Large Rockets & Getting them into Orbit

[BR00NER]

Soo.

Bill is stuck on the mun with a ton of science and a probe, that is miles away from Bob, which also has a ton of science. I am trying to arrange a rescue of both the science and Bill however all of a sudden I am having real difficulty getting into Orbit.

I think it may be because my rocket is to big however I need some clarification. Basically the problem starts just after I dump my first stage. My rocket catches up with the Apopasis, over takes it and starts falling back to Kerbin. I think I am tilting correctly as I can achieve orbit no problem with a smaller rocket but I need a bit more muscle to rescue Bobbyo and the science. I have watched some people on youtube launch bigger rockets than me and get them to orbit so I must be doing something wrong or the rocket is to heavy. I am fairly new to the game (on xbox) so maybe further down the tech tree I will be able to do this.

Any advice would be great and any example of a rocket that can get you to the mun (and beyond) would be awesome.

 

Cheers

Kerbanout BR00NER

 

[Bill Phil]

Try boosters.

Really, though, you probably need more thrust on your second stage.

[BR00NER]

Ah okay I see. That could be it. When the first section detaches and I am left with one engine I have it at 50% thrust. I think I watched a video that said it was more efficient. Do you think this could be the problem? 

 

Cheers Phil 

[Steel]

28 minutes ago, BR00NER said:

Ah okay I see. That could be it. When the first section detaches and I am left with one engine I have it at 50% thrust. I think I watched a video that said it was more efficient. Do you think this could be the problem? 

 

Cheers Phil 

It's only more efficient if you have too much thrust, you have too little, so try setting that engine to 100% first. If that's not enough try more/better engines

[Deddly]

Hi @BR00NER, welcome to the forum! I moved your post to Gameplay Questions. Hopefully you'll get some more good advice in here.

[GoSlash27]

I design my upper stage to have .7 t/w. An ideal second stage burn should keep your apoapsis about 45 seconds ahead. If you can't keep Ap ahead of you that far, you're probably short on thrust. It could also be that your first stage was too steep or lacking DV.

Best,
-Slashy

[BR00NER]

cheers man, when you say to "steep" do you mean the tilt or to powerful? And what's the best way to work out t/w?

[LN400]

12 hours ago, BR00NER said:

Ah okay I see. That could be it. When the first section detaches and I am left with one engine I have it at 50% thrust. I think I watched a video that said it was more efficient. Do you think this could be the problem? 

 

Cheers Phil 

What you can do to find what throttle cap to set by working out the total mass of the whole show going to space, payload, fuel, tanks, engines, everything.

Let's call the total mass M.

Figure out what your initial thrust to weight ratio (TWR) should be. Some like 1.30, others like 1.50. Let's call this TWR A.

Look at the engine data in the VAB and look for max thrust, both for vaccuum and atmosphere. Decide which one of these is most relevant for the stage you're building. Let's call this max thrust T.

 

Now work out:

A * M * 9.81 / T

The number you get is the sought after cap so if you get say, 0.52 then cap the throttle at 0.52. If you get a number larger than 1, get a bigger engine.

 

EDIT: Changed the word acceleration to TWR. I guess I was in a hurry the fist time.

Edited August 14, 2016 by LN400
Corrected a silly error in the post

[BR00NER]

Thanks man. I'll have a look at that. If they let us play KSP at school I might have passed maths 

[ForScience6686]

This is the first time I'm hearing that you should limit throttle on ascent.  I've always heard that you're bringing too much engine if you aren't running at full throttle.  However, I'm generally only on srbs until at least 10km, then full throttle from there.  

[LN400]

1 hour ago, ForScience6686 said:

This is the first time I'm hearing that you should limit throttle on ascent.  I've always heard that you're bringing too much engine if you aren't running at full throttle.  However, I'm generally only on srbs until at least 10km, then full throttle from there.  

One problem with running at full throttle is that each rocket is more or less unique with its own mass, its own drag profile etc. Full throttle could mean a sensible acceleration all through the dense atmosphere, and a sensible rate of change of TRW, for one design and the same engine on another rocket with the same mass but a very different drag profile, as an example, could mean lost dv. Problem arises when there can be quite a leap in engine performance. You could find that the engine you need can deliver say 500 kN but the only engines to consider either deliver 450 kN or 900 kN (again, I am pulling numbers out of my big hat but as an example they do just fine).

Edited August 13, 2016 by LN400

[foamyesque]

The only throttle you should need on ascent is the VAB. There's a bit of an awkward gap thrustwise for the liquid engines between 20kn and 200kN, but beyond that you should be able to find something that matches up with what you need. @GoSlash27's suggested second stage TWR is good; aim for 0.6-0.8 and you should be able to complete the orbit insertion burn in good order.

[GoSlash27]

9 hours ago, BR00NER said:

cheers man, when you say to "steep" do you mean the tilt or to powerful? And what's the best way to work out t/w?

BR00NER,

 LN400 outlined the process for figuring t/w above. I don't bother limiting my throttle on upper stages (I consider 0.7 the minimum for this job), but I do limit it on boosters so that it wants to follow an efficient gravity turn.

When I say too steep on the boost stage, I mean the tilt... although that can also result from too much thrust.
 Basically a boost stage that is too steep will leave your transstage too high and slow. You don't have enough time to circularize before you're past apoapsis and falling back down.

HTHs,
-Slashy

[BR00NER]

Awesome, cheers everyone. I've got my rescue probe in the Muns orbit. Now just need to land next to bob and RTB ha

[ForScience6686]

23 hours ago, LN400 said:

One problem with running at full throttle is that each rocket is more or less unique with its own mass, its own drag profile etc. Full throttle could mean a sensible acceleration all through the dense atmosphere, and a sensible rate of change of TRW, for one design and the same engine on another rocket with the same mass but a very different drag profile, as an example, could mean lost dv. Problem arises when there can be quite a leap in engine performance. You could find that the engine you need can deliver say 500 kN but the only engines to consider either deliver 450 kN or 900 kN (again, I am pulling numbers out of my big hat but as an example they do just fine).

So in that case, wouldn't it be better to use the 450 engine with less mass as well as some srbs to make up the needed thrust?  Maybe I've learned incorrectly, but srbs have always been my first stage as they are cheap and only really useful in the denser atmosphere. 

 

Sorry to derail this discussion a bit...

[LN400]

9 minutes ago, ForScience6686 said:

So in that case, wouldn't it be better to use the 450 engine with less mass as well as some srbs to make up the needed thrust?  Maybe I've learned incorrectly, but srbs have always been my first stage as they are cheap and only really useful in the denser atmosphere. 

 

Sorry to derail this discussion a bit...

I see this as on topic myself so: The use of SRBs depends. Their main problem is you have no way of reducing their thrust once they fire meaning you can end up going too fast in the dense atmosphere wasting dv or even worse, seeing the ship tear itself apart or overheat. Their burn time is often in the 20-50 seconds range. Combining SRB with liquid for 20 seconds might mean you won't be able to burn enough fuel for the main to have enough TWR after the SRBs burn out. Again, this is all down to specific desigs and each design need its own solution for thrust. Some do well with a too weak engine combined with SRBs, other designs are sub optimal or just plain madness. Then there is price. Sometimes an single oversized main is the cheaper option.

Edited August 14, 2016 by LN400

[bewing]

SRBs give you a lot of dV for the kerbuck. In exchange, you probably want to give up on the idea of a gravity turn in the first place. So just launch things straight up instead. The word "efficiency" gets thrown around a lot, but it actually has several meanings -- and a vertical launch with cheap SRBs is very "efficient" when calculated as price per kg to munar orbit, for example.

[foamyesque]

It's entirely possible to perform a gravity turn with SRBs. What on earth are you talking about, @bewing?

[Magzimum]

1 hour ago, foamyesque said:

It's entirely possible to perform a gravity turn with SRBs. What on earth are you talking about, @bewing?

I think @bewing means to literally go straight up, and nothing else (not the straight up to space, then turn 90 degrees to make it to LKO). So, straight all the way to e.g. the Mun, removing the need to turn into a LKO at all.

 

 

 

[bewing]

Correct, Magzimum. And foamyesque: a stack of SRBs tends to be very tall and noodly, and can easily be unstable even with only tiny steering inputs. At burnout, each stage is often significantly over a twr of 3. Yes, if you add lots of struts and control surfaces and gimbaling you can still force it to turn, but all that added BS wastes the tiny extra bonus you get from a gravity turn. If you launch vertically, you want as much speed as you can get right off the pad -- the more the better until you blow up. Because if I can get my speed higher than 2300 m/s before 70km altitude, then I have more Oberth than you.

[foamyesque]

4 hours ago, bewing said:

Correct, Magzimum. And foamyesque: a stack of SRBs tends to be very tall and noodly, and can easily be unstable even with only tiny steering inputs. At burnout, each stage is often significantly over a twr of 3. Yes, if you add lots of struts and control surfaces and gimbaling you can still force it to turn, but all that added BS wastes the tiny extra bonus you get from a gravity turn. If you launch vertically, you want as much speed as you can get right off the pad -- the more the better until you blow up. Because if I can get my speed higher than 2300 m/s before 70km altitude, then I have more Oberth than you.

 

Oh, we're talking about stacks of SRBs now, not just a first stage or co-firing boosters? In any case, I'm still not gonna believe you without some actual evidence. SRBs crappy Isp make using SRBs to lift SRBS blow out your mass ratio in a hurry, and on a four minute burn time, spending two minutes of that horizontal instead of vertical saves you around 1200m/s of deltaV, so the bonus for executing a gravity turn and horizontal insertion to Mun orbit instead of going straight up is pretty large. It also lets you get away with much lower TWRs, hence better mass ratios, hence smaller rockets.

 

So basically: Show me the money. What is your cost to Mun orbit per kg with your approach?

[luizopiloto]

You can also try built the spacecraft in orbit... launching each module on medium sized rockets... this is the method I use for interplanetary missions... specially after 1.0, when the game started to get pretty laggy with high part count crafts...

[Spricigo]

17 hours ago, bewing said:

Correct, Magzimum. And foamyesque: a stack of SRBs tends to be very tall and noodly, and can easily be unstable even with only tiny steering inputs. At burnout, each stage is often significantly over a twr of 3. Yes, if you add lots of struts and control surfaces and gimbaling you can still force it to turn, but all that added BS wastes the tiny extra bonus you get from a gravity turn. If you launch vertically, you want as much speed as you can get right off the pad -- the more the better until you blow up. Because if I can get my speed higher than 2300 m/s before 70km altitude, then I have more Oberth than you.

I'm pretty skeptical that can be a general case instead of works for me kind of thing. Also seems you really didn't bother enough with gravity turn or lower TWR designs to have a good parameter to compare your approach. 

 

Like @foamyesque said show me the money.  Give us an example craft or at least some info so we can test your idea. 

21 hours ago, bewing said:

SRBs give you a lot of dV for the kerbuck. In exchange, you probably want to give up on the idea of a gravity turn in the first place. So just launch things straight up instead. The word "efficiency" gets thrown around a lot, but it actually has several meanings -- and a vertical launch with cheap SRBs is very "efficient" when calculated as price per kg to munar orbit, for example.

You casted doubt in the meaning other use of "efficiency"  and "efficient"  and end up using it in the exactly same meaning everyone else in this discussion.  Why?  It don't change how true or relevant your point is but surely make it less clear. 

[bewing]

@foamyesque, @Spricigo:  OK guys, let's do the old "compare gravity turns vs. direct ascent" thread again. Over on this other thread, to avoid hijacking the current one any worse.

http://forum.kerbalspaceprogram.com/index.php?/topic/146000-just-how-much-better-is-a-gravity-turn-than-direct-ascent/