Fuel Usage and dodgy sas

[kknight13]

I have noticed when i launch one particular rocket it begins to slowly lean to one side even with sas turneed on. the sas units don\'t seem to be at max strength either. can anyone explain why and how to prevent?

Also i found that if i don\'t have the first stage at full power (70-85%) the rocket doesn\'t slowly lean.

do you get the same benefit for the fuel if you are using it for longer but with less strength?

[duckunlimited2]

Do you have an Advanced SAS on the rocket (A.S.A.S.)? Its a better SAS unit that can control both RCS and regular SAS units to better keep a rocket stable.

[kknight13]

i have an advanced sas unit on the rocket as well as 8 standard sas units

i try to keep the rcs off for the takeoff so i can use it for small changes and mun landings

[tjoreilly]

Could you post a pic, or better yet, a pic and .craft file of your rocket?

[kknight13]

new to the game, even more confused by the internet, how do i get a picture of the rocket?

i think this is the earliest rocket with the problem, most of my latest are variations on this design

[tjoreilly]

You can take a screenshot by pressing [F1]. I have been using Imgur to host my screenshots. It seems to work well.

I\'m taking a look at your rocket now. My first impression is that it looks too heavy and the bottom stage looks unbalanced with the four tricoupler setup, but I haven\'t flown it just yet.

[kknight13]

as long as you keep the initial thrust below 90% it is pretty stable and it is only about getting the next stages a better start.

with it i have gotten the next stage into orbit around minmus reach escape velocity and return to a 500km kearth orbit before it ran out of fuel.

also that may have gotten the staging wrong, the 3rd stage rocket fires with the 4 2nd stage rockets

[kknight13]

here are photos

just re testing the rocket myself, bit unstable but as long as thrust is just above lift off sufficent it goes fine

[tjoreilly]

The thing with ASAS is that on increasingly larger crafts it needs modules to send its commands to, e.g., winglets, sas, rcs, thrust-vectorable engines.

In your case, you have abundant SAS units, so roll is kept well enough under control. However, as your rocket drains more and more fuel it becomes unbalanced and your ASAS has no modules to command that can make the minor course corrections to keep the craft flying straight. You might try replacing four of the engines on the lowest stage with LV-T45s or adding AV-8 Winglets to your rocket.

That being said, it might be worth seeing if you could reduce the overall height of your craft. The ASAS at the top of your rocket gets worked pretty hard.

[kknight13]

how important is the height of the rocket?

wouldn\'t changing the rockets at the base so that some produce more thrust than others make the rocket even more unstable?

thanks for the suggestions

[tjoreilly]

In the case of your rocket, replacing one of the engines on each tricoupler did make the craft more stable as a result of added control.

As far as height goes, the larger the craft is, the harder it can be to control, even more so as the height to base width ratio increases.

*on a lark, I cut your rocket in half, removing everything below the 2nd stage engines. She\'ll get to Minmus with fuel to spare.

[kknight13]

the first stage is just good for getting through the thicker nastier part of the atmosphere. great to know it is really effective with only part of the rocket

[purple100]

My understanding is that SAS modules keep the craft from rolling, but not pitching or leaning over. Is that correct?

[UmbralRaptor]

SAS will also try to damp out pitch and yaw.

@kknight13 I\'m going to have to second the need for winglets or vectored thrust on the first stage to give the ASAS something to work with. Also, replacing the last stage engine with an LV-T45 (the thrust vectoring one) will provide more control later on. Probably enough to cut down on the number of SAS units.

And since one one else answered it, fuel usage with the current stock rocket engines is directly proportional to thrust. There are designs where throttling down may make sense in the atmosphere (to reduce drag), but otherwise, lower throttle is only useful for greater control. (Full throttle minimizes gravity drag)

[kknight13]

*on a lark, I cut your rocket in half, removing everything below the 2nd stage engines. She\'ll get to Minmus with fuel to spare.

i have heaps of trouble with my rockets (complete failure) so the more fuel the better

i haven\'t managed a mun landing yet

(at least one where the command capsule was still attached to a rocket)

[jayther]

Yeah, that\'s a lot of fuel tanks. You would probably do a lot better with less fuel tanks on that second (middle) stage if you don\'t want to cut out the entire bottom stage and adding control fins aren\'t working for you. Keep in mind that the bottom stage has to carry the fuel tanks in the top stages where the engines are not being used. Less fuel tanks an early stage has to carry, the better (for both fuel efficiency and control).

You don\'t need that many SASs. 9 SASs is overkill. You can get rid of the SAS at the bottom and the SAS in the middle below the ASAS. Gotta trim the fat.

Also, your approach of how you exit the atmosphere and achieve orbit is important to fuel efficiency.

[Khrissetti]

how important is the height of the rocket?

wouldn\'t changing the rockets at the base so that some produce more thrust than others make the rocket even more unstable?

thanks for the suggestions

The taller your rocket is, the less stable it is, it\'s all down to the centre of gravity on the thing. The CoG is the point around which all your forces act, like the pivot point on a see-saw.

The further a force is from the CoG, the more rotational moment it applies, the same as the see-saw, a very light weight can lift a heavy weight if the heavy weight is near the pivot and the light weight is at the end of the see-saw.

On a rocket of 10m with its CoG in the middle, if you pitch over by one degree then at the end your engines move to one side by 9cm (approx.), if the engines have an upwards force of 100KN then your ASAS has to counteract 8.73KNm of moment trying to turn your rocket to one side

if your rocket is 50m long then that becomes 43.63KNm, at 100m long that becomes 87.26KNm

(That maths isn\'t totally right as the angle of that rocket\'s thrust will increase with the rocket, this is just to demonsrate why a taller rocket is less stable.)

If you look at your stats for the SAS and ASAS it gives you a measure of the maximum torque they can apply which is (to our purposes) the rotational energy it can use to counteract the rotational energy of your off-centre rocket. if the turning moment is greater than the torque your SAS can apply then it will tip over, regardless of SAS.

There are three ways of countering this:

1) Make your rocket squatter. the shorter it is, the smaller the distance between force and CoG

2) reduce your power. As you discovered.

3)add winglets and control surfaces. This gives your SAS more \'torque\' to play with to counteract the rotation.

[kknight13]

Also, your approach of how you exit the atmosphere and achieve orbit is important to fuel efficiency.

got any specific advice for that?

[jayther]

That was just a general comment in fuel efficiency (because when to start tilting determines how much of the thrust is going to the horizontal direction versus how much is going to break through the atmosphere and gravity). It has nothing to do with why your ship is tilting when you\'re going full power. I was just thinking that if you\'re having trouble with smaller designs, having a good approach to get to an orbit is important and one of the key things for some minimalist designs you may have seen. Also, smaller designs have less drag, so that\'s another thing to think about when you\'re make ginormous rockets.

Anyway, I did huge rockets before as well when I started. Moar boosters, right? But after achieving orbit for the first time, I tried looking for ways to make the design smaller because there is no way in hell am I going to make it to the moon when I\'m having trouble getting into an orbit with behemoths.

I\'ve encountered tilting problems as well with big designs, but sometimes it\'s just the smallest thing (and maybe even some weird random hitch with alignment), so I make the rocket again from scratch and usually it turns out to be stable (and not tilting at launch).

[UmbralRaptor]

got any specific advice for that?

Start turning early (~10 km altitude, possibly lower!), but slowly. Perhaps in steps. Somewhere around 35-60 km altitude, you want to be at a shallow (0-20°) angle above the horizon, with an Ap of 60-75 km. Keep burning until your Ap is out of the atmosphere (~69.1-75 km, towards the higher end if you\'re going to be lower in the atmosphere for longer time), then wait.

You\'ll want to start burning again somewhere between ~10 s and 2 minutes before Ap, depending on your TWR. This time, purely horizontally, until both Ap and Pe are above the atmosphere. Don\'t worry if this results in alternating between short burns and coasting for a while.

Also, don\'t worry about spending up to half an orbit in the upper (60-69 km) atmosphere during ascent. Drag losses at this altitude are less than you\'d expect.

[rkman]

I build and flew that craft (with stacks of 2 tanks instead of 3 on the lower stage), and found that while it was on average stable, it did slowly sway back and forth by a considerable amount (between at least + and -5 degrees) while the 1st stage was burning. It looks like there are forces at work that want to push the craft off course.

I did not dear to tilt it over until after the 1st stage was jettisoned.

I suspect a problem is that due to the tricouplers the 1st stage is not perfectly symmetrical, possibly inducing a slow roll on the craft that the SAS units can only barely compensate for.

At any rate, when it comes to attitude control of a craft that has its engines burning, gimbal engines are far more efficient than SAS.

Most of my craft even large ones have only 1 or 2 SAS units (and one asas or mechjeb), and at least one gimbal engine on each stage. No RCS except for rendezvous craft. Sometimes a few fins on the 1st stage.

Where SAS is really needed is when you want attitude control when the engines are off, and the craft is of a such a size that control by the command pod sas alone is to slow.

Same craft modified:

I used shift Q and E in the VAB to place the tricouplers perfectly symmetrical, replaced 3 main engines with gimbal engines.

Replaced the center engine on the upper stage with a gimbal engine. Removed all but one SAS.

That\'s much more stable on ascent. One SAS is still enough for good control over the upper stage including the lateral mounted stacks, with engines off. 1st stage is slightly underpowered, can probably be resolved by removing a few tanks from the top stage, ie replace 4 or 5 tanks with small tanks.