The ... X-38 CRV?

44 minutes ago, wumpus said:

but I'd also have to point out that:

• The engine is single use (I assume there was a reason to not attempt to recover a small expensive craft that never left the atmosphere).  Kind of defeats the point of an SSTO.
• The entire plane was critical for feeding the engine and/or providing the needed lift.  You can't just splat a scramjets engine on LEGO style (of course you can't build anything else KSP/LEGO style.  But scramjets are much worse).

Yep. Spaceplanes are easy in KSP.

[wumpus]

4 minutes ago, regex said:

The ... X-38 CRV?

Sorry, the X-43 Scramjet test.  I need to google these things better.

23 minutes ago, tater said:

They are hard, or inefficient. Minus the "airbreather" part, what's the point, exactly? I consider a "spaceplane" something that functions like an aircraft, but can reach space, I guess. I don't consider Shuttle a spaceplane, for example. If it were efficient to make rocket spaceplanes (or just not far harder than rockets) we'd have seen one by now. I'm happy to change the definition I use if there is a well-established usage that I haven't paid attention to.

If you walk into the Smithsonian, you will see both the X-15 and Spaceship-1 hanging in the "milestones of flight" entry hall.  Both are designed to use lift and both managed to get into space (but not orbit).  I'd insist that using [fixed] wings for lift is a better definition of a plane than breathing air.  Even the shuttle used lift to slowly land (even though it still fell like a brick.  And since it was obviously mostly rocket by weight even without the fuel, I doubt it qualifies as a spaceplane).

KSP is just goofy in that it allows you to get nearly to orbital velocity using jets (which is pretty much required to make jets work at all, although I'd expect conventional jets to explode around mach 1* and even supersonic jets to fail in amusing was past mach 3.  It encourages an "impossible" definition of spaceplane.

* I think there is at least one Boeing jetliner that has a tested Vne > mach 1.  The story is that it was designed for mach .90 flight or so and FAA regulations required at least 10% over that, so it had to be tested beyond the sound barrier.  Don't expect most craft to handle the stresses of transonic flight (although modifying the engines is probably the easiest part: just widen the compressor enough to slow down the air, just less than "real" supersonic engines).

[tater]

I should have added orbital space flight, not just hitting the Karmen line---which is a great milestone/achievement, but basically useless, particularly for a crewed vehicle.

I agree on lift making more sense in the definition than air breathing, but functionally, I don't think non-air breathers make any sense in this role (X-15 didn't generate huge amounts of lift I think, I'd imagine a rocket slung under a B-52 would do just about as well).

Edited August 24, 2016 by tater

[Guest]

7 hours ago, TheDestroyer111 said:

//EDIT, to the guy below me: This is a test flight, and this is a small tourist plane with only one well-trained pilot and no soft cargo.

Aren't pilots themselves considered soft cargo?

[Van Disaster]

1 hour ago, wumpus said:

Rudder in front: wildly popular in KSP, but almost certainly unstable.  Didn't the Rutan Vari-eze do this as well?  It presumably *can* be tamed, but I doubt control theory was understood at all in 1903.

Rail required for start - how many runways were there in 1903?  I don't think a modern plane could take off anywhere in 1903

A tail at the back is as old as flying creatures, and certainly centuries of rocketry - not to mention other arenas dealing with lift and stability like sailing craft and airships - would have demonstrated natural stability. They did put the vertical tail at the back and there are very good reasons for canards, importantly that they contribute lift.

Runways - you can land a Piper Cub *anywhere*; fun couple of hours on youtube watching people do that if that sort of thing interests anyone.

[Sharpy]

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.

 

Edited August 24, 2016 by Sharpy

[Diche Bach]

2 hours ago, 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.

 

This may be a stupid question but . . . why don't rockets melt then? They have to accelerate through the atmosphere too, eh?

[Sharpy]

1) ablator, isolating materials

2) they leave atmosphere pretty fast, before they get too much horizontal speed. Amount of conductive heat transfer depends on pressure.

On reentry, there's ablator and timing - a pretty rapid descent so that heat doesn't get to penetrate deep inside. One reason why they can't gently glide to a stop.

You'd think, that similarly to KSP, you can make the spaceplane enter gently - whenever it gets too hot, nose up and rise above atmosphere. Unfortunately, by the time you get enough lift to do that, you're already heating like crazy.

[Diche Bach]

So it seems to me the solution to the space plane paradox: behave like a plane until that is no longer tenable, then behave like a rocket, i.e., go totally radial (or nearly so) and kick in the rocket engines. I suppose the limitation here in real life is: getting enough fuel / big enough rocket engine up that high?

Chemistry of explosives would thus seem to be the real limiter for space planes?

[Sharpy]

Honestly - I don't know.

[Majorjim!]

I find the opposite..

 Then again it depends on what you want to do with your rocket and how complex you want the rocket/payload/mission to be. 

 Aircraft are, for me, a total breeze. Easy as pie. A walk in the park. So to speak. There are far more complexities and skills involved in building and flying a complex rocket mission. 

[Corona688]

8 hours ago, Sharpy said:

I have a feeling you're not quite seeing the point I'm arguing.

Spaceflight is vastly harder than aviation.

It certainly is -- for an airplane.

 

 

[Sharpy]

5 minutes ago, Corona688 said:

It certainly is -- for an airplane.

Did you factor in things like

- cold welding
- thermal gradients
- ullage
- airtightness (can't be 'mostly', any leak will act as a thruster)
- material performance in vacuum
- material performance with zero UV shielding
- CME procedures
- debris
- space sickness
- radio ranges
- fuel boil-out
- fuel corrosiveness
- CO2 removal
- CO2 local concentration
- range safety
- microgravity-safe food
- docking port operation
- pressure equalization
- Blood nitrogen saturation
- microgravity toilets
- air filtering
- sound energy reflection
- hydrogen accumulation
- CMG desaturation
- cosmic radiation
- muscle atrophy
- retina deformation
- ablative sealant performance

and thousands other such problems not present in aviation?

[Diche Bach]

LOL, I think the game was designed to be a rocket game, thus the learning curve tailors to that. Having played a few other flight sims, I think the way the physics engine works for airplanes is "not quite right" . . . or at least, it makes it more difficult to get it "just right."

The gear, the balance, the angles . . . on a plane it all needs to be within a narrow margin of error for the plane to fly easily, but it can be done. The stuff some of these guys taught me in this thread helped me a crap ton. I went from a plane that was a constant struggle, to ones that fly damn well. Including a big ass liner that could make it half way around Kerbin with 5 passengers, and which was so stable, I literally just got it into the air, put it on 4x time warp and walked away. It just kept on doing its thing! (will have to post that craft file one of these days).

But lets keep in mind here: what we are doing in KSP is a lot different than any other flight sim of which I'm aware: DESIGNING planes, then flying them, versus just flying them.

Constrast this with a rocket in KSP: yeah, its difficult. But the margin of error just seems less. MOAR boosters can solve a lot of problems, though more refined techniques are generally better at it. Not so with planes: brute force generally is not an option (though the guy above mentioned setting up a plane with a TWR > 1 and yeah, that does seem to make it a lot easier doesn't it).

[Corona688]

49 minutes ago, Sharpy said:

...

and thousands other such problems not present in aviation?

Nobody said manned rockets.

If you want to stay in an airplane for days at a time, those are issues there too, anyway.

You're trying to overcomplicate what's really quite a simple question.

Edited August 24, 2016 by Corona688

[Sharpy]

1 hour ago, Corona688 said:

Nobody said manned rockets.

If you want to stay in an airplane for days at a time, those are issues there too, anyway.

You're trying to overcomplicate what's really quite a simple question.

space rockets. What I wrote about is mostly for launch and LEO. Deep space probes come with its own slew of problems.

Or do you really want to limit airplanes to RC models?