This rocket will not fly, why?

[cicatrix]

I'm just too stubborn to let the air win, plus going faster saves dV to orbit if you can stay in control. So I have just been learning to design rockets that can punch through the transonic region with enough stability and control to not spin out. At a fundamental level "cicatrix" is right that is seems to be about enough control authority, especially for heavy rockets. However, he is wrong that TWR needs to remain low to make it work. My most effecient rockets to orbit tend to have TWR in the 1.7-2.2 range, but I admit it can be a challenge to keep them flying true. Challenged Accepted!

You can design a rocket with TWR=10 if you want, but any excess TWR you have is simply a waste of fuel. You're figuratively banging your head against a brick wall. Increasing TWR gains you nothing you just spend more.

[cybersol]

You can design a rocket with TWR=10 if you want, but any excess TWR you have is simply a waste of fuel. You're figuratively banging your head against a brick wall. Increasing TWR gains you nothing you just spend more.

Going faster enables more thrust lower, which actually SAVES you fuel. Terminal velocity is much higher than you think now, and you save gravity drag by going faster. Its a fact, and you can empirically test it to find it out for yourself. Also, there is a video around of someone with starting TWR of 4+ using a ridiculously bad ascent profile and still getting 3150 dV to LKO in 1.0.2.

Cheers,

CyberSoul

[Amram]

I'm just too stubborn to let the air win, plus going faster saves dV to orbit if you can stay in control. So I have just been learning to design rockets that can punch through the transonic region with enough stability and control to not spin out. At a fundamental level "cicatrix" is right that is seems to be about enough control authority, especially for heavy rockets. However, he is wrong that TWR needs to remain low to make it work. My most effecient rockets to orbit tend to have TWR in the 1.7-2.2 range, but I admit it can be a challenge to keep them flying true. Challenged Accepted!

drag is a square function, the basic point of that is that going 10% faster, from 100%, to 110%, will cost you 21%(1.1²-1² = .21) more drag. The faster you go, the worse it gets and the faster it gets worse with increases. The faster you go in thicker air, the more you will lose to that air. Go too slow and you lose too much to gravity.

Of course, thats nitpicking, too fast is well, crazy fast before atmo losses outweight the savings in gravity loss reduction. Don't worry about it, it'll be a rare thing to see someone lose more to aero on the way up than they gained from reducing gravity losses.

So, on a scale of 1 to 10, how detrimental is breaking mach 1 at 6km alt? Thats about where I'm hitting it and soon as I do, I lose all control and the thing flips out.

Giant post, but you did ask why, its hard to do why without explaining, and explaining tends to get big.....so.....

10k hours on stock atmo? You know how all of us far players detested stock aero so much, and couldn't find enough bad things to say about it? There's a reason. Forget everything you know about KSP's aero that you learned prior to 1.0, none of it will help you now in any way, and nearly all of it will hinder you. It was that bad. Everything you think should launch fine based on past experience? Don't bet on it.

What stock really should show the player is Q. Q is the dynamic pressure your rocket is experiencing, and is a damned good indicator of just how much the atmosphere is resisting. Those very same effects you get down low will also happen at high altitudes, if your moving fast enough. Less air doesn't mean they can't happen, just that the forces are lower so they are easier to keep controlled, but raise your speed and they WILL occur despite lower densities.

Stock should beat everyone over the head with this concept with a non skip-able tutorial, the headaches understanding this would solve....

If your losing it at ~mach 1 at 6km, then assuming mach 1 as 340m/s, density at 6km of of 0.369kg/m³, Q is 21326, pretty low, which means your really draggy, or deviating too much, or lack control authority for your piloting, pick your answer, they'll all solve it.

first we need density of the air. iirc, that is calculated as e^(-alt/5000), so e^(-6000/5000) = .301194, that gives us the ratio, sea level is 1.225, so at 6km its 0.301194*1.225 = 0.369kg/m³.

Next we need velocity, you said mach 1 is when you lose it, and afaik KSP doesn't moderate mach with altitude, so its 340 all the way up.

Q is 0.5 * density * velocity squared. so 0.5 * 0.369 * 340^2 = 21326.06

228m/s @ 2km

278m/s @ 4km

340m/s @ 6km

415m/s @ 8km

507m/s @ 10km

620m/s @ 12km

757m/s @ 14km

924m/s @ 16km

1130m/s @ 18km

1379m/s @ 20km

2273m/s @ 25km

3748m/s @ 30km

6179m/s @ 35km

...

Still, same forces doesn't mean loss of control, your mass distribution is very different as your burning fuel the whole way up, so the proportions change, the centroids move, your ability to control those forces changes. Its almost impossible to compare.

To finally answer the question though, how bad is breaking mach 1 at 6km?

NaN......or 10, depending on your flavour.

Seriously, it doesn't have an actual answer, especially not in a 1 to 10 form. Although your launch is failing, clearly a 10 for that combination of design, piloting, and launch profile.

How bad is it for a given design, with a given piloting skill, with a given launch profile? That has an answer, but its troublesome to figure out, and any changes to the design, piloting, or flight profile invalidate the answer and you have to figure it all out again....much easier to just launch it, and adjust if necessary. Piloting and launch profile are both player skill, and widen the margin for design variety. Pre-1.0 piloting will not help you here, pre-1.0 launch profiles will terminate every launch for you. Its a whole new game.

What is bad is generating more force(Q times drag coefficient) than you can control, and then putting it where you can't control it. The torque the nose is going to impose on you depends entirely on how far from prograde it is, increased by the leverage you give it by placing the nose far from the center of mass. if its 100% perfectly aligned with zero error, the net torque is zero, and you'll fly just fine. As long as you stay dead straight, try to move, and the force offsets, produces torque, and if you can't control it, you lose it.

If you reduce your drag, you reduce the force on the rocket, and that means you could go faster for the same controllability and drag losses, reducing gravity losses. Or you might maintain control of a launch you couldn't control before. Don't underestimate how draggy something can be, and for a change, stock is more punishing than far is(both because it doesn't occlude when you think it will, and it does nothing to inform you of such control problems and far does)

The more you deviate, the more leverage you give that force. This is why you can't do the old "up 10 over 45" routine anymore. This is made worse by exposing the side of the rocket to these forces as well. Ever try to hold something large and flat on a windy day? Your rocket behaves the same.

Once you deviate, it pushes on your entire length on the exposed side. If everything were equal, as it was in the old aero, it wouldn't matter, but its not. Heavy engines in the rear, huge drag force on the nose. The engines want to come forward and take the lead, the nose wants to fall back and slow down, and together they'll win every time with every rocket if the pilot and engineer let it happen. Whether the pilot even has a chance is up to the engineer.

Rule one:

Keep the forces control-able. Slow down if your design isn't aerodynamic enough. Improve the design if you want to go faster - more control authority and/or reduce drag.

Rule two:

Keep the forces under control. Put them too far out of alignment, while they are too large to handle, and your screwed. Don't deviate from prograde unless the forces are small, which means either low velocity, or little/no air density.

Violate either rule and aero will violate your launch.

Violate neither and you can do massive launches on nothing but engine gimbal and pod torque, no dead weight reaction wheels or fins.

Reaction wheels and fins just widen the margin for error by increasing control authority, they don't make the problem go away, nothing does that.

The short answer is this:

Fly as fast as you can reasonably control it, and yes, that means slow down if you can't. But understand that the faster you go, and the more draggy it is, and the more you deviate from prograde, the harder it will be to keep under control.

Less deviation, or more aerodynamic, or less speed, and the forces will remain lower, and you are more likely to keep it controlled. If everything goes perfectly, you could keep it flying straight on 100g's of accel right up until drag crushes it, good luck with that, you WILL deviate, you WILL lose it, but theoretically possible.

There is no hard fast rule anymore, and there never should have been. Stock taught you a broken system, left you with it far too long, and then pulled the rug out from under you, your a 10,000 hour newbie......can't say I've ever played a game where that was the case before.

Edited May 15, 2015 by Amram