I wonder why Gs are higher during reentry than launch

[Pawelk198604]

For example once when i did suborbital flight in ksp, i get 4 G during launch and 11.5 during reentry?

[Kulebron]

You pull high Gs because of ballistic reentry, basically the stronger braking speed is, the steeper your ship dives into the atmosphere, and this in turn increases the braking force even more. On takeoff you avoid this (either high drag or high Gs) because you usually don't have enough thrust to do this. It's really hard to make a rocket this powerful and still keep it practical.

In real life, G loads top at 4-5 just before staging, and reentry is not ballistic, the capsules glide a bit to have a moderate acceleration all the way down.

Last ballistic reentry I can remember: https://en.wikipedia.org/wiki/Soyuz_TMA-11

Saturn V ascent profile:

Also, Soyuz 18A had an unusual abort: the 3rd stage failed, when the ship was very high above the atmosphere at almost orbital speed. When reentry started, the orientation radar couldn't lock correctly in those conditions, and turned the capsule to glide down rather than up, increasing G loads higher than those of ballistic reentry, up to 21.3 g, according to Wikipedia.

Captions: (left) Lift force prolongs breaking, lowering the G loads. (right) Lift force increases braking, raising G loads.

Edited October 28, 2014 by Kulebron

[Brotoro]

For example once when i did suborbital flight in ksp, i get 4 G during launch and 11.5 during reentry?

Because you took longer to speed up with the rocket than it took the atmosphere to slow you down when you hit it on the way back.

[Starman4308]

For example once when i did suborbital flight in ksp, i get 4 G during launch and 11.5 during reentry?

During ascent, you have control over your acceleration, and that ascent is usually not too fast because of issues which crop up when you ascend too quickly. During reentry, you're quite ballistic: if you've got FAR installed, you can control your reentry a little bit with a lifting reentry, but you're still mostly ballistic.

In stock KSP, you're ascending at ~2 G to avoid exceeding terminal velocity (going faster makes you waste fuel fighting atmosphere). In the real world, or when playing with FAR/NEAR, you generally start your ascent at 1.2-1.6 Gs of acceleration because, were it any higher, you would risk aerodynamic failure. In either, during re-entry, you're on a ballistic course for lower atmosphere, and no matter what tricks you play to bleed off as much velocity as possible in the upper atmosphere, sooner or later you will slam into lower atmosphere with a lot of velocity left, where drag will hit very, very hard.

[Kulebron]

There's one case when you can have about 3g quite evenly, even with current aerodynamics: on a return to Kerbin from Mün or other planets, you can plough down to 29-30Km, still high, but get moderate Gs, then flare a bit to 35-50Km flying at about 1700 m/s, then descend again, hitting about 3g again, not more. In this case, the very eccentric orbit lifts you instead of a lifting body.

[magnemoe]

There's one case when you can have about 3g quite evenly, even with current aerodynamics: on a return to Kerbin from Mün or other planets, you can plough down to 29-30Km, still high, but get moderate Gs, then flare a bit to 35-50Km flying at about 1700 m/s, then descend again, hitting about 3g again, not more. In this case, the very eccentric orbit lifts you instead of a lifting body.

Yes you aerobrake into an ballistic trajectory, so you get 500 m/s less to brake.

Spaceplanes also get far more gentle braking as they generate lift

[K^2]

In stock KSP, you're ascending at ~2 G to avoid exceeding terminal velocity (going faster makes you waste fuel fighting atmosphere).

This is only strictly true during the vertical stage of the ascent. Once you begin gravity turn, you briefly need a bit higher TWR. But that tends to work out, since your rocket is getting lighter.

[rdfox]

For the record, in the real world, the acceleration loading on launch depends on the exact vehicle, while the G-loading on entry depends on the spacecraft AND the trajectory.

For example, the Redstone, Atlas, and Titan boosters used in the Mercury and Gemini programs were converted ICBMs, designed to launch (relatively) tough nuclear warheads that didn't care much about high G-loadings. As a result, they made no effort to limit TWR in the later stages of ascent--the closest they came was the Atlas's "stage-and-a-half" staging where the two outboard booster engines (which made about 3/4 of the total launch thrust) were jettisoned two minutes into flight, but that was less for TWR control and more for a more efficient use of propellants. All three of them saw G-loadings exceeding 7g during the ascent; Titan II-Gemini actually did this twice, once near the end of the first stage burn, and once near the end of the second stage burn. I don't know much about the Saturn IB used for Apollo 7 and the Skylab and ASTP flights, at least in ascent G loads, but when it came to the Saturn V, as Mike Collins put it, "Saturn is a gentleman," with maximum G loadings of about 4.5g during ascent, thanks to the planned early shutdown of the center engine in the first and second stages--which was done solely for TWR control without the added complication of a throttle. Shuttle maintained a 3g maximum through throttles on the SSMEs and careful shaping of the SRB propellant castings to control thrust levels.

As for entry G loadings, KSP re-entries are currently ballistic, as the aerodynamic model doesn't simulate spacecraft body lift, only wing lift. In reality, the only manned spacecraft designed for pure ballistic re-entries were the Mercury and Vostok spacecraft. While I don't have any access to the Vostok's entry G-loading data, I do know that Mercury's G-loading soared on entry, particularly on the two suborbital flights (which, by nature, had a steeper entry angle than the orbital flights). If you listen to the cockpit tapes of Al Shepard's Mercury flight, you'll hear him reading out G-loadings during the entry. "Three... five... seven... *grunt* nine... ELEVEN... I'm OK... OK... OK..." After the parachute had deployed, he reported to the ground a peak G-loading of about 11.5g during the entry.

All other capsules, as mentioned above, had/have their center of mass *not* along the vehicle centerline, so that their aerodynamically neutrally stable attitude is tilted slightly, to generate some lift and reduce the entry G-loading. For return from low Earth orbit on a normal lifting entry trajectory, there's a handy rule of thumb--the peak entry G-loading is equal to the reciprocal of the spacecraft's lift-to-drag ratio. For example, the Gemini capsule had a lift-to-drag ratio of 1:4, which meant that the peak entry G-loading in it was about 4g. (I'm not sure this held true for the Space Shuttle, but it flew an extremely shallow entry profile designed to reduce peak thermal loading at the cost of a much longer thermal soak, to suit the reusable thermal protection system employed; the ablative TPS used on capsule-type spacecraft can handle a much higher peak thermal load, but is more limited in the length of the acceptable exposure.)

However, attempting to use a "normal" shallow entry profile on Apollo wasn't an option, except on 7, 9, and the Skylab and ASTP missions. Coming back from the Moon, there was no braking burn into Earth orbit before entry; they flew a direct entry trajectory that saw them plummet straight into the atmosphere to land. Using that shallow of a profile would have resulted in the spacecraft "skipping" off the atmosphere and off into a high Earth orbit that would not return before the command module's consumables were used up and the crew was dead (some "skip" profiles would even see them end up in an orbit that would never re-enter the atmosphere!). As a result, a much steeper profile had to be used, and the L:D rule of thumb didn't work any more. A normal Apollo lunar re-entry would hit about 7.5g, and if the trajectory was a bit off but still within survivable limits, that could soar as high as 14g. (As a result, all Apollo Command Module Pilots had to be trained to fly the entire entry--solo, in case the Lunar Module crashed or couldn't launch from the surface--at 14g in a centrifuge, which, according to Collins, was "no fun at all.")

(As a side note, there *was* a contingency plan for Apollo, in case the designated landing zone was forecast to be in a typhoon at landing, to deliberately use a trajectory that was shallow enough to create a small atmospheric "skip" before the terminal entry and shift the landing point a few hundred miles into hopefully better weather. Since this was never tested unmanned, the crews were understandably a bit apprehensive about actually using it...)