Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[ank]

What is a good level of acceleration to have while still in the atmosphere? I'm sucking really hard at fundamental rocket design right now.

You want to apply infinite thrust at the beginning(in practise: as much as possible), then throttle down when you reach about 200 m/s in the lower atmos.

The calculations are waaaay complicated, I began to model it, but there is just so many factors, so I kinda gave up.

[Skyrunner]

The basic rules would be (1) go at a fast enough speed to get out of the atmosphere as soon as possible without suffering too much from drag (usually round 200 m/s) (2) go full throttle as soon as you get out of the first 1/4th of the atmosphere until your apoapsis is high enough to touch your intended orbit. XD

[Sean Mirrsen]

A good suggestion is to aim for a TWR of 2. I.e. you want to "fall upwards". Just aim for the second mark on the G-meter.

[Girlinhat]

What does JATO mean? Jeb's Ass To Orbit?

www.lmgtfy.com/?q=JATO

Also, some hilarity?
http://youtu.be/yyDocdJp_gs

No regrets.

[BigFatStupidHead]

What is a good level of acceleration to have while still in the atmosphere? I'm sucking really hard at fundamental rocket design right now.

I aim for 100m/s till about 11-13 km (which is where the atmosphere gauge drops below the thick blue bar) up, then open the throttle up with my gravity turn. I -believe- the ideal speed for any given altitude is equal to terminal velocity at that altitude.

[ank]

That kinda makes sense.
Though 100 m/s is too slow, if your rocket is capable of going faster. tests have shown it's somewhere around 200-230 m/s depending on the craft.

[Sean Mirrsen]

Terminal velocity is a good ballpark estimate, which is why a TWR of 2 is the easiest way to gauge your performance. If you accelerate upward at two gees, you are falling upward - therefore going roughly at terminal velocity.

[Nao]

Adding to the acceleration (TWR) discussion.

Kerbal-X (which is craft file shipped with stock game) is very good example on how to set up staging with the right amount of power. The TWR of 2, is also a very good guideline.

But if you want to maximize efficiency a TWR 1,7-1,8 is great for longer staging. Reason being that you burn fuel as you go so just before burnout of the stage you get 2,2-2,4 TWR. So the average is around 2.
The theory of launching with 2 TWR and throttling down to maintain terminal velocity is sound but in reality you haul useless mass in form of not used engines.
If you design a rocket with stages starting at for example TWR 1,7 its important also to keep 100% throttle above 2 TWR to accelerate further for the next stage.

Also a small guideline for ascents- try to fly 200m/s vertical when at 8km, then start slowly turning east, going around 60deg attitude at 300m/s and 45deg at 500m/s. From that try to fly prograde (where your orbital velocity vector is pointing) and watch your Apopsis (in map view) and not go below 30deg untill your Ap is 30km and from then try to get to 0deg attitude at 50km Apopsis. Keeping 0deg is generally best while increasing Ap until desired one, after that is just standard circularization burn when reaching Ap altitude.

[Twi]

We interrupt this discussion to bring you something silly and moddy and quite the opposite. Submarines!

...well, attempts at submarines. They're still the 'really hard to control' phase, but they're technically functional without requiring absurd amounts of mass or anything like that. As you may have guessed from the GUI, this is Hooligan Labs airships stuff! ...But modded to have negative volume. Which makes the controls and stats all wonky, but surprisingly does not break the game! It's just that it's really, really twitchy and kind of tough to control.
Here, have a bunch of pics.

Spoiler (click to show/hide)

<sub>An early prototype

Another prototype, rolling on the ocean floor! They're pretty good at that, actually.


This is where they stopped being underwater rovers and started being subs.

Down she goes! just testing, mostly.

These things are kinda hard to control. The 'ballast' is REALLY twitchy.

SSTO- Single Stage To Ocean.

Good enough!

RAMMING SPEED</sub>
[
<sub>Underwater views are weird.

This thing behaves a bit better near the surface, but it tends to roll, which is annoying. Still not finished. Need a more powerful torque and such.</sub>

[BigFatStupidHead]

Hey, that's awesome! How are you dealing with the underwater camera problem?

[Twi]

Hey, that's awesome! How are you dealing with the underwater camera problem?

Not very well.

On the plus side, underwater rovers+ Mk1 Cockpit= actually looks pretty neat.

Also, the sub behaves weirdly in regard to depth. Your 'buoyancy' has relatively little effect on the water's surface, and it takes some time to sink fully, especially if you have a lot of bits that, well, float. As long as you're barely underneath the surface, you can actually go around quite well and the sub tends to stay mostly horizontal. Once you get down lower, though, it's sort of like breaking surface tension: suddenly the 'buoyancy' becomes far more responsive, and the sub tends to go vertical one way or another, and usually starts heading down. The fact that the force exerted by the 'balloons' increases as you get lower may be part of the cause of this. There's also the fact that those winglets in the back do generate lift and tend to point you in the direction of travel, something not always preferable. 

Oh, and if you roll enough you go forwards XD

In short:

Boating on surface: controllable just fine.
Subbing just under the surface: reasonably controllable.
Going at depth: Can be difficult to control.
Roving underwater: you have traction.

[ank]

Terminal velocity is a good ballpark estimate, which is why a TWR of 2 is the easiest way to gauge your performance. If you accelerate upward at two gees, you are falling upward - therefore going roughly at terminal velocity.

But you won't be going at terminal velocity right at takeoff.
The idea is burn max and then throttle down to 2 TWR, once you hit terminal velocity.
Every second you are not in orbit, Kerbin pulls you down with a force of 1 G, so we gotta get into orbit quick!

[Sean Mirrsen]

Like Nao said above, throttling down from the start is a bad idea. Well, it's sound compared to a full throttle burn with TWR>2, but it's a bad design if it has a TWR of more than 2 and has to do that. Basically, the only reason you are hauling up the mass of your engines is because they provide you with thrust. If you throttle them down, you are hauling up extra mass, because for the same thrust as you throttle down to, you could have just installed less engines.

The solution is actually simple, and very kerbal. Just add boosters! SRB's are cheap, cheerful, and provide a good enough kick for the initial ascent stage that your rocket can resume a TWR 2 burn upwards for the rest of the flight.

[Dutchling]

Why is a TWR of 2 ideal?

[Sean Mirrsen]

Because something around terminal velocity (i.e. the speed you'd normally fall at) is a decent estimate at the optimal ascent speed to balance delta-V losses against atmospheric drag and gravity. If your thrust-to-mass ratio is 2, you can accelerate upward at two G's. One G is counteracted by Kerbin's gravity, and the practical upshot of a TWR of 2 is that you are essentially "falling upwards", naturally hitting terminal velocity on your way up, and probably being significantly more efficient than if you had a much higher or lower TWR.