Tips for Spaceplanes with Realistic Atmospheres?

[Geschosskopf]

I'm currently playing in @GregroxMun's wonderful Alternis Kerbol system.  In the fine print, it says this mod has atmospheres based on @OhioBob's "Realistic Atmospheres".  As I understand things, this means that atmospheres fade to nothing in a smooth curve instead of the stock way of not thinning as rapidly with altitude and then being "decapitated" at some arbitrary height.  This has all sorts of fun and useful effects (not to mention realism) and I like it very much.

However, it also entails something I'm finding a bit irksome:  Jet engines conk out at rather lower altitudes than in stock, but the atmosphere is still the same height in total.  Thus, spaceplanes need a rather higher ratio of rocket fuel than in stock, more of which has to burned in air.  This is giving me fits.  While I've been able to get uselessly small spaceplanes (2 seats, no docking port, no mission equipment) to orbit at Alternis Kerbin, the rocket fuel load for anything bigger overloads the jets, so the thing finds it difficult, if not impossible, to get supersonic.  Of course, I'm no spaceplane guru.  Still, I manage to brute-force my way around in the stock system without too much trouble, so I'm not entirely unfamiliar with the concept

So, any tips?  Are there any known "magic ratios" for spaceplanes in the Alternis Kerbol and/or just Realistic Atmospheres in the stock system?

I have also considered tweaking the atmcurve and velcurve of the jet engines, which were intended for the stock atmosphere.  But I'm not sure if that's a legit answer.  I don't fully remember what those values mean but IIRC, they're based on pressure, so would be independent of the atmosphere model.  If that's the case, then I wouldn't consider changing them legit.

Thanks.

 

[Guest]

Probably not the answer you want as I haven't used that mod, but... have you attempted rocket-only spaceplanes? I'm in a "planes only [almost]" career, and used them exclusively for a lot of things before I reached the RAPIER. My mainstay for a long while was the Skipper; you can make lifters good for 40 ton payloads or so with that fairly easily. If the atmosphere thins more quickly, rocket-powered spaceplanes ought to work better than in the stock atmosphere. 

So in your shoes I'd probably start with simplifying the problem by making a rocket spaceplane and once that's done, see if I can squeeze a bit more efficiency out of it by swapping the rockets for RAPIERS and tuning the fuel load. I would expect that the launch profile for that would end up pretty similar to how it works for marginal SSTOs in stock: climb while accelerating to the highest altitude your engines can still breathe, level out to squeeze out as much speed as you can out of the jet fuel, point yourself upwards, switch mode, and go.

[OhioBob]

1 hour ago, Geschosskopf said:

In the fine print, it says this mod has atmospheres based on @OhioBob's "Realistic Atmospheres".  As I understand things, this means that atmospheres fade to nothing in a smooth curve instead of the stock way of not thinning as rapidly with altitude and then being "decapitated" at some arbitrary height.

The stock atmospheres fade to nothing as well.  The main difference between stock and Realistic Atmospheres is that the RA atmospheres use the ideal gas law.  RA takes into account the surface pressure, the gas molecular weight, the planet's gravity, and the temperature-height profile to compute how the atmospheric pressure changes with increasing height based on real life principles.
 

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However, it also entails something I'm finding a bit irksome:  Jet engines conk out at rather lower altitudes than in stock, but the atmosphere is still the same height in total.  Thus, spaceplanes need a rather higher ratio of rocket fuel than in stock, more of which has to burned in air.  This is giving me fits.

I'm really surprised to read this.  While some of the atmospheres in RA vary quite a bit from the stock versions, Kerbin's is changed very little.  I would expect the Kerbin atmospheres in stock and RA to be almost indistinguishable.  There are some differences, but not enough to produce the glaring differences that you describe.
 

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So, any tips?  Are there any known "magic ratios" for spaceplanes in the Alternis Kerbol and/or just Realistic Atmospheres in the stock system?

I'm afraid I don't have any tips.  I'm not familiar with Alternis Kerbol, so I don't know for sure what atmosphere it's using.  But if it's using the Kerbin atmosphere from RA, I'm at a loss to explain the problem you're experiencing.
 

[Geschosskopf]

2 hours ago, Brikoleur said:

Probably not the answer you want as I haven't used that mod, but... have you attempted rocket-only spaceplanes?

Well, no, I had not.  Thanks for the idea, though.  That just might do it.  See, the only reason I want a spaceplane is to have something that can land more accurately on Alternis Laythe than dropping a rocket lander through an atmosphere, and be used more than once.  This isn't about getting stuff to LKO    

 

17 minutes ago, OhioBob said:

The stock atmospheres fade to nothing as well.  The main difference between stock and Realistic Atmospheres is that the RA atmospheres use the ideal gas law.  RA takes into account the surface pressure, the gas molecular weight, the planet's gravity, and the temperature-height profile to compute how the atmospheric pressure changes with increasing height based on real life principles.

Hmmm.  I seem to recall the birth of RA coming out of a discussion of the problems of aerobraking/capturing in the traumatic period when the heat and aero models were changing every couple months shortly after 1.0.  That discussion revealed the "truncated" nature of stock atmospheres, which is what was causing everybody grief.  You went from space to incineration instantly and RA came along to fix that.  But that was long ago.  Maybe stock has since changed to have more of a curve.

But anyway, what effect does RA using the IGL and stock not actually have on the air?  Say at 15km ASL?  What would be the differences in pressure, temperature, etc?

 

17 minutes ago, OhioBob said:

I'm really surprised to read this.  While some of the atmospheres in RA vary quite a bit from the stock versions, Kerbin's is changed very little.  I would expect the Kerbin atmospheres in stock and RA to be almost indistinguishable.  There are some differences, but not enough to produce the glaring differences that you describe.

Well, maybe the Kerbin in Alternis Kerbol has some differences from stock Kerbin, and RA thus produces more noticeable differences there?  I don't know.

I find quite a few differences from stock, actually, all of which support the notion that the atmospheres I'm dealing with in the AK system are thinner at lower altitudes.  Not only to jet engines die at lower levels, but wings don't work as well as you get higher.  When entering the atmospheres, you have to get considerably lower for this to have any real effect on your trajectory, etc.

A notable difference between Alternis Kerbol's Kerbin and stock Kerbin is that in AK, Kerbin is a tide-locked moon of Jool about as far out as stock Tylo.  So you don't have the fling from the planet's rotation.  Despite this, however, rocket launch dV is about the same as in stock, which is also consistent with the air thinning out starting lower down.  But the lack of fling might also hinder spaceplanes.

Anyway, it's an interesting challenge.

[Spricigo]

4 hours ago, Brikoleur said:

Probably not the answer you want as I haven't used that mod, but... have you attempted rocket-only spaceplanes?

Another idea is to have a bit of rocket power devoted to help the jets. A short activation to break transonic and when the jets are about to flame-out can make wonders for a not so powerful spaceplane.

Also, the usual moar boosters approacch. Be it engines that you carry all the way to the orbit or just droppable panthers to help with low velocity performance. If the plane is intended for Laythe  it makes a lot more sense to rely on some unusual contraptions for the single time it will be launched from Kerbin.

[OhioBob]

22 hours ago, Geschosskopf said:

Hmmm.  I seem to recall the birth of RA coming out of a discussion of the problems of aerobraking/capturing in the traumatic period when the heat and aero models were changing every couple months shortly after 1.0.  That discussion revealed the "truncated" nature of stock atmospheres, which is what was causing everybody grief.  You went from space to incineration instantly and RA came along to fix that.  But that was long ago.  Maybe stock has since changed to have more of a curve.

There was a problem like that with the stock atmospheres at one time, but changes were made in one of the releases to help fix it.  The upper atmospheres of Jool and Eve were revised so that they began a more rapid thinning at a lower altitude.  But Kerbin's atmosphere was never really a problem.  The stock atmosphere is based on the U.S. Standard Atmosphere, so it's already quite realistic.  For RA I based the atmosphere on a set of models developed by the U.S Air Force that breaks it down by latitude and season.  But both are based on Earth, so they are obviously very similar.
 

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But anyway, what effect does RA using the IGL and stock not actually have on the air?  Say at 15km ASL?  What would be the differences in pressure, temperature, etc?

I did a plot of LOG(pressure) vs. altitude for the stock Kerbin atmosphere and the RA Kerbin atmosphere.  The lines are almost a perfect overlap.  Where there is a tiny bit of separation between the curves, it's the RA atmosphere that has the higher pressure.  But for most practical purposes, there no difference between them.

There is a bit more variation between the models in terms of temperature.  The U.S. Standard Atmosphere uses the mean northern hemisphere temperature a latitude of 45 degrees (at least that's what I recall).  It is just a single temperature-height curve, so the KSP developers had to fill in the missing data with educated guesses.  My research turned up better and more complete data to work from, so I believe my global temperature model is more realistic than stock's.  This means there's more variation in air density between stock and RA than there is in air pressure.  But again there just isn't that much of a difference between the models that there should be noticeable differences to the extent that you're describing.

As to you're specific question, the air pressure at 15 km ASL is 6722 Pa in stock, and 6818 Pa in RA.  The temperature varies depending on location and time, but the "base temperature" at 15 km is 216.65 K in stock, and 211.75 K in RA.  Base temperature is the temperature without any latitudinal, diurnal, or seasonal temperature variations applied.

Why don't you take a flight with the Aero GUI open and see what the temperature, pressure and density are actually reading?
 

Edited February 1, 2018 by OhioBob

[Guest]

12 hours ago, Geschosskopf said:

Well, no, I had not.  Thanks for the idea, though.  That just might do it.  See, the only reason I want a spaceplane is to have something that can land more accurately on Alternis Laythe than dropping a rocket lander through an atmosphere, and be used more than once.  This isn't about getting stuff to LKO  

Hey, I just did that on stock Laythe. Built a whole Kosmodrome there. Since according to @OhioBob the problem isn't with the atmosphere, it certainly ought to work. It was the most fun project I've done in KSP since my first Mun landing.

(full gallery https://imgur.com/a/4SRLj, craft https://kerbalx.com/hangars/29774)

[foamyesque]

The last time I can recall Kerbin's atmosphere ending abruptly was back when it stopped at ~35km. Now that was a sharp transition... but it went away when the atmosphere was booted up to 70km and they significantly reworked the drag model, back in 2011.

[Geschosskopf]

7 hours ago, OhioBob said:

Why don't you take a flight with the Aero GUI open and see what the temperature, pressure and density are actually reading?

Never knew it did that.  So sure, I'll give that a go.  I learned to shun the Aero GUI shortly after it 1st appeared in the game because it caused lag and crashes, so have only glanced at it briefly a few times since.

Anyway, thanks for your explanations of how RA works.  That will help when I have some data to look at.

 

18 hours ago, Spricigo said:

Another idea is to have a bit of rocket power devoted to help the jets. A short activation to break transonic and when the jets are about to flame-out can make wonders for a not so powerful spaceplane.

Also, the usual moar boosters approacch. Be it engines that you carry all the way to the orbit or just droppable panthers to help with low velocity performance. If the plane is intended for Laythe  it makes a lot more sense to rely on some unusual contraptions for the single time it will be launched from Kerbin.

Good ideas, thanks.

[OhioBob]

@Geschosskopf, I just downloaded Alternis Kerbol and looked at its atmospheres.  Alternis Kerbol is not using the "Realistic Atmospheres" mod for its atmospheres.  @GregroxMun writes in the thread that he used my atmosphere calculator to create the atmospheres, but they are his unique creation.  And from what I can tell, I don't think he changed anything regarding Kerbin's atmosphere.  Unless there is something in the configs that I missed, Kerbin should have its stock atmosphere.  You probably should report your problem in the Alternis Kerbol thread.

[Geschosskopf]

30 minutes ago, OhioBob said:

You probably should report your problem in the Alternis Kerbol thread.

Thanks for the info.

[Whirligig Girl]

I don't remember the details of my implementation of atmospheres, but I know at least Tylo uses OhioBob's calculator. I'm pretty sure the other atmospheres do as well.

Kerbin's atmosphere appears to be stock in the configs.

Edited February 1, 2018 by GregroxMun

[Geschosskopf]

8 hours ago, GregroxMun said:

Kerbin's atmosphere appears to be stock in the configs.

Hmmm.  Well, I'll have to look at this more closely then.  I'll get back to you on this in a while.....