Launch accuracy of KSP?

[Zatie12]

I just watched the livestream of tonight's Ariane V launch, delivering the ATV to the ISS.

On the livestream (assuming I remember this correctly) they reported the vehicle reached 65km altitude in about 2 minutes after launch. After 3 min and 20 seconds (approx), the vehicle was already at 101km altitude.

I did a little test in KSP, with an Ariane V mockup (KW Rocketry) if I launch vertically with full thrust, I can only reach 20km altitude after 2 minutes. If I used the stock parts with the mainsail engine, I think the numbers would be roughly the same - KW Rocketry is fairly similar in performance?

Do we have some inaccuracies? or am I making obvious mistakes?

Thanks

[cesarcurado]

Maybe it's because the Ariane V launch was on Earth and in KSP launchs are in kerbin.

Maybe also because it's a game and like how it is currently it would require more patience.

[ManaaniWanderer]

KSP is at about 1/10 scale for astronomical disances, and our atmosphere ends at 70km where Earths is higher(100km?)

[Mode7]

Everything in KSP is scaled down a bit. Kerbin's atmosphere is much smaller than Earths as well so the numbers might line up pretty well if that were taken into account.

Also, Kerbin isn't Earth. Everything is a little different in KSP so it is really apples to oranges.

[Zatie12]

I was only wondering.. and yes I can't argue against the fact this is Kerbin rules and not Earth rules.

[duncan1297]

another thing is kerbins magic atmosphere which makes terminal velocity about 100 m/s.

[Stealth2668]

It's Earth's physics that are wrong not KSP's

[Rickenbacker]

Stealth, I smirked at that one .

OP, you have to remember that everything in KSP is scaled down, but the atmosphere is MUCH thicker. The atmosphere at sea level would probably feel more like water on Earth...

[CalculusWarrior]

And the water on Kerbin is more like gelatin; I remember seeing a calculation for its density and the person doing it got some crazy big number.

[OrbitusII]

The "real-life-accurate" rockets in KSP are actually scaled down in power as well as size. If you had a full-size, full-power Ariane V in KSP, getting to orbit would be trivial (you could possibly even use it to go interplanetary!).

[Uberick]

Yea, the numbers don't line up,but the mechanics are the same. If you go play orbiter you will notice the difference between Earth and Kerbin. Earth is bigger, launches take more time, all the planets are farther away. I assume they've shrunk KSP to make it easier to navigate(driving a rover around Duna takes a fraction of the time it would take to drive that same rover around Mars), and I think that's the right decision.

[roosterr]

Since you are comparing real launches to KSP, I noticed the recent Falcon 9 launches to the IIS start their gravity turn in less than the first minute after launch. I try to mock this with stock orbital vehicles and it is pretty tough to make it work smoothly. The old Mechjeb set its default gravity turn at about 10k and that is a comfortable zone for most big KSP rockets. BUT now the new Mechjeb default gravity turn starts at 5K. I actually like this better, although it is more challenging, I think it may be truer to life. All you gotta do then, is make sure your launch packs a mean punch, then scale it back as you gain altitude, but make sure you are hauling a$$ when you make that turn. Fun!

[Schweehog]

Perhaps you might want to try with this: http://forum.kerbalspaceprogram.com/showthread.php/12106-0-20-Lionhead-Aerospace-Inc-ATV-Ariane-5-released-now-%21%21

[rdfox]

Personally, I have MechJeb start all gravity turns at 1km (unless I'm launching from a planetary body with higher mountains nearby). Not just for realism (honestly, real-world rockets tend to start their roll program at tower clear, and their gravity turn as soon as the roll program's complete), but because I've never liked the idea of having my cast-off boosters bombard the KSC area; by starting the turn at 1km, I'm safely out over the ocean before anything gets jettisoned.

[Firov]

The reason that it's recommended to start your gravity turn so late in stock KSP is because the stock aerodynamic system is hopelessly innacurate. It's a very simplified system where drag, for whatever reason, is related to the object's mass, and is the same no matter it's orientation or position. For example, in stock KSP adding a nose cone to a fuel tank not only won't reduce reduce drag, it will actually increase it, since the nose cone's drag is added to the fuel tank's drag.

All of this means that the terminal velocity in Kernin's atmosphere is extremely low, and as such it's more efficient to gain altitude before starting your gravity turn so you can minimize losses to drag.

If you're looking for something more realistic, I'd suggest you look into the FAR mod. It adds a much more realistic aerodynamic system, where things like nose cone's and payload fairings actually make a difference. Furthermore, drag is no longer related to an object's mass, but rather it's actual shape and size.

FAR is most helpful for aircraft, but it has a big impact on rockets too. For example, with FAR, it's generally recommended to begin your gravity turn as soon as you clear the docking clamps. It also encourages you to design aerodynamic rockets, and to put your payloads in aerodynamic fairings (KW Rocketry is great for this). If you do, you'll find it requires less fuel to get into orbit than stock KSP. Of course, on the other hand, if you try to fly some of the unaerodynamic monstroties you see in this forum with FAR, you'll be punished severely.

Edited June 6, 2013 by Firov

[RoboRay]

Personally, I have MechJeb start all gravity turns at 1km (unless I'm launching from a planetary body with higher mountains nearby). Not just for realism (honestly, real-world rockets tend to start their roll program at tower clear, and their gravity turn as soon as the roll program's complete), but because I've never liked the idea of having my cast-off boosters bombard the KSC area; by starting the turn at 1km, I'm safely out over the ocean before anything gets jettisoned.

It's nice to see I'm not the only one with range-safety concerns.

I do the same thing, executing a slight pitch-maneuver of about 5 degrees after rolling to align the NavBall vertical and lateral axis with my view orientation just after coming off the pad.

Of course, what we are doing in KSP is not an actual gravity turn. I wish MechJeb hadn't used that as a generic term, because it's led to a lot of misuse. I only recall one person attempting an actual gravity turn (where gravity provides the pitch-change for the turn, not player control inputs) in KSP.

Edited June 6, 2013 by RoboRay

[Fel]

I just watched the livestream of tonight's Ariane V launch, delivering the ATV to the ISS.

On the livestream (assuming I remember this correctly) they reported the vehicle reached 65km altitude in about 2 minutes after launch. After 3 min and 20 seconds (approx), the vehicle was already at 101km altitude.

I did a little test in KSP, with an Ariane V mockup (KW Rocketry) if I launch vertically with full thrust, I can only reach 20km altitude after 2 minutes. If I used the stock parts with the mainsail engine, I think the numbers would be roughly the same - KW Rocketry is fairly similar in performance?

Do we have some inaccuracies? or am I making obvious mistakes?

Thanks

As others have noted, Kerbin is 10.6x smaller than earth,

That means you reached an altitude equivalent of 212km in 2 min, hence were going 3.26x faster.

KSP "ROCKET COMPONENTS" are NOT SCALED TO EARTH. They have some similarities, but are really quite powerful (To keep launchtimes low, that is.)

KSP is at about 1/10 scale for astronomical disances, and our atmosphere ends at 70km where Earths is higher(100km?)

HarvesteR said that the atmosphere as well is 1/10th scale so... ~700km?

[Savage117]

@Fel and others,

I give you the Karman Line, this will help clear up some questions about the extent of Earths atmosphere.

http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line

I am very aware of the similarity of this guys last name to Kerman, I wonder if Squad was aware of it? I have now named one of my kerbals Theodore von Kerman as a result of this wiki.

[Fel]

neverminds...

You're still talking about a HEAVILY disputed topic. Wikipedia is not being fair with trying to consolidate the whole argument down like that.

*And it really isn't the edge of the atmosphere... just a proposed "Edge of Space", different concepts.

Edited June 6, 2013 by Fel

[Savage117]

Ok,

Its not much of an argument if you ask me, its pretty apparent that the atmosphere above 100km on Earth is so thin that an object no matter how unaerodynamic it may be can stay up their for a very long time. That is pretty clear cut, it may not be in "Space" but it is in a stable orbit, which is what we drive for in KSP. I was just pointing out the accepted Earthly altitude that scientists consider "safe" orbital altitudes since it seemed some forum goers were not aware of the Earthly orbital altitudes. So, in conclusion its aproximatley 100km for Earth and 70km for Kerbin.

[purpletarget]

I'd be leery of calling anything at 100km being in a stable orbit for earth, as the decay times for anything that low, probably will be inside the orbital period time for that distance. ie, it'll crash before it completes an orbit. The Karman line is an interesting notion, but I think is more directed to issues of administration of aerospace for the purposes of traffic controls, treatese and international legal issues type stuff.

In terms of objects in orbit, the decay times are well within a few days under a couple hundred km. Unlike KSP atmos with a hard cutoff, now you're in it, now you're not, Earth's extends out quite a ways, such that most objects in LEO still have to deal with some aspect of drag compensation as part of their mission plan. And it also expands and contracts on occasion, so it's not like it's entirely predictable either.

So while KSP has been scaled down to around 1/11th or so of IRL, not everything scales properly, and Kerbin's atmospheric thickness is one of those cases. It's far thicker wrt the plantary radius than in the case of Earth, which is why we can't get away with highly eliptical orbits like Molniya's, because the Pe would have to be deep inside Kerbin's atmosphere to get the same kind of eccentricity.

So I'd mostly suggest not worrying too much when launches have different timings than IRL. Launching will take as long as it takes to get from your origin to your destination given the terrain and vehicle involved...and time is one of those things that doesn't scale very well.

(Also wrt to the OP questions about the IRL rockets...the aerodynamic model is a big factor in this as well...between streamlining to allow less drag, and mass to allow inertia to bulldoze it's way through atmo, I think the IRL rockets are able to manage much higher comparable speeds than the current KSP model allows)

[rodion_herrera]

Do we have some inaccuracies? or am I making obvious mistakes?

That line of questioning would best suit Orbiter ( http://orbit.medphys.ucl.ac.uk/ ) better, because it simulates launches from the REAL earth. And there are indeed downloadable Ariane V's for Orbiter too...

[khyron42]

From wikipedia:

Given the rapid orbital decay of objects below approximately 200 kilometers (120 mi), the commonly accepted definition for LEO is between 160 kilometers (99 mi) (with a period of about 88 minutes) and 2,000 kilometers (1,200 mi) (with a period of about 127 minutes) above the Earth's surface.

Based on that alone, we could put LKO of 70 km = Earth LEO of 200 km, since orbits below there on Kerbin also experience "rapid orbital decay".

[SkyRender]

Real space is much harder to define in real life. The atmosphere of Earth gets practically nonexistent by terrestrial standards by 100KM (the Karman Line and "official" boundary of space), but as far out as 400KM we have detected trace quantities of atmosphere still present. It's less a question of where space starts and more a question of how far up you're willing to go before you decide you're in space.

[Savage117]

Thats what I was trying to say, the Karman Line is like the 70Km mark in KSP, only KSP makes it a definitive cutoff intead of a gradual lessening of atmosphere like it is IRL.

I think it would be neat to see that kind of realism in KSP but if they were gonna go that far they should go all the way and do Lagrange points and other orbital anomalies, maybe even micro debris. Maybe make it a hardcore mode or something. But im just brainstorming and they have already stated they very likely wont go that far.

I apologise for not being clear earlier.