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1.0.4 suborbital flights: too slow to land safely ?


Gaarst
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While messing around in a new career mode in 1.0.4, I had some contract asking for suborbital flights, and also orbits around Kerbin. After having accomplished a few, I came to realise something rather counter-intuitive concerning aerodynamics and reentry:

For the same reentering capsule, an reentry from LKO will be slowed down enough to deploy chutes and land safely; while for some obscure reason, reentry from a suborbital flight (~75km apoapsis), even though going slower initially, will lose less speed and end up too fast too low, leading to unavoidable crash, even when using chutes as soon as possible.

I haven't conducted methodical tests with different flight profiles to determine the exact circumstances of this event, but I have had a few suborbital flights that ended up crashing, even in the sea, at alt 0, due to being too fast too low.

I thought a some time about it, and have come up with two possibilities explaining this:

1- (the most plausible to me) Vertical velocity issues:

The craft does not slow down much as it is slower when coming back from suborbital flight, but the reentry being steeper, the vertical velocity ends up being higher than for an orbital reentry. This would explain this issue, and the fact that going "faster" (with higher horizontal velocity) in a suborbital flight prevents it. The thing is, when getting down to under 10km after reentry, whether you come back from LKO or suborbital flight, your ship has basically the same trajectory, almost vertical, so there would be no reason why speeds are different between the two.

2- KSP aerodynamics being inaccurate:

The title speaks for itself: for some reason, aerodynamics is inaccurate and creates this issue inevitably. I know that KSP stock aerodynamics is not an exact model of the Earth's atmosphere, but even when considering approximations, the drag induced by the reentry should make the two profiles end up the same way. What could also cause this, is the fact that drag in 1.0.4 was reduced, and for some aerodynamic reason, this creates the issue discussed.

I made this thread to see if anyone else has encountered this, or would like to discuss it, or even come up with a solution to it. :wink:

(I may add screenshots if anyone asks for some.)

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The answer is #1.

Straight up and back down again is dangerous in 1.04. The velocity vector is entirely in the vertical direction, so drag does not have much opportunity to act on the ship before it gets too close to the ground.

However, a suborbital flight that has a substantial horizontal speed is pretty safe.

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Yep, it's answer one. Compare your vertical velocity at interface in an orbital reentry and a suborbital reentry.

Probably more illuminating to compare the vertical velocity at, say 10,000 meters. At that point you have a lot of drag acting on the ship, but if you are screaming straight down at 2 kps you have only 5 seconds or so before impact. If you are dropping at 100 mps while screaming along at 2 kps sideways, you have a LOT of air to go through before you hit the ground, and all that air will be trying to slow you down.

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but that's what people complained about in earlier days.

for example: land an asteroid on kerbin. pre 1.0, that was quite easy. fly to an asteroid, rendevouz with it, bring it on a trajectory that intersects kerbin (not only the SOI, but the ground). and then ride a small craft with the asteroid through kerbins atmosphere. even if the asteroid hits straight downwards (no lateral velocity), the asteroid would just decelerate from >3km/s to 100m/s. and the asteroids speed is lower than its maximum impact velocity, resulting in an asteroid which hits the ground like a ball.

by the way, such a suborbital flight is no problem on earth. for example space ship 2. it launches on white knight 2, goes up into a suborbital flight with an apoapsis of >100km, and falls straight back to earth. and that thing has virtually no heatshields.

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by the way, such a suborbital flight is no problem on earth. for example space ship 2. it launches on white knight 2, goes up into a suborbital flight with an apoapsis of >100km, and falls straight back to earth. and that thing has virtually no heatshields.

That thing has wings. It doesn't fall straight down, it glides down.

That's the usual technique used in SSTO. As your speed increases, and you drop into thicker atmosphere, so does your lift. Then you lift your nose, and glide up, until the atmosphere gets too thin and your speed too low to support you. Then you fall into thicker layers, gaining speed (and heat), and glide up again. Instead of trajectory consisting of a dead drop from 100km down, you have the trajectory flattened to about what an orbital reentry would give you, except using wings instead of orbital centrifugal force to stay "afloat".

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I was surprised to see this happen to Kurtjmac.

What do you mean my a Mercury-Redstone type flight? You have to travel in a parabolic arc, you can't just go straight up and down. Mercury didn't do that. You can also use engines by the way, parachutes are not the only answer.

Mr3-flight-timeline.png

Edited by Alshain
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I was surprised to see this happen to Kurtjmac.

The reason why it works in RL but not in KSP is twofold:

1.) moar atmosphere! Sure, going into orbit around Earth takes massive amounts of speed more than it does on Kerbin, but a suborbital launch goes to roughly the same heights and has roughly the same speed coming back down. But Earth's atmosphere extends much higher, so the craft falling back down is slowed down sooner, especially if you give it a little bit of horizontal velocity as well (like Mercury-Redstone did). But, while that helps, the main reason is:

2.) moar chutes! Capsules returning to Earth have the option of using drogue chutes, which can be opened higher up and at higher speeds than regular parachutes. Even though they don't offer enough drag to allow a safe landing, they still offer more than enough drag to lower the terminal velocity by a huge amount and slow the craft down to safe main chute levels long before approaching the surface. In KSP, we have drogue chutes too... late in the tech tree, for 2.5m craft sizes. Except, nobody does suborbital flights with 2.5m craft during late career mode. We do them with 1.25m craft during the very first flights! And there are no drogue chutes available to fill that niche.

Both issues can be addressed by installing mods, but for the stock game, the situation is indeed less than ideal.

You have to travel in a parabolic arc, you can't just go straight up and down.

Of course you can. Blue Origin is doing it, and the capsule does return safely (they have already done full-scale test flights).

You just need a good way to slow down, like drogue chutes.

Edited by Streetwind
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by the way, such a suborbital flight is no problem on earth. for example space ship 2. it launches on white knight 2, goes up into a suborbital flight with an apoapsis of >100km, and falls straight back to earth. and that thing has virtually no heatshields.

100km is not THAT high considering the size and sphere of influence of earth. Travel 10% of the distance to the moon, let yourself fall back to earth, and then we'll talk about heat shielding again.

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even if the asteroid hits straight downwards (no lateral velocity), the asteroid would just decelerate from >3km/s to 100m/s. and the asteroids speed is lower than its maximum impact velocity, resulting in an asteroid which hits the ground like a ball.

Well, this rapid decceleration is completely stupid and was only possible due to the wrong aerodynamic calculations before 1.0. KSP approximated the shape of an object by it's mass. That resulted in every object having the same terminal velocity. Ridiculous.

What do you expect would happen if a giant asteroid hits earth head on?

such a suborbital flight is no problem on earth. for example space ship 2. it launches on white knight 2, goes up into a suborbital flight with an apoapsis of >100km, and falls straight back to earth. and that thing has virtually no heatshields.

You are comparing apples to oranges in many ways here.

1.) The problem is not about heating, it is about braking.

2.) Space ship two and and White Knight 2 are planes, using lift to get into shallow trajectories.

3.) Suborbital trajectories are totally possible in KSP, just not straight up and down. (depending on the aeodynamic qualities of the vehicle)

4.) Kerbin is very different from Earth, as it is way smaller, while the atmospheric hight is similar. Orbital speed is way higher in LEO (7.8km/s) than in KEO (2.4km/s). On earth that means that you need a heatshield to bleed of that horizontal velocity. In KSP, using realistic aerodynamics, there would be no dangerous heating at all, due to the lower speeds. Ontop of that the path through theatmosphere is quite long, so there is plenty of time to slow down on a shallow trajectory. Squad had to increase heating to get some heat going on.

a suborbital launch goes to roughly the same heights and has roughly the same speed coming back down.

no.

Edited by Chaos_Klaus
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Another thing that caught me is following the recent update, my sub-orbital joy-ride which was perfectly safe now turned Kerbal tourists into meat-paste because the Mk16 parachute, which now needed 700m to finishing deploying was still deploying at 500m since my ship had been designed before the update. This wasn't generally an issue with the orbital craft, as that had been repeatedly tweaked and had the 'new' parachutes.

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Probably because the descent profile is too steep for the mass/cross-section.

This. A long, thin ship will not slow down enough to be salvageable by regular chutes. Use drogues, airbrakes, or just detach everything except the pod.

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The real elephant in the room that nobody's mentioned yet is that the Mk1 pod has the same mass as Mercury but one quarter the drag (it's 1.25m in diameter, Mercury was roughly 2m in diameter). So of course it's going to have more trouble slowing down.

This holds equally for the Mk1-2 pod (kitted out it's the same mass as Apollo but 1/4 the drag).

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Funny thing: I did all my early suborbital tourist contracts with shuttles. I built a small shuttle with a Mk1 plane cockpit in the front and a MK1 command pod in the back. It's launched like a mini Buran straight up, using 6 "thud" engines in the main stage (these have a nice gimbal range), 2 in the plane and 2 SRBs. It can reach orbit, but I just use it for the suborbital tourist contracts, which are faster to complete.

Turns out falling back into the atmosphere with a plane after flying straight up until around 80km isn't that dangerous. The plane decelerates fine, barely heats up at all, and glides safely back to the runway. It lands a bit fast, but a parachute helps me brake before running out of landing strip.

Sounds like shuttle is a better option for suborbital than a capsule in 1.04...

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I made this thread to see if anyone else has encountered this, or would like to discuss it, or even come up with a solution to it. :wink:(I may add screenshots if anyone asks for some.)
In short see answer 1.

However if it worth doing. It KSP it worth overdoing. Stock 1.0.4 radiators make good early tech air breaks. Pictures here http://forum.kerbalspaceprogram.com/threads/126293-Career-Mode-Start-Impossible-as-of-1-0-3-drag?p=2037813#post2037813

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What do you expect would happen if a giant asteroid hits earth head on?

a large enough asteroid (diameter greater than about 2Km) will be moving so fast it'll go through the atmosphere without hardly slowing down. Then again I'm not sure it said asteroid was placed in "rest" above the atmosphere

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An easy rule of thumb for cooling down your steep-decent craft; is to attempt to "aerodynamically stall" it.

Aim at least 10 degrees above prograde, to turn some of your velocity into lift. You'll make your decent path shallower to compensate. Sacrificing airspeed for altitude is a win-win for reentries.

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