How low can the TWR on the injection stage be?

[Bartybum]

I'm still refining my heavy lifters, so I'd like to know.

[Monkeh]

I've found a starting TWR on a lifter of around 1.6-1.8 is nice. That will obviously go up as the fuel is burned. Any higher than 2-2.2 and the air resistance works against you and you lose efficiency.

Anything from 1.2-2.0 as a starting TWR can work well though. Less than 1.1 and you'll burn a LOT of fuel to go not very far, also, if you use asparagus staging watch for decreases in your velocity after each separation, if you see your speed drop between stages this is a waste of fuel as well, burning to get back velocity you've lost is not very efficient.

With that in mind I also try to keep my speed increasing even when waiting to get to the apoapsis for the main thrust of the orbit insertion burn, just so you don't have to regain lost delta v, a very low burn can keep your speed increasing slightly and save you fuel in the long run.

[blizzy78]

Monkeh, just a minor correction: I think you are talking about acceleration instead of velocity. If your actual velocity is dropping during ascent, then you have bigger problems.

Edit: Nevermind this post. As others have stated correctly, velocity may drop just fine when staging. Shouldn't be dropping too much, though

Edited October 10, 2013 by blizzy78

[Temstar]

Injection stage as in the upper stage of a booster that puts the payload in final orbit right? I use TWR of 1.2 at the start of the stage burn but you can actually do <1 TWR, it's just trickier to circularise.

[Scarecrow88]

I thought the injection stage was the bit that took you from orbit around one body to orbit around another. If that is the case, TWR will only affect your rate of acceleration. How long are you prepared to wait for your burns to complete?

[Kasuha]

Upper stage of my lifter has very low TWR, it wouldn't even lift the stage with no payload from Kerbin surface. Its task is not to lift anything, its task is to make the last push to convert suborbital trajectory into safe orbit, make rendezvous and dock.

I am not good in calculating TWR but it has two LV-909 engines (actually four, two prograde and two retrograde), X200-32 fuel tank and a crew module. And it can pull a full orange tank into its orbit quite well.

Monkeh, just a minor correction: I think you are talking about acceleration instead of velocity. If your actual velocity is dropping during ascent, then you have bigger problems.

My lifter's velocity always drops after a stage runs out of fuel. I considered that normal because the payload remains the same while the thrust decreases by dropped engines - and only the first stage has enough power to actually lift it all with full fuel tanks. Now that you mentioned it I guess I'll give it a second thought, maybe I could make my lifter smaller...?

[Monkeh]

Monkeh, just a minor correction: I think you are talking about acceleration instead of velocity. If your actual velocity is dropping during ascent, then you have bigger problems.

Yes, you are, of course, correct. If I see that 150m/s drop down to 130m/s than I get angry with my stupid lifter.

[arq]

My lifter's velocity always drops after a stage runs out of fuel. I considered that normal because the payload remains the same while the thrust decreases by dropped engines - and only the first stage has enough power to actually lift it all with full fuel tanks. Now that you mentioned it I guess I'll give it a second thought, maybe I could make my lifter smaller...?

I'm with Kasuha here. Velocity should drop when you ditch a stage because of the reduced thrust. This doesn't happen so much at altitude, but early when drag is high it is nearly unavoidable. Essentially, if your TWR is above 2.2 (the little extra allows you to accelerate to terminal in the thinning atmo) or so below 10km, you are wasting fuel and should throttle back (which means you are wasting engine mass). So really I have my lifters start each stage around 1.7-1.8 so that they end around 2.1-2.3. Except for the last 1000m/s or so of my launch delta-v, which really doesn't matter so usually I just burn what's left at whatever TWR it gives me, whether that's 0.3 or 3.

[NGTOne]

I usually build 3-stage rockets - stage 1 gets dropped during vertical ascent, stage 2 part-way into the gravity turn, and stage 3 after my apoapsis has reached the desired height. Stages 1 and 2 tend to have TWRs in the 1.5-2 range, whereas stage 3 tends to be about 0.7-0.9. Tends to be more efficient. My usual ascent time, between liftoff and MECO, is about 4:30. I tend to use my transfer stages to circularize, to cut down on debris (stage 3 falls back down).

[rodion]

Some of my heavier lifters have transient drops in velocity between initial stages but it doesn't bother me too much, as long as it's before I start turning for lateral acceleration. I assume it is taking a suboptimal ascent through the atmosphere if it does that, but as long as it gets the intended gear to orbit, it's fine by me.

[Kasuha]

After giving it some thought I noticed that what I constructed for the first stage of "Orange to Orbit to Orbit" challenge (which is just optimized asparagus lifter) uses less fuel to lift two full orange tanks than my normal lifter uses to lift one.

The difference is this.

Classic asparagus staging (unfolded and displaying only half of it) looks like this:

>O[----FUEL----][Payload]

>O[----FUEL----]

>O[----FUEL----]

>O[----FUEL----]

With each stage feeding fuel to stages above. The chalenge lifter looked like this:

>O[FUEL][Payload]

>O[FUEL]

>O[------FUEL------]

>O[------FUEL------]

The idea is that only one engine - the one right below the payload - is what lifts the payload. Everything else just lifts additional fuel for this engine. With such architecture the ship's velocity really shouldn't drop with staging, or the effect should be minimal - just what's caused by drag. Each stage individually should be able to lift the payload from the ground.

I guess I'll give it a try.

[blizzy78]

I'm with Kasuha here. Velocity should drop when you ditch a stage because of the reduced thrust.

Err. Somehow I must have been confused and been mixing things up. Guess I need more coffee.

[Diazo]

Err, now I'm confused.

Velocity is how fast your ship is traveling at this instant, acceleration is how fast it changes.

The only reason your velocity would drop is if your thrust fell below a TWR of 1, or you are going so fast you are losing a significant amount of thrust energy to drag.

For a vessel to go from 150m/s to 130m/s in that second after a stage burns out and before it is jettisoned, is your thrust actually below a TWR of 1 for you guys during that time?

I'm now going to go off and see what actually happens in game, but for velocity to drop you have to experience a negative acceleration from somewhere.

D.

[blizzy78]

The only reason your velocity would drop is if your thrust fell below a TWR of 1, or you are going so fast you are losing a significant amount of thrust energy to drag.

must... resist... the urge... ah well, here goes. Let's hope I have it right this time

Assuming asparagus staging, your thrust will be lower than before when a stage burns out. Given enough drag, it will slow you down, thus lowering your velocity. TWR doesn't necessarily have to be below 1 at this point.

[Diazo]

I understand that (see my comment about TWR<1 or drag in my previous post).

I suppose it is a question of how much you are dropping per stage. I tend to take off at a pretty low TWR and dropping tanks quite often so I guess I just don't see the high drag effects being talked about.

My mid range design is typically 7 engines, 1 center and 6 symetrically mounted. I will stage 3 times on my way up, only dropping 2 engines each time as the tanks empty until only the center engine is left to circularize.

That means my first stage I go from 7 to 5 engines so I don't lose that much thrust (relatively) so the drag effects are not enough to give me a negative acceleration I guess.

(Still have not had a chance to check what my designs actually do in-game.)

D.

edit: I suppose to clarify, because I launch with a low TWR, my vertical velocity never stops increasing. I don't plateau at 150m/s for a few seconds to clear the lower atmosphere, I set my launches up so I just keep on going faster and faster vertically.

Which is where my surprise comes from, for my launches to lose vertical velocity, I'd have to experience -20m/s^2 to -30m/s^2 acceleration (including gravity in that) and that is a huge number.

edit the 2nd: To clarify further, my vertical acceleration certainly drops when a stage burns out, I am carrying a bunch of dead weight until I can jettison it. What I don't see is my velocity drop, it increases at a slower rate until I can jettison the stage, but it never stops increasing.

Edited October 10, 2013 by Diazo

[Kasuha]

That means my first stage I go from 7 to 5 engines so I don't lose that much thrust (relatively)

7 to 5 is loss of almost 30%. And empty tanks and engines you toss are definitely less than 30% of the current craft weight/drag.

so the drag effects are not enough to give me a negative acceleration I guess.

That doesn't depend on how much thrust you lose, rather on how fast you are going. If you are limited by drag at the moment, the loss of thrust will definitely slow you down. If you're going slow, you'll probably not lose anything.

[arq]

You really want to go roughly terminal velocity for the first 10km of ascent, otherwise you're wasting delta-V fighting gravity. If you're going terminal velocity, drag=weight, so drag forces are very significant in efficient ascents.

[RoboRay]

All that launch and staging info is good, but Bartybum was asking about injection stages, not launch stages.

For transfer injections, there is no minimum TWR requirement. Note, however, that craft with low TWRs may require several orbits to complete their injections.

Any time your total transfer injection becomes a significant fraction of your orbital period (for low-Kerbin orbit, more than 5 minutes or so) you can greatly improve your efficiency by breaking your injection up into a series of short burns (or periapsis kicks) split across subsequent orbits. This results in your apoapsis rising higher with each kick until you finally reach escape velocity and can complete the remainder of the injection normally, with a longer burn.

In such a case, your sequence of orbits will look something like this:

Edited October 10, 2013 by RoboRay

[blizzy78]

All that launch and staging info is good, but Bartybum was asking about injection stages, not launch stages.

In his post he was about "heavy lifters"...?

[RoboRay]

Well, how big an injection stage you need would drive your lift requirement.

I don't think he would say "injection" if only talking about launch requirements.

[rhoark]

It depends on the agility of the first stage. If you can be nearly horizontal by 25km, your periapsis will be off the ground by the time your apoapsis reaches the target, and you can circularize easily with .1 TWR. Something that spontaneously disassembles with a turn like that needs a higher TWR, or only launch to higher orbits. (My first rocket that could carry an orange tank as untapped payload couldn't turn well, so could not launch to a circular orbit under 150k)