Parachute & aerodynamic forces

[paul23]

Well I used to play ksp way back in 0.20. Now after a bit over a year I got a new laptop which can hopefully withstand the forces that my huge lunar bases put on it .

But before that I need to be able to launch & get myself out of the air. Well that's not the problem, heck getting into an orbit might be doable with the basic launch pad size. But what I can't understand is the simple landings.

Just dropping the standard mk1 command, a hammer engine & a simple mk16 parachute from a altitude of (say) 30km, in quite steep orbit (flight path would put me in a "parabole" around 50km of kerbal base).

What happens each time is the same: the "module" locks itself in a head-first trajectory around 10km altitude. Then when finally aerodynamics go away the part is at a height of ~5km above ground, moving at a speed of 300-400 m/s. If I then open the parachute it instantly rips off.

So how do you guys do this? How to successfully bring back larger craft? Is the key stability (making sure the parachute is not in the front of the aircraft), or is the key speed (making sure even non leaving/sub orbitals have a very shallow angle & a very low density so the drag is huge compare to the mass). I don't particularly look forward to constantly have to manage opening parachutes at exactly the right altitude and speed....

[FlyingPete]

You need a fairly shallow re entry profile. The idea is to burn off excess speed in the upper atmosphere- sounding rocket trajectories (to nip out of the atmosphere for early science) are difficult to bring back in one piece. You can't get away with deploying the parachute and letting them open whenever they want with the new aero.

For larger craft, try using drogue chutes before deploying the main chutes. These can be deployed at higher speed/altitude, and help to stabilize the craft as well as scrub off more airspeed.

[Clipperride]

The first few sub-orbital hops can be tricky to land due to the steep angle of the descent. Your parachute can be opened once below 250m/s (or a little faster if you are feeling brave/foolhardy). As you've found, you can run out of sky before you get to that speed!

Try stacking two or three command pods on top of each other (which you'll need to do for tourist contracts until you unlock the Apollo style command pod). Then, once below 1000m/s switch from keeping your craft pointing retrograde and try to get it sideways, horizontal to your flight path. Increasing the drag will slow you down more effectively, thanks to the new drag and atmosphere models.

Once you have liquid engines you'll have much more control of your flight path and will be able to fly a shallower descent.

[Snark]

Yah, you're running into a few problems:

First, you're entering really steeply, as FlyingPete points out. This wouldn't be such a problem if it weren't for the other points, mentioned below.

Next, your craft is too aerodynamically stable (what a problem to have, huh?) You have that heavy command pod in front, whereas the booster trailing behind is super light after it's exhausted its fuel, so it acts like tailfins and keeps you pointing forwards. That's great for launch, but on re-entry it means that you get "locked" nose-first, which in turn means you're too streamlined so you're still going super fast when you get down low.

There are a few ways around this.

One is FlyingPete's excellent advice about following a shallower reentry curve, and also using drogue chutes. The regular chutes have to be under 250 m/s, but drogues can go up to around 500 m/s so you can use those first.

Another option is to reduce the aero stability. For example, put a stack decoupler in between the command pod and the booster, so you can jettison the booster when you're done with it. That will get rid of the problem of being locked nose-first, so the pod can torque itself around to be butt-first. It will therefore be much less streamlined and will slow itself down to under 250 m/s, at which point you can use the regular Mk16.

Incidentally, are you running that Hammer at full power? A Hammer with just a command pod should be able to get you up to over 60km, not just 30-- as long as you run it at lower throttle. At full power, it's way overpowered for a ship of that size, so you end up going too fast too low and waste most of your fuel fighting drag. In the VAB, you can right-click on the booster and set the thrust limiter to something lower than its default 100%. Aiming for a TWR of around 1.5 tends to work fairly well, which for a craft like yours would be around 30-35%.

Edited October 15, 2015 by Snark

[paul23]

Yah, you're running into a few problems:

First, you're entering really steeply, as FlyingPete points out. This wouldn't be such a problem if it weren't for the other points, mentioned below.

Next, your craft is too aerodynamically stable (what a problem to have, huh?) You have that heavy command pod in front, whereas the booster trailing behind is super light after it's exhausted its fuel, so it acts like tailfins and keeps you pointing forwards. That's great for launch, but on re-entry it means that you get "locked" nose-first, which in turn means you're too streamlined so you're still going super fast when you get down low.

There are a few ways around this.

One is FlyingPete's excellent advice about following a shallower reentry curve, and also using drogue chutes. The regular chutes have to be under 250 m/s, but drogues can go up to around 500 m/s so you can use those first.

Another option is to reduce the aero stability. For example, put a stack decoupler in between the command pod and the booster, so you can jettison the booster when you're done with it. That will get rid of the problem of being locked nose-first, so the pod can torque itself around to be butt-first. It will therefore be much less streamlined and will slow itself down to under 250 m/s, at which point you can use the regular Mk16.

Incidentally, are you running that Hammer at full power? A Hammer with just a command pod should be able to get you up to over 60km, not just 30-- as long as you run it at lower throttle. At full power, it's way overpowered for a ship of that size, so you end up going too fast too low and waste most of your fuel fighting drag. In the VAB, you can right-click on the booster and set the thrust limiter to something lower than its default 100%. Aiming for a TWR of around 1.5 tends to work fairly well, which for a craft like yours would be around 30-35%.

Thanks for the advice: but I don't have drogues (yet?). Maybe I shoot myself in the foot, I've put science & money rewards both at 10%, so jettison the hammer before landing kind of removes any profit I would make from a mission. - I put it at these low number to have a "reason" to go out there and earn science from other planets. I remember my previous run having all science finished a long time before even reaching minus.

[Snark]

Thanks for the advice: but I don't have drogues (yet?). Maybe I shoot myself in the foot, I've put science & money rewards both at 10%, so jettison the hammer before landing kind of removes any profit I would make from a mission. - I put it at these low number to have a "reason" to go out there and earn science from other planets. I remember my previous run having all science finished a long time before even reaching minus.

Well, bear in mind that a Hammer is pretty darn cheap (especially when spent)-- recovering it doesn't get a whole lot. If you can make a reasonable, fun game at only 10% money rewards, I gotta say my hat's off to you-- I find money to be a drudge and a grind even at considerably higher numbers than that. 10% is an incredibly tough funds multiplier. Have you consider cranking it up higher? You can still leave the science set really low, if you like-- that'll still force you to go out beyond Mun / Minmus for science.

Another option that may help, if you're dead set against jettisoning the booster:

1. Do you have liquid-fuel engines unlocked yet? That will make it easier to fly butt-first without getting locked nose-first on reentry, since you have that heavy engine on the back of your rocket. Plus you can save a smidgeon of reserve fuel to slow yourself down before impact, if necessary.

2. Do you have any reaction wheels unlocked? Adding some torque to your craft could allow you to muscle it off of that locked nose-first attitude.

3. Do you have a pilot at level 1 yet? If so, you can turn on SAS to "hold surface retrograde" when you're up at the peak of your trajectory. It may be able to hold you butt-first as long as the aero forces don't get too big.

4. Do try adjusting the thrust of the Hammer down to a level appropriate for your ship mass-- you'll get a lot more bang for your buck, which means not only can you go higher, but it also gives you more wiggle room for choosing your trajectory (e.g. launching with a bigger horizontal component so you can enter at a shallower angle, as FlyingPete suggests).

[Right]

I'm having trouble replicating you're problem. I'm going at a faster speed and opening the chute at a lower altitude too.

Regardless, set you're minimum pressure to its lowest, and your deployment altitude to 1km (2km returning at a very steep angle). This allows your parachute to be in its semi-deployed state for as long as possible, reducing your speed without threatening to rip itself off.

Just to doublecheck, you're running 1.0.4?

[paul23]

I'm having trouble replicating you're problem. I'm going at a faster speed and opening the chute at a lower altitude too.

Regardless, set you're minimum pressure to its lowest, and your deployment altitude to 1km (2km returning at a very steep angle). This allows your parachute to be in its semi-deployed state for as long as possible, reducing your speed without threatening to rip itself off.

Just to doublecheck, you're running 1.0.4?

Uh if I deploy higher I go even faster, and the chutes break even more violently

[Snark]

I'm having trouble replicating you're problem. I'm going at a faster speed and opening the chute at a lower altitude too.

Regardless, set you're minimum pressure to its lowest, and your deployment altitude to 1km (2km returning at a very steep angle). This allows your parachute to be in its semi-deployed state for as long as possible, reducing your speed without threatening to rip itself off.

Just to doublecheck, you're running 1.0.4?

I was able to reproduce the problem pretty easily-- just put together a 3-piece ship as described, launch to a high altitude on a steep trajectory. The craft comes down just as the OP describes-- it's already falling much faster than 250 m/s when it gets down to where it's possible to semi-deploy the chutes, and stays nose-down and way too fast all the way to ground level.

The problem would be fixable with just a little bit of tech (e.g. use a liquid fueled rocket) or just a little bit of cash (e.g. jettison the SRB), but the OP is running a career with just 10% rewards on both science and funds, so options are limited.

Without some ship redesign, I'd say that the best bet would be to try flying a more horizontal trajectory so as to reenter at a shallower angle, as has been previously suggested.

[Kryxal]

There's nothing for it, really ... with a really light booster (because it's now empty) you're going to lawn-dart in nose-first. You'll need to toss the booster, then you can easily hold in a more reasonable orientation.