Re-Entry heat too easy?

[Pappus]

So I built a rocket and throw a couple of jet engines on to create a kerbin ferry for those contracts that keep popping up and didn't want to waste anything so since I only have jet engines it obviously isn't a SSTO, but still I am wondering while de-orbiting this think as to how simple it is to avoid entry heating.

When I reach roughly 55km the ship will start to show those "burn" effects all around and all I need to do is nose up and my wings will de-accelerate me from ~1800ms to <1000ms in matters of seconds and obviously I won't heat up again nor did it last long ~5-6 seconds.

I feel like lift might be too effective at slowing a craft down at high altitudes or is it really meant to be this easy to avoid feary death? I play on moderate difficulty and didn't touch anything so I don't know if the re-entry heat is lower than 100%, but I suppose even 200% wouldn't matter when re-entry heating effect lasts a couple of seconds never to be seen again.

[antbin]

My feeling is heat and risk of unplanned disassembly are both too low in 1.0.2

[Snark]

Yes, re-entry heat is definitely easy-mode. I love that they added heat shields, I love that orientation matters, that the heat shields are ablative, all of that stuff... except that it's possible to re-enter without a heat shield without experiencing any ill effects, as long as you're careful about it.

In particular: airbrakes are way overpowered. Put some airbrakes on a ship and you can slam into atmosphere at several kilometers/second and it decelerates you as gently as a feather way up high in the atmosphere with no problems-- indeed, without even any graphic effects at all. Don't get me wrong, I love that they added airbrakes, and I love the way they look, and the fact that they're tied to the brakes hotkey... it's just that they're massively overpowered, they shouldn't be a free ticket to atmospheric reentry.

Also, a kerbal in EVA can reenter with no overheating problems at all. That's just wrong.

Myself, I like to play by just imagining that reentry heat is deadly, and putting heat shields on things even when I don't really need them. But I wish the game made it more of an actual challenge rather than an imagined one.

[jarmund]

My feeling is heat and risk of unplanned disassembly are both too low in 1.0.2

This. While I'm OK with it being possible to return a pod or a simple craft from LKO without a heat shield because of the simplicity and entertainment value, I find the reentry heat to loose its meaning when I can return my minimus miner to kerbin without losing anything else than a ladder to heat. No, I didn't aerobrake over multiple orbits, I did what I almost always do: Plese periapsis around 30k, burn if it looks like I'll skip through.

I played with deadly reentry pre 1.0, and from the looks of it, I'll be going back to it again unless the reentry effects will be made tougher on the crafts. I'm waiting a few patches before deciding to see where any balancing measures will converge.

[Reactordrone]

Reentry heating is appropriate for the speeds being encountered. As a test I sent a ship out to the limit of kerbin's sphere of influence and boosted back towards the planet getting an entry speed of slightly over 4500m/s and burned through 60% of the small heat shield. At Earth entry speeds of nearly 7000m/s you'd need the heat shields but kerbin's small size means you just aren't going fast enough for long enough for them to be necessary. The only way Kerbin entry would be deadly is if they made the spaceships out of inappropriately low melting point materials.

[Snark]

Reentry heating is appropriate for the speeds being encountered. ... At Earth entry speeds of nearly 7000m/s you'd need the heat shields but kerbin's small size means you just aren't going fast enough for long enough for them to be necessary. The only way Kerbin entry would be deadly is if they made the spaceships out of inappropriately low melting point materials.

From the standpoint of physics, you're entirely correct... but I think the real issue here is more one of gameplay. After all, we already know that the Kerbal universe has properties very different from our own; there's no realistic way for a planet 600km in radius to have Earth's gravity.

It's a toy solar system, and they made the right decision in doing so. KSP aims at giving us the experience of "what's it like to fly rockets," making it real enough to be challenging and to give an appreciation of what's hard, while simplifying enough to make it fun to play without requiring an advanced degree in aeronautical engineering. It's a tricky balancing act, and for the most part, Squad has done a brilliant job with this.

And part of flying rockets is that reentry is supposed to be hard. This is a game, so it's fine if it's not as hard as real life, but it should at least be hard enough that you have to care, and your ship design should have to make at least a reasonable nod in that direction.

So if we can accept planets that are 10x denser than reality, I think it's fine if we also postulate reentry heating above normal, for the sake of fun.

[n0xiety]

The bad part is that i can reenter with an ssto while going 3000m/s and show the whole body 90 degrees to prograde for 10 seconds to slow down immensly and get away with the g forces the craft endures yet if i were to go directly at prograde the craft starts overheating slowly then explodes...... If i am not loosing speed while looking at prograde that means there is not much friction to slow me down meaning there can't be that much heat anyways........... So it is better to just keep looking at prograde untill you drop under 25km altitude and then do a 90 degree body slam into the airflow which will slow you down in 5 seconds and don't explode anything....... I find this too arcade sorry...

[magnemoe]

From the standpoint of physics, you're entirely correct... but I think the real issue here is more one of gameplay. After all, we already know that the Kerbal universe has properties very different from our own; there's no realistic way for a planet 600km in radius to have Earth's gravity.

It's a toy solar system, and they made the right decision in doing so. KSP aims at giving us the experience of "what's it like to fly rockets," making it real enough to be challenging and to give an appreciation of what's hard, while simplifying enough to make it fun to play without requiring an advanced degree in aeronautical engineering. It's a tricky balancing act, and for the most part, Squad has done a brilliant job with this.

And part of flying rockets is that reentry is supposed to be hard. This is a game, so it's fine if it's not as hard as real life, but it should at least be hard enough that you have to care, and your ship design should have to make at least a reasonable nod in that direction.

So if we can accept planets that are 10x denser than reality, I think it's fine if we also postulate reentry heating above normal, for the sake of fun.

yes, reentry heat is too weak in normal orbital speeds. Problem is that making it more realistic will make it very hard for spaceplanes.

Already heating around 1000-1200 m/s limit how long you can fly at 20 km at that speed to around an minute and an plane will have to ascend in an shallow trajectory.

Now you could always use plane parts as heatshields to avoid the issue with reentry heating anyway. Even deadly reentry mod let you use engines and heatshields who is very unrealistic.

More idiotic, using radial LV-N around an core with an heatshield at bottom makes sense, however the nukes heat up the tank to 1500 degree and this melts the heatshield after landing on Mun.

[kurja]

The bad part is that i can reenter with an ssto while going 3000m/s and show the whole body 90 degrees to prograde for 10 seconds to slow down immensly and get away with the g forces the craft endures yet if i were to go directly at prograde the craft starts overheating slowly then explodes...... If i am not loosing speed while looking at prograde that means there is not much friction to slow me down meaning there can't be that much heat anyways........... So it is better to just keep looking at prograde untill you drop under 25km altitude and then do a 90 degree body slam into the airflow which will slow you down in 5 seconds and don't explode anything....... I find this too arcade sorry...

Tried that the other day when I was going to badly overshoot my re-entry / approach ksc and the Mk1 cockpit exploded :/

[georgTF]

It is too easy most of the time since there are so many unrealistic ways to manage re-entry. But it's not 100% safe either. I lost a craft returning from the Mun possibly because i set the Pe too low thinking that heat is a non-issue as well as having a heatshield. The result? A small-ish return lander burned through 200 units of ablator in seconds. It was frankly, shocking to see anything actually use ablator nevermind burn right through it! Eventually a Science Jr. overheated and exploded severing the craft in half and killing one kerbal. This was on default hard settings.

Not quite too easy, just devs erring on the side of caution possibly because of re-entry issues with 1.0 which featured a deadlier athmosphere.

[DarkGravity]

I'm old enough to remember the Apollo missions clearly. It was always very tense. Walter Cronkite saying to America: if they come in too steep, they'll burn up on re-entery, if they come in too shallow, they'll skip off the atmosphere like a rock tossed across the top of a pond, and they'll be lost in deep space forever.

It would be nice if KSP captured some of that drama for re-entry beyond the admittedly very cool fire effects. Also, I've never heard of anyone ever in game skipping off the atmosphere like a stone on a pond. Is that even a thing?

Edited May 29, 2015 by DarkGravity

[georgTF]

... I've never heard of anyone ever in game skipping off the atmosphere like a stone on a pond. Is that even a thing?

Doubt it's possible to even prevent accidentally raising Ap much less bouncing off into space. That said Earth re-entry speed is higher and the athmosphere different. IIRC it is possible to bounce off a layer of athmopshere though i fail to see how it could send a craft into deep space. Surely it would still have some kind of egg shaped orbit around Earth with a Pe low enough to be captured eventually.

That may just be the problem. If the craft were to bounce off, it might take long enough to return to Pe to starve the astronauts of oxygen or power. More worryingly, the following re-entry could certainly end up being too steep and lethal.

It was incredibly tense in no small part due to the fact it's never been done before, at least not enough to know where the limits are.

In KSP though, airbraking will allways lower Ap and the Pe will stay inside the athmosphere so that eventually the craft will re-enter once it has slowed down enough to drop Ap into the athmosphere. On Earth, possibly the gravity assist would be such that "bouncing" could raise Ap and if the craft was near Earth escape velocity it might just be possible to lose it forever.

[Snark]

...if they come in too steep, they'll burn up on re-entery, if they come in too shallow, they'll skip off the atmosphere like a rock tossed across the top of a pond, and they'll be lost in deep space forever... I've never heard of anyone ever in game skipping off the atmosphere like a stone on a pond. Is that even a thing?

Doubt it's possible to even prevent accidentally raising Ap much less bouncing off into space.... IIRC it is possible to bounce off a layer of athmopshere though i fail to see how it could send a craft into deep space. Surely it would still have some kind of egg shaped orbit around Earth with a Pe low enough to be captured eventually.

If you're on an orbit that's already captured to Earth (or Kerbin, or anywhere)-- i.e. you're not on an escape trajectory to start with-- then it's physically impossible to aerobrake into an escape path. Conservation of energy requires this. If you're in a captured orbit, it means you do not have enough energy (kinetic + potential) to escape. Period, full stop. The only way to escape would be to add energy; contact with atmosphere reduces your energy. So you can't aerobrake your way to escape. Q.E.D.

If the craft were to bounce off, it might take long enough to return to Pe to starve the astronauts of oxygen or power. More worryingly, the following re-entry could certainly end up being too steep and lethal.

I could certainly see that being a concern. Doesn't explain the "lost in space forever" comment, though.

In KSP though, airbraking will allways lower Ap and the Pe will stay inside the athmosphere so that eventually the craft will re-enter once it has slowed down enough to drop Ap into the athmosphere.

And in real life. Conservation of energy says that you can't raise apoapsis to escape; conservation of angular momentum says that you can't raise your periapsis out of the atmosphere. Cannot be done.

On Earth, possibly the gravity assist would be such that "bouncing" could raise Ap and if the craft was near Earth escape velocity it might just be possible to lose it forever.

This is physically impossible. If you're orbiting a planet, the only kind of "gravity assist" you can get from the planet itself is if you fire your engines at periapsis to take advantage of Oberth effect. You can't bounce your way to escape, any more than you can drop a rubber ball and have it bounce higher than you dropped it.

...Of course, none of this explains the "lost in space forever" concern that DarkGravity raised initially. That piqued my curiosity, so I rummaged around on the web a little. Most of what I could find (Wikipedia, NASA) is maddeningly coy about return-trip details (e.g. exact time of trans-earth burn, exact time of reentry), but I finally tracked down this:

http://history.nasa.gov/ap11fj/index.htm

Rummaging through this account, I find the following timestamps (these are hours/minutes/seconds of mission time from launch):

To the Moon:

- Translunar injection: 002:44:18

- Lunar orbit insertion: 075:49:51

Back to Earth:

- Trans-Earth injection: 135:23:44

- Reentry: 195:03:06 approx

...if I'm doing the math right, I make that a bit over 73 hours for the outbound journey, but a bit under 60 hours for the homeward leg. On the other hand, if I just plug in the numbers for how long a minimum-DV burn would take (i.e. half of an elliptical orbit, with periapsis at LEO and apoapsis at the Moon's orbit), I come up with 126 hours.

So it sounds as though they were making the transfers between Earth and Moon a lot faster than optimum dV conservation would suggest (perhaps to save consumables?). If that's the case, it may be that they were actually on an escape trajectory when they were coming home, which totally explains the "lost forever" comment: if you don't aerobrake enough, you don't kill enough velocity to capture, and you end up flying off into the solar system.

[John FX]

If you think the heating is too easy try to put a probe down on Eve with no heatshield.

Kerbin should be easy as it is the place the most inexperienced pilots will be experiencing first.

[n0xiety]

I actually experienced this skipping effect only with an ssto while returning from duna fast. I set the entry height to 35km. Normally you would expect periapsis to drop down even more than 30km when you do a move like this as you loose speed right? That didn't happen. I passed the periapsis and hit 35km mark, after that the periapsis started going up again and it was once again infront of my craft. Periapsis gone up to 60km when i finally left the atmosphere. I noticed that i lost speed according to the periapsis gain. Well i guess we could call this a strange ksp drag caused atmosphere skip.

[georgTF]

If you're on an orbit that's already captured to Earth (or Kerbin, or anywhere)-- i.e. you're not on an escape trajectory to start with-- then it's physically impossible to aerobrake into an escape path. Conservation of energy requires this. If you're in a captured orbit, it means you do not have enough energy (kinetic + potential) to escape. Period, full stop. The only way to escape would be to add energy; contact with atmosphere reduces your energy. So you can't aerobrake your way to escape. Q.E.D.

...

I could certainly see that [running out of life support power] being a concern. Doesn't explain the "lost in space forever" comment, though.

...

And in real life. Conservation of energy says that you can't raise apoapsis to escape; conservation of angular momentum says that you can't raise your periapsis out of the atmosphere. Cannot be done.

Here's why the "lost in space forever" comment still may apply:

Low Earth Orbit velocity is >9400 m/s

Earth Escape Velocity is 11200 m/s

I don't know what lunar escape velocity they used, but it seems plausible they needed to make a single burn to leave the Moon and position themselves at a correct point for Earth athmosphere capture. If at that point their velocity relative to Earth exceeded 11200 m/s then they certainly had only one chance to return or would be lost to space. Not interstellar space, but they'd end up in some interplanetary orbit.

The Kerbin - Mun - Minmus system is modelled differently and it seems to be a lot easier to transfer around. Esp. with OP fuel/engines and patched conics. Not to mention quicksaves. And still we get it wrong very often.

I'd still like to know why the return craft had to risk a such a dangerous trajectory. If that is indeed what happened.

[jandcando]

I do think that the system is way too easy. I play on 120% reentry heating difficulty, and I did an aerocapture on Eve and did not burn though very much Ablator at all.

[asmi]

If you're on an orbit that's already captured to Earth (or Kerbin, or anywhere)-- i.e. you're not on an escape trajectory to start with-- then it's physically impossible to aerobrake into an escape path. Conservation of energy requires this. If you're in a captured orbit, it means you do not have enough energy (kinetic + potential) to escape. Period, full stop. The only way to escape would be to add energy; contact with atmosphere reduces your energy. So you can't aerobrake your way to escape. Q.E.D.

It's actually more subtle. Anyone who ever played Orbiter and went to the Moon knows that significant part of journey happens when the Earth is NOT a major body that affects the motion, but the Sun is - this is "computer-melting" © (Squad) n-body physics for you, which oddly enough works OK even on stone-age computers without melting them. Second - craft returning from the Moon has velocity of over 10 km/s, and as such can produce enormous lift if it's aerodynamic enough (remember lift is proportional to velocity squared!). So the vehicle can bounce off atmosphere almost without losing any speed, and therefore its' orbit apogee will be close to the distance to the Moon. Now, if it's lucky (or unlucky - depending on whether it's intended or not) and reaches apogee when the Moon is nearby, it's gravity will pull the craft towards it raising perigee out of atmosphere (again, this is n-body physics for you), and this spells doom for the crew. If you think it's too far-fetched - look at the Spektr-R radio telescope mission, which exploits such Lunar gravity boosts intentionally to raise its' perigee over time.

So in KSP terms the Moon is outside Earth's SOI, which may sound rather bizarre until you recall that the concept of SOI is only applicable to objects with mass that is insignificant compared to the main orbiting body, and the Moon's mass is obviously not insignificant.

Edited June 6, 2015 by asmi

[MarvinKitFox]

So I built a rocket and throw a couple of jet engines on to create a kerbin ferry for those contracts that keep popping up and didn't want to waste anything so since I only have jet engines it obviously isn't a SSTO, but still I am wondering while de-orbiting this think as to how simple it is to avoid entry heating.

WOW.

What an honor to meet the person who wrote the text for KSP contracts!

[Orbital Vagabond]

So it sounds as though they were making the transfers between Earth and Moon a lot faster than optimum dV conservation would suggest (perhaps to save consumables?). If that's the case, it may be that they were actually on an escape trajectory when they were coming home, which totally explains the "lost forever" comment: if you don't aerobrake enough, you don't kill enough velocity to capture, and you end up flying off into the solar system.

I think this is the most likely explanation. The "skipping off the atmosphere" probably referred to how the capsule generated lift to modify it's trajectory and ease re-entry. In fact the pod is supposed to escape the atmosphere after it's initial re-entry, but at suborbital speeds, so that it re-enters again more slowly and at a steeper angle. If it was travelling in excess of escape velocity and didn't kill enough velocity on the first re-entry phase, then yeah, it would "skip" out into interplanetary space.

[DarkGravity]

I'd still like to know why the return craft had to risk a such a dangerous trajectory. If that is indeed what happened.

I remember it like it was yesterday. Trust me, it is not something you forget.

Start at the 2:40 time mark.

Why they had to take such a dangerous trajectory I don't know, but I do know that every single aspect of the mission was nail-biting dangerous. It was unknown if the landing site would swallow up the lander in a pocket of quicksand-like powder or if maybe the astronauts would succumb to such a fate on EVA. Only a few years before, 1 in 5 Titan rockets blew up. Astronauts and test pilots were dying with alarming frequency and only 5 years earlier some bookies were offering 1000 to 1 odds against man setting foot on the moon in the next 6 years. This was not a high-tech society, and Robbie the Robot was nearly state-of-the-art for science fiction cinema, and pop culture held that the moon was made of green cheese. This was long before Star Wars ever came out and before pocket calculators even existed. Imagine a time when a simple pocket calculator (that could only add, subtract, multiply and divide) was revolutionary -- this was before that! Slide rules were in common use and all telephones were corded, and it was a big deal to get a "long distance" telephone call (one from another area code). People talk about how 9-11 changed the world, but not nearly so much as the moon landing. As Walter Cronkite said 4 1/2 minutes before Apollo 11 touched down "4 and 1/2 minutes left in this era."

The way I look at it is that they probably choose such a dangerous trajectory because that was the safest path they could achieve given the incredibly limited technology of the time. And compared to the other risks they were taking in this mission, this risk didn't seem out of line. This was an era when all recorded music was analog, and 8-track tapes were nearly as common as 12-inch vinyl records, and senior citizens were confused by the new term "stereo." Cars did not have air bags, anti-lock brakes, fuel injection, front-wheel-drive or even seatbelts that went across the shoulder/torso. Even watches with a digital display were cutting edge exotic and hadn't yet been perfected. If you wanted to change channels (there were 4 to choose from) on the T.V., you got out of your chair and walked over to the T.V. Anyone who was alive then can tell you where they were and what they were doing at the time of the moon landing. Going to the moon and back was incredibly dangerous, and there was no small measure of luck involved. That is just the way it is when you push the envelope that hard.

Edited June 5, 2015 by DarkGravity