How to make an efficient payload rocket and get it into orbit?

[Sakai]

Hi there!

I really need help with this... The frustration builds up in me that I literally raged quit xD

I've been trying to send space station modules up in the orbit of Kerbin however I always fail to even reach an orbit because the rocket is too weak to bring it all up, when separating solid booster stage it always smashes into my rocket and main stage even when using sepatrons, my rocket loses control when moving to East.

So in simple way I really need some tips and help on building a payload rocket to Kerbin's orbit. Yes I do use mods, rather a lot, here is a picture of a full list if that can help. I do not plan on taking any mods out, this is the way I love playing and I learned most of things but this is just too hard and I need help!

[Taki117]

A picture of your craft and payload would really help, as well as if you are using aerodynamic mods (NEAR, FAR, Stock Drag Fix) However, it sounds like your main stage doesn't have a high enough TWR, I suggest getting a mod like Kerbal Engineer to show you the TWR of each stage.

[Alshain]

Well it would certainly help to know which mods. The simplest way is to design a combination of TWR and DeltaV. You always need a TWR greater than 1, but how much TWR and dV depends on the aerodynamics and how well you pilot. The SRBs destroying your craft is a bug caused by the ejection creating rotation, sepatrons often make this worse. The best non-mod way of working around it is to cut your engines before staging, wait for the SRB's to clear, then fire them again. That little bit of no engine activity won't hurt your ascent and if the SRB's hit the rocket while traveling at the same relative speed, there shouldn't be any damage.

[capi3101]

Some general information about your designs would be helpful. Let's go basic information only - how massive is the payload? If you don't know that, take what bits you're using for the booster off the payload, put it on the pad by itself, go to the map view and click the little information tab on your craft; the part count and mass will be there. Once you've got that information, let us know. The specifics of your payload don't matter so much as that one key piece of data.

Then we can pick which route to go - rocket or spaceplane. That's your preference - rocket's generally easier to build, spaceplane is generally more efficient and cost-effective. Your choice.

[Sakai]

Here is my craft, I edited it a bit, managed to fix the booster problem somehow. I back my words of "I know how to build rockets" I just realized how hard it is and I have no clue how to even build them properly Now I can't even turn east because gently pushes with RCS/SAS will make it spin and break in half... When should I turn with this?

http://i.imgur.com/11XmdyQ.jpg

http://i.imgur.com/RuBv2s8.jpg

http://i.imgur.com/hMCSwo3.png

Thing about spaceplanes, yes I could use them but I never used them before, last time I tried I broke into small pieces >.< However if I were to learn how to use a space plane I think I would of do that but rockets somehow seem "easier"? or is it just me? And yea I did the rocket like that as I plan to... Make bigger payloads later

Edited November 3, 2014 by Sakai

[Alshain]

Do you have an in-game screenshot? Either that mod is putting the fairing in the wrong place or you have it put in the wrong place. If you are using NEAR/FAR that could cause some problems, and if your not using NEAR/FAR... why are you using fairings?

EDIT: Also what's with all the thrust? 3.12 TWR? That's not good in any aerodynamics.

Edited November 3, 2014 by Alshain

[Sakai]

I am using FAR that is why I use fairings. I have no clue on that one actually, it most likely could of be me placing it wrong but it all seems fine when flying upwards, only problem is when I want to turn east around 10000 or is it wrong to turn east with such big rocket towards east at 10000? Should I go more?

[Alshain]

Ok, if you are using FAR you have WAY too much thrust. When you end up going too fast, you can't turn quick enough without tearing the ship apart and you miss you ideal mark. Drop that TWR down to 2.0 maximum, though I prefer even lower, closer to 1.25 on the launchpad.

[Sakai]

So I take it as lowering TWR means lowering the speed yes?

Sorry I am new to all those mathematical things and such >.>

[Capt. Hunt]

RCS doesn't help much in low atmo, you might have better luck with fins for guidance on the first stage, also, you may want to add struts to the boosters.

[Alshain]

Yes, TWR (Thrust to Weight Ratio) is essentially the amount of power you have on your craft. The less you have, the slower you will accelerate. You must have a TWR of 1.0 to leave the ground, but too much makes it hard to maneuver especially when FAR is in play.

Engineer is showing you TWR on the launch pad, as you ascend your TWR will increase due to lower gravity and less drag as you leave the atmosphere. As this occurs you can even throttle back to save fuel (in flight there is a TWR (current) on Engineer). Try to design your craft around 1.25 TWR on the launchpad and keep it around 1.5 as you ascend by using the throttle. This is a very general approach, but one that should work and save a decent amount of fuel.

EDIT: Oh and double check that fairing. Any sudden wind break is going to kill your efficiency. I'm not sure it can even attach like that so I doubt it's really a problem, but probably the image mod.

Edited November 3, 2014 by Alshain

[CarlosMaximus]

Also, if you're using FAR, don't wait till 10,000 m and then throw the rocket over, start your turn at about 1,000 m, and try to stay within the prograde marker, keep the turn gentle, aim to be flying horizontally by about 32,500-35,000 m.

[Alshain]

Sorry, it's TWR (throttle) you want to look at. I'm not looking at the game at the moment.

[Sakai]

Thanks! I will check and try fixing it soon and will let know how it went!

[John FX]

My first thought was `that will spin without some fins at the bottom.`

One thing to always remember is FAR stands for Fins Are Required...

I put mine on the SRB`s so they eject at the right point and don`t waste my fuel too much. If the craft spins out after that then put some on a later stage too. Sometimes I just put them on docking ports...

I tip over right away and then hold about 20-25 degrees (using SAS) from vertical until 300m/s when the SAS stops working at which point I release all controls. The craft then flies a gravity turn kept stable by the fins on the bottom until about 30km by which time your Ap should be in the 50`s with a time to Ap of over 1 minute. Throttle down if your time to Ap goes over 1:10. Letting this get too high means you are going up too steep.

By this time (35-40Km) I have dropped the aero fins and I point prograde (orbital not surface) and control thrust to keep time to Ap around 1 minute until my Ap is in space (70km+) and my Pe is getting near 0 or over 0 if I have flown well.

Then a small circularizing burn at your Ap and bingo, orbit in FAR.

Here is a little tutorial I did to put together a station. The mods used are FAR, DRE, Proc fairings, Karbonite and the ALCOR capsule.

http://forum.kerbalspaceprogram.com/threads/97475-Standard-Utility-Construction-Kit

You end up with this

http://forum.kerbalspaceprogram.com/threads/97475-Standard-Utility-Construction-Kit

All the craft there launch to orbit fine in FAR, you may find something there to help you with yours.

Edited November 3, 2014 by John FX

[Alshain]

It shouldn't spin unless the SRB's are flexing the decouplers. In that case you need struts, not fins.

The docking ports are an interesting approach. Why not use radial decouplers? Realistically you don't need either, the mass of those winglets during space flight does not outweigh the mass and drag cost of the docking ports. Your actually hurting your design by decoupling them at all, might as well just leave them on. I'm not sure decoupling them with radial decouplers would make that any better either.

When I do use stabilizing fins (which isn't often) I usually plan the rocket so the stage that has them detaches in high atmosphere when they are no longer necessary.

[Jeff Bird]

Hi Sakai,

Quick question. Are you using struts to brace whats under the fairings? I had I problem like yours once. I found that my problem was the payload under the fairs was wobbling. This sent my rockets off course and spinning sometimes.

[Sakai]

Well I tried but failed and made it even worse! Is there anyone who perhaps has a .craft file of a payload rocket made using KW Rocketry mod by any chance? >.< So I could take a look at it and learn from it and re-creating similar one. That is the kind of... Way I tend to learn.

[Streetwind]

Define "payload rocket". Any assembly consisting of a rocket engine, a fuel tank and one or more additional parts is a rocket that carries a payload. It's like, you're asking for a "drivable car"

It would be a start if you could tell us how much payload you want to lift. How heavy is the object you're trying to get up there?

[Jeff Bird]

Hi Sakai,

Using the pictures you posted I've recreated your rocket as best I could and then tried to fly it. First thing I noticed was that those four SRBs are overkill. At 1000m the thing was going way too fast. Even with the main engine at 0 throttle. Without struts everything under the fairings does wobble too. So I took off two of the SRBs, ran struts from the second stage tank to the fairing base. It flew, I got it in orbit, USI parts and all. Had to be real careful pitching over though. I'd ditch the RCS thank and thruster on the fist stage altogether. Maybe put an SAS where the RCS tank was. Use a shorter 2.5m tank on the second stage. Procedural fairing are a a little bit easier to the use then the KW ones.

If I can get some time tonight I'll work with your rocket some more. Take some screenies and send you a .craft file.

Edited November 4, 2014 by Jeff Bird

[OhioBob]

Ok, if you are using FAR you have WAY too much thrust. When you end up going too fast, you can't turn quick enough without tearing the ship apart and you miss you ideal mark. Drop that TWR down to 2.0 maximum, though I prefer even lower, closer to 1.25 on the launchpad.

I've found that the optimum TWR on the launchpad is about 2. Less than that and the gravity drag is too high, greater and the atmospheric drag is too high. A TWR of 2 gives a good balance.

Gravity drag arises from a rocket having to hold itself up in a gravity field. A rocket needs a TWR of 1 just to counteract gravity, and anything above 1 begins to propel the rocket upward. If the TWR is 1.25, then 80% of the thrust is simply counteracting gravity. With a TWR of 2 we have only 50% of the thrust counteracting gravity, and 50% of the thrust making us go fast.

On the other hand is atmospheric drag, which arises from the resistance of the air. Drag force is proportional to velocity squared, so going fast means more air resistance. Having to overcome atmospheric drag is wasteful. The solution is to go slower to reduce the drag force.

We therefore have to competing objectives - accelerate fast to reduce gravity drag, but go slow to reduce atmospheric drag. Somewhere in the middle is a compromise that maximizes overall performance. I created a computer simulation to test this and found that optimum performance is achieved with a liftoff TWR of about 2.

FYI, the test was performed assuming the basic game, i.e. no mods.

[Pecan]

I've found that the optimum TWR on the launchpad is about 2...

You've found results that are different to everyone else.

For mass-efficiency (stock) a pad TWR of 1.5 - 1.7 is generally considered ideal, rising as fuel is burnt so the stage has an average TWR of 2 during its burn.

With FAR (which the OP is using) or for cost-efficiency a very low pad TWR, around 1.2, is usually optimal. In the former case because FAR's drag-model is much stricter and in the latter because you replace expensive engines with cheap fuel.

[OhioBob]

What's FAR?

[OhioBob]

You've found results that are different to everyone else.

For mass-efficiency (stock) a pad TWR of 1.5 - 1.7 is generally considered ideal, rising as fuel is burnt so the stage has an average TWR of 2 during its burn.

Obviously do what you think is right, but I get slightly higher losses at 1.5-1.7. I just ran a couple simulations for a sample rocket with all things being equal except for the throttle setting and the pitch rate. With the throttle set to give me a pad TWR of 1.6 my losses were: drag 857 m/s, gravity 1234 m/s, and total 2091 m/s. With the throttle set to give me a pad TWR of 2.0 my losses were: drag 1029 m/s, gravity 1048 m/s, and total 2077 m/s. That's not a big difference, but TWR 2.0 was a little better.

(edit to add) For the particular design I just tested, the optimum TWR came out to be 1.85, though that was only about 4 m/s better than the 2.0 TWR.

Edited November 7, 2014 by OhioBob

[Alshain]

What's FAR?

Ferram Aerospace Research is a mod that adds a realistic aerodynamics model to the game. The current model is to add up all the drag values of all parts on the ship and create an artificial drag value. Of course if the part is behind another part, it's still counted where in the real world it would be aerodynamically shielded. This is kind of stupid. Lift is also done similarly, so you can stack wings ridiculously and they would count toward lift even if they weren't in the airflow. FAR (and NEAR) fix that. For more info read this wonderful little guide, if you just want the drag info and don't care about planes, scroll down quite a ways till you see "Drag" in big letters with blue lines.

One common example of how bad it is in stock aerodynamics is that you can stack a bunch of wings, encase them in structural panels and literally make a flying box.

Welcome to the forums!

Edited November 7, 2014 by Alshain