Squad on the recent Aero changes

[sephirotic]

Just some random information for the discussion:

Current sea-level speed record on a manned controlled airplane was set in 1976 on an F104:

Lockheed test pilot Darryl Greenamyer built a F-104 out of parts he had collected. The aircraft, N104RB, first flew in 1976. On 2 October 1976, trying to set a new low-altitude 3-km speed record, Greenamyer averaged 1,010 miles per hour (1,630 km/h) at Mud Lake near Tonopah, Nevada. A tracking camera malfunction eliminated the necessary proof for the official record. On 24 October 1977 Greenamyer flew a 3 km official FAI record flight of 988.26 miles per hour (1,590.45 km/h).[73]

The F104 is a very streamlined single-engine interceptor built in the 50s, it weights around 9 tons and it's engine has a maximum thrust of ONLY 79KN.

A quick dirty replica in KSP weighting 8 tons and with throttle limiting the engine thrust to just under 100kn got the Following absolute max speeds:

1.0.2 aero: 265m/s

1.0 aero: 316m/s

Half the 1.0 aero (0.3, 0.030 1.000) 356m/s

1600km/h, which is the speed we were aiming here is 450m/s

However, at the half 1.0 aero, a pod reentry from a 200km orbit was stil at mach 2 from only 2km to the ground.

Well, I hope they could balance this better.

[Scoundrel]

Mach 3 sustained at sea level... serious fun! Ferram is right in that it is the gas turbines that are the limiting factor: mass flow would make the turbine inlet temperatures go far beyond what any current or even near future turbine technology would allow. A variable bypass cycle engine could do it, but I shudder to think of the amount of fuel it would gobble (15:1 stoic to start plus burning rich... ugh!).

As for landing speeds, it's mostly wing planform and wing loading (it's more complicated than that, but nobody is really interested in every single factor) that determines landing speeds, not the type of engine. Modern jet engined fighters have high landing speeds because of their swept/delta wings (and compensate for it with flaps and slats), not because they lack propellers.

IMHO, increasing drag and reducing lift all across the board isn't the solution to kludging blunt body physics, nor is "fiddling with it till it feels right" because that is simply using bias to solve an engineering issue that should have been solved mathematically from the beginning. Work the system until it works, as they say.

That said, I guess it's good that they're still mulling aero over. I'd hate to have been stuck with 1.0.2 aero and heating... even if it will break my latest round of SSTOs and spaceplanes.

[uglyduckling81]

Recovering a Pod in 1.0 was impossible without a heap of chutes. 1 chute on top of a capsule was a guaranteed dead Kerbal.

I haven't played since 1.0 after testing and discovering 1.0 just wasn't ready to be played. I'm going to wait a few months and revisit. Hopefully the worst of the problems will be sorted.

I don't understand all the rage about having to redesign craft. The main aspect of this game is designing rockets and planes. People going nuts about having to redesign. If you only want to fly craft and not design them, go and download the craft that others have made and stop bitching.

[KerbMav]

The dragmodel is fine for me, but please Squad... fix the asymmetric flameouts.

It is the same for rover wheels it seems - whenever I am running out of power my rover turns right.

[sephirotic]

Recovering a Pod in 1.0 was impossible without a heap of chutes. 1 chute on top of a capsule was a guaranteed dead Kerbal.

I haven't played since 1.0 after testing and discovering 1.0 just wasn't ready to be played. I'm going to wait a few months and revisit. Hopefully the worst of the problems will be sorted.

Then you were doing something wrong, probably were using drogues or trying to chute very large vessels, because even a single Mk-16 can stop the Mk1-2 pod with the OLD 1.0 aero but with the REDUCED drag of the chutes from 1.0.2, Take a look:

Of course 18.0 m/s is not exactly a smooth splashdown but the Mk1 pod can survive that, with the XL chute it touches down at a gracefull 5m/s

[uglyduckling81]

Then you were doing something wrong, probably were using drogues or trying to chute very large vessels, because even a single Mk-16 can stop the Mk1-2 pod with the OLD 1.0 aero but with the REDUCED drag of the chutes from 1.0.2, Take a look:

http://i.imgur.com/XHWjdP2.png

Of course 18.0 m/s is not exactly a smooth splashdown but the Mk1 pod can survive that, with the XL chute it touches down at a gracefull 5m/s

I wasn't doing anything wrong. I've played this game enough (couple hundred hours) to understand how it works. The standard chute given when starting a new career in 1.0 just refused to slow the craft enough to prevent it exploding.

If you had a different experience I'm glad for you but that was my experience. After a dozen tries and a bit of experimenting I gave up and have just been stalking the forums and waiting for patches to come through.

Also I'm waiting for all the mods to updated to make the career mode harder and more interesting.

[mikegarrison]

Where there is much less heat. Heat is the limiting factor, not drag. Jet thrust is broadly proportional to dynamic pressure, so outside of heat (and excess intake drag) there isn't much difference between low and high in terms of max speeds. You need to make sure your airframe doesn't melt and your compressor doesn't melt, which means sprints speeds far higher than sustained speeds, and tighter thermal limits below 50,000ft.

I think the 1st stage turbine is actually the part that is most likely to be destroyed by the heat first. Or maybe the flameholders in the afterburners.

[Kuu Lightwing]

Wait, can somebody explain why engine would heat more in thicker atmo?

[mikegarrison]

I don't disagree in the slightest that drag is too high for planes. I do disagree that it's too high for capsules, and I challenge you to compare capsule terminal velocities in KSP 1.0.2 stock, and KSP 0.90 with FAR. At sea level, since the atmosphere up high changed.

I played around with landing capsules using the 1.0 atmosphere and the 1.02 (detuned) parachutes, and it worked just fine.

The problem that was killing everyone with 1.0 on re-entry (unless they used the magic indestructible 1.0 chutes) was the bugged heat shield physics flag, not the drag.

- - - Updated - - -

Wait, can somebody explain why engine would heat more in thicker atmo?

PV=nRT --> the higher the pressure, the higher the temperature.

[Kuu Lightwing]

That's not that simple. It's not only pressure that changes, amount of substance (n in the gas equation) would also be different at low and high altitudes.

[ShadowZone]

So... basically we have to "thank" Scott Manley for the atmo changes?

*cracksknuckles*

/jk

[mikegarrison]

That's not that simple. It's not only pressure that changes, amount of substance (n in the gas equation) would also be different at low and high altitudes.

No, it's not that simple, for a lot of reasons, but that's the basic issue.

[Kuu Lightwing]

I'm not convinced. Engine compressors have something that's called "pressure ratio" which shows how much it compresses the gas and I'm not sure if it changes with altitude all that much. So, the exact dependance on external pressure is not that clear from the gas equation.

[Scoundrel]

You are correct Kuu, it isn't the compressors (though it can and has been an issue) but in modern engines it is the turbine inlet temperature which limits gas turbines. This is because the turbine inlet temperature increases in direct relation to mass flow.

[eddiew]

The dragmodel is fine for me, but please Squad... fix the asymmetric flameouts.

I thought they had? I've had very few of these, and the solution tends to be to remove and refit all your engines and intakes

One RAM or shock cone intake seems to be enough to run an engine up to its flight ceiling. Having less than 1:1, or trying to fly level at the peak of altitude, might cause the engine to suffocate, but by and large I find they shut down before they run out of air.

[mikegarrison]

I'm not convinced. Engine compressors have something that's called "pressure ratio" which shows how much it compresses the gas and I'm not sure if it changes with altitude all that much. So, the exact dependance on external pressure is not that clear from the gas equation.

Yeah, but that's the problem. The OPR of something like a 777 is around 40:1. That's why you can't run them at full power for very long, because the heating from that kind of pressure ratio is intense.

I haven't worked with military engines in a long time, but I think their OPRs are lower. However, they can have a lot more ram pressure, which somewhat offsets that.

http://en.wikipedia.org/wiki/Overall_pressure_ratio

Edited May 8, 2015 by mikegarrison

[uglyduckling81]

Wait, can somebody explain why engine would heat more in thicker atmo?

The way I understand it is it's similar to the way an airconditioner works.

There are X molecules of heat in a given amount of air. If you increase the pressure your putting more air/heat molecules into the same amount of space.

For instance if 1 cubic metre of air has 1 heat molecule under regular conditions asl. Compress that same air to double it's pressure so now you have 2 molecules of heat in 1 cubic metre of air meaning its twice as hot.

Now if you think of that as the atmosphere, the higher you go the less pressure and the less density so the less heat in the same space than asl.

Thicker air = more heat for the same amount of space.

The air in front of a fast moving vehicle (aircraft) has to compress and move out of the way. The faster you go the more than air has to compress in front of the vehicle meaning the vehicles is exposed to more heat.

Thats my understanding of how it works anyway in laymans terms.

[NathanKell]

The F104 is a very streamlined single-engine interceptor built in the 50s, it weights around 9 tons and it's engine has a maximum thrust of ONLY 79KN.

Not to pick on you in particular since I've seen a lot of people do it, but...once again, static thrust isn't maximum thrust. The number you're quoting is the static thrust: it's the thrust at sea level on a standard day. It is most certainly not the thrust at Mach 1 at sea level, nor is it the thrust at Mach 2 a bunch of kilometers up.

[basic.syntax]

The way I understand it is it's similar to the way an airconditioner works.

There are X molecules of heat in a given amount of air. If you increase the pressure your putting more air/heat molecules into the same amount of space.

The way you write it makes the point clear, I just wanted to nitpick (I have no good knowledge of Aero stuff to say much else in this thread) or clarify that atomic & molecular motion is, or what creates a heating effect. So I think it's enough to say that thicker, denser air has more atoms and molecules bumping against each other and turbine parts, causing a greater heating effect, than lower density air.

[_Aramchek_]

Not to pick on you in particular since I've seen a lot of people do it, but...once again, static thrust isn't maximum thrust. The number you're quoting is the static thrust: it's the thrust at sea level on a standard day. It is most certainly not the thrust at Mach 1 at sea level, nor is it the thrust at Mach 2 a bunch of kilometers up.

Actually according to wikipedia the engines max thrust is only 59kn without ab, and 79 kn with afterburner.

[LostOblivion]

What I don't understand is why, with jet engines on a conventional aircraft, I am given incentive to fly low and fast instead of high and slow. At normal airliners' cruise altitude around 11000 m I only have a surface speed of around 230 tops, while close to the ground I easily get just short of mach 1.

[m4v]

I played around with landing capsules using the 1.0 atmosphere and the 1.02 (detuned) parachutes, and it worked just fine.

The problem that was killing everyone with 1.0 on re-entry (unless they used the magic indestructible 1.0 chutes) was the bugged heat shield physics flag, not the drag.

yeah right, everyone is so picky about the exact m/s of a plane at sea level yet the supersonic terminal velocities of re-entering pods is just fine. There's no perfect setting that will fix planes and pods, is time to go with a compromise and move on.

[Temeter]

What I don't understand is why, with jet engines on a conventional aircraft, I am given incentive to fly low and fast instead of high and slow. At normal airliners' cruise altitude around 11000 m I only have a surface speed of around 230 tops, while close to the ground I easily get just short of mach 1.

Did you try 7.5 km? Kerbins atmosphere is a 66% model of the real one. At least that's what it was initially.

[mikegarrison]

The way you write it makes the point clear, I just wanted to nitpick (I have no good knowledge of Aero stuff to say much else in this thread) or clarify that atomic & molecular motion is, or what creates a heating effect. So I think it's enough to say that thicker, denser air has more atoms and molecules bumping against each other and turbine parts, causing a greater heating effect, than lower density air.

This might be a good time to explain that there are two different models for how gases work. Gases are actually little individual molecules. This is true. And when the molecules are spread out far enough, we think of gases in that way. Instead of density and pressure, we talk about mean free path.

When a spacecraft is in the upper atmosphere, it's generally more appropriate to talk about mean free path. Essentially, you think of the atmosphere as a bunch of little particles bouncing off the ship.

But lower in the atmosphere, like where you fly planes, we don't really think of the atmosphere as being a bunch of little particles. We think of it as being a fluid.

So yeah, while what you say is true about denser air having more molecules that bounce off each other and also the airplane, aero engineers don't think of it that way. We think of it as a mathematical construct: a potential field with emergent properties like density, pressure, viscosity, vorticity, etc.

[Elthy]

There was a Squadcast? It wasnt friday yesterday... Is there a summary somewhere?