Let's talk staging efficiency.

[Colonel_Panic]

Hi, so for most missions so far I've been using a radially decoupled 4-stage 7-stack lifter either 1 or 2 tanks tall depending on mass needs (which I just today learned that you folks colloquially refer to as 'asparagus' staging), and often feeding fuel from the tanks of whatever ships I'm lifting into the second or third stage booster pair reasoning I can lift more with less this way and just refuel it later via orbital rendezvous.

What I'm getting into now though, is trying to build some extremely low-mass, high-ÃŽâ€v lifters for lander return vehicles, and I had a few questions with that regard, since ÃŽâ€v isn't the whole picture. There's also things like gravity and air drag to deal with, and fine tuning the TWR and ÃŽâ€v of each stage can make a difference in the total ÃŽâ€v needed to get into orbit. Take 'asparagus' staging for example. Is there a 'sweet spot' I should aim for in TWR per stage? do I want TWR to start high and get lower, or start lower and get higher, or stay the same throughout the ascent?

Will I get more total ÃŽâ€v by adding more fuel weight to the later stages thereby reducing the TWR/ÃŽâ€v of earlier ones, or the other way around? Is there any proven formula for maximum efficiency?

[J.Random]

imho, TWR should be constant or slightly decrease with every stage (excluding final stages). If it's increasing, it means that you were hauling excess mass on previous ascend stage for too long.

[EasyAce]

Hi, so for most missions so far I've been using a radially decoupled 4-stage 7-stack lifter either 1 or 2 tanks tall depending on mass needs (which I just today learned that you folks colloquially refer to as 'asparagus' staging), and often feeding fuel from the tanks of whatever ships I'm lifting into the second or third stage booster pair reasoning I can lift more with less this way and just refuel it later via orbital rendezvous.

What I'm getting into now though, is trying to build some extremely low-mass, high-ÃŽâ€v lifters for lander return vehicles, and I had a few questions with that regard, since ÃŽâ€v isn't the whole picture. There's also things like gravity and air drag to deal with, and fine tuning the TWR and ÃŽâ€v of each stage can make a difference in the total ÃŽâ€v needed to get into orbit. Take 'asparagus' staging for example. Is there a 'sweet spot' I should aim for in TWR per stage? do I want TWR to start high and get lower, or start lower and get higher, or stay the same throughout the ascent?

Will I get more total ÃŽâ€v by adding more fuel weight to the later stages thereby reducing the TWR/ÃŽâ€v of earlier ones, or the other way around? Is there any proven formula for maximum efficiency?

I've been experimenting with the ascent stage - comparing TWR and ÃŽâ€v. Since there is less gravity the higher you get, TWR is less important the higher you get.

I haven't played that much, but I usualy try to have TWR of 2.0 - 2.2 or higher at liftoff.

To get to orbit you need a total of about 4500ÃŽâ€v. To reach 13 km, where the atmosphere/drag and gravity realy looses its grip and the turning phase often starts, you usualy burn about 2000ÃŽâ€v.

My plan, since I want to save the Kerbals on ground ... is to not have the first stage to decouple before 2000ÃŽâ€v has been burned. This way I am sure that the first stage will land away from the Space Center

[EasyAce]

Uhm... and to answer your question. First of all make sure you have more than 2.0 TWR at liftoff. After that your TWR should stay at about 2.0 until you get to space (40km or higher). Then TWR isn't important until you once again need boosters and rockets to fight gravity.

In space you should focus on ISP and ÃŽâ€v.

[Horn Brain]

One thing I can tell you is not to feed fuel from your payload back to the lifter unless you need the thrust from the lifter. If the payload has enough thrust to do the job of finalizing your orbit, then it will almost surely be more efficient than a mainsail. This will mean less orbital refueling for you later.

The staging process is a trick to increase your effective mass ratio in the rocket equation, thereby giving you more dV. Drop as much dead weight as soon as possible. Ideally, you would drop every tank as soon as it is emptied, and drop every engine as soon as your TWR without it is around 1.5 to 2.0 (during the vertical ascent portion).

The problem with asparagus staging is that when you drop engines you are increasing the fuel fraction of the remaining vehicle. That's good for overall dV, but bad for TWR right after staging, and the lower your TWR, the more gravity losses you incur. The trick is to find the balance.

From my experience, the ideal ascent profile goes like this (ignoring noble objectives like not raining death on the launchsite):

Launch: High TWR first stage should sprint up to terminal velocity and then run out of fuel immediately afterwards and be dropped. This is where solids come in handy, but the first stage of a large asparagus stack does this job just as well.

Climb: To maintain terminal velocity, you only need a TWR of about 1.5 or higher in the lower atmosphere. This is when you can dump engines and tanks to save weight because you don't need the thrust until you pass 10 km or so.

Gravity Turn: I usually use an actual gravity turn where I learn when to tilt over and follow the prograde vector (first surface for aerodynamic reasons, then gradually shifting to orbital as drag diminishes) and still end up with a nice ascent profile. Here you want high TWR again because the atmosphere stops being important and you really just want to be going as fast as possible as soon as possible.

Apoapsis tuning and Circularization: At this point, I'm usually moving at 70 to 80% orbital velocity or higher and TWR is no longer important. If I did my gravity turn right, I should have plenty of time to apoapsis so I usually just use an LV-T45 or 30 for this stage, or maybe a poodle if it's heavy enough that weight is not a factor.

Those are my methods. I hope they are useful to you. Anyone who thinks I'm doing something very stupidly should feel free to chime in. I'm always trying to improve my lifters.

[magnemoe]

Uhm... and to answer your question. First of all make sure you have more than 2.0 TWR at liftoff. After that your TWR should stay at about 2.0 until you get to space (40km or higher). Then TWR isn't important until you once again need boosters and rockets to fight gravity.

In space you should focus on ISP and ÃŽâ€v.

Twr 2 at liftoff sound to much for me unless you use trashcans to give it an push. for very large designs you might want to have the outer asparagus with litle fuel to give you an kick. Else I'm happy with 1.6-1.7 yes it might be a bit low but it increases fast.

Over 10 km you want high twr for some time up to 60 km, after this you want high isp.

If you launch something with nuclear engines its typicaly an good idea to let it take over from here.

If you launch something with an powerful nuclear engine setup you might want to put this in the center of the asparagus. either in center or put the LV-N around an 2.5 meter tank so it can burn between the tanks below.

This will increase twr and isp. Pipe fuel up to its tank so its topped at seperation.

[ComradeGoat]

2 is the upper reaches of where I put my TWR for liftoff. With Asparagus Staging, you want your core stages to provide a disproportionate amount of the thrust, so you don't suddenly find yourself thrust deficient when a staging event robs you of two engines. Lately I've been doing this with the last asparagus stage consisting of a couple of mainsails on tiny fuel tanks. The outer tank engines do little more than provide enough thrust to lift the weight of the fuel in the stack above them. This reduces stresses on the rocket too.

[Temstar]

I find TWR of around 1.7 to be optimal at lift off for asparagus staging. You want a big core stage for the asparagus (I personally recommend a core that makes up about 22% of the total lift off thrust) to ensure a smoother TWR curve through the different stages.

Here's an example of one of my asparagus staged lifters, note the TWR for each stage:

With payload engines, if possible you want to also fire them at lift off to contribute their thrust to the whole vehicle. What you'll do is then to pipe the fuel from the asparagus core stage up to the payload to make sure it's fully fuelled at payload release. By firing the payload engine you increase your TWR for all stages, smoothing out the TWR curve as well as possibly letting you get away with less engines on the rocket while.

[EasyAce]

Ok, I guess my goal of having >2.0 TWR is a little steep, but I feel like getting of the ground and as fast as possible into low gravity is fuel efficient.

2.0 lookes great as your rocket immediatly propell from the ground.

[Temstar]

Ok, I guess my goal of having >2.0 TWR is a little steep, but I feel like getting of the ground and as fast as possible into low gravity is fuel efficient.

2.0 lookes great as your rocket immediatly propell from the ground.

The problem is for the ideal ascent profile you want your average TWR to be 2. Since your TWR is going to go up as you burn fuel you wan to start below 2 so that the low end and high end balance out to be around 2. And that's also why asparagus staging ditches engines and gain efficiency - if you don't ditch engine you either have too much TWR as your rocket becomes lighter, or if you're throttle back to keep the rocket at TWR=2 you're hauling up more engines than you need to maintain TWR=2.

[EasyAce]

The problem is for the ideal ascent profile you want your average TWR to be 2. Since your TWR is going to go up as you burn fuel you wan to start below 2 so that the low end and high end balance out to be around 2. And that's also why asparagus staging ditches engines and gain efficiency - if you don't ditch engine you either have too much TWR as your rocket becomes lighter, or if you're throttle back to keep the rocket at TWR=2 you're hauling up more engines than you need to maintain TWR=2.

Good point

[ComradeGoat]

You want to go slow nearer the ground as air resistance increases with the square of the velocity. Gravity near the surface barely drops off at all: astronauts on the ISS weigh almost the same as they do on Earth. They're "weightless" because they're in orbit, I.e. free fall. The way to do less work against gravity is by turning east after takeoff, but that has the balanced with the longer time spent in thicker air as a result, which is why we generally go straight up for the first ten thousand metres.

[JaySmoka]

I find TWR of around 1.7 to be optimal at lift off for asparagus staging. You want a big core stage for the asparagus (I personally recommend a core that makes up about 22% of the total lift off thrust) to ensure a smoother TWR curve through the different stages.

After many tries, I found that Temstar's method is the best for me. I made up some table to help building my rockets and it's quite helpful... I had no more problem to get my things in orbit. The lifter get me to an orbit 85KM X 30KM. The tug is hauling the payload to a circular orbit while the lift fall back to Kerbin. No debris! yay!

[Nao]

Agree with temstar on the TWR, you want to have around 1,7 at launch so you approach 2,1-2,2 TWR just before stage burnout, and the next stage should have 1,7 TWR as well to keep the craft as close to terminal velocity as possible.

If you want to maximize Dv from staging make sure each stage has the same burn time. This is assuming same ISP, if you have ie: LV-N at the last stage, making it longer is ok.

It's also important to design in such way that you use 100% thrust of your engines all the time. If not then you just flying with dead engine weight. What that means for a rocket, especially one with long stage burn time is that starting TWR as low as 1.6 can be used and then it can go above 2.5 just before burnout. It's also important to Keep thrust at 100% even above terminal velocity if the next stage has low starting TWR (something MechJeb fails to do).

As for the TWR ascent profile, for me it mailny goes like this:

1) as much as resonable for initial acceleration, so if you have short 15-30s first stage 2.0 TWR and even more is good. (longer stages do well with 1,7-1,8)

2) ascent until 8-10km (start of gravity turn) around 2,0 TWR mid stage (that means 1,8 TWR at the start)

3) gravity turn (from 220m/s to 1100m/s or even 1600m/s if you use low thrust LV-N for last stage) needs actually the biggest TWR of around 2-3. This isn't a "hard" requirement and you will only shave 20-50m/s of Dv compared to standard 1,8 TWR but it's just something to look at, as i've seen and flown some really underpowered rockets during gravity turn loosing more than 100-200 Dv by that.

4) last stage up to orbit, TWR should be enough to complete circularization in one burn - usually 0.5-1

One last thing i would point out that Dv isn't 100% best way to judge rocket efficiency. Dv only shows current rocket capability. For example you can have a rocket that can deliver 10 ton load to LKO using 4200m/s Dv with every stage having 2.0 TWR. That's great but it would be possible to construct a 1.6 TWR rocket that would send 10 ton load in 4500m/s (seems worse) and having less takeoff mass. This is because engines weight a lot in KSP and even by burning more fuel you need to haul to space a lot less mass.

Launch weight isn't that important for Kerbin ascent's but its critical for landers. I would take a lander that uses more fuel (and Dv) any day if it were lighter for the same payload and mission.

Another interesting element of ascent is how TWR can change the attitude angle in relation to velocity vector.

Of course the most fuel efficient way to ascent is to keep the ship pointed as close to velocity vector as possible. But as the ship has finite number of stages that have different thrust and ISP's so its often not possible to follow the best path exactly, and a ship that can usually is heavier since its part's arent working at 100% all the time. This means that for a more tightly designed ship it can be beneficial to burn in other directions.

For example: Using an general design with low TWR LV-N last stage and LV-T30 gravity turn stage. You can use LV-T30 up to 1700m/s, stage and use LV-N low TWR to circularize.

But you can set up ship and flight profile in another way: use LV-T30 up to only 1300m/s (using less fuel) and then burn with low power LV-N at an high attitude angle (60deg angle will still be more fuel efficient in adding horizontal velocity since 800s ISP * cos60 = 400s -> more than 370s of LV-T30). The LV-N stage will slowly accelerate into orbit and burn more fuel itself but the net fuel used can be similar or lower.

There will be a Dv difference for the designs as the second one will have much more of it (longer burning LV-N stage) but it will use more to ascent to orbit, so the resulting Dv in orbit will also be similar.

This relation allows for more varied designs while still maintaining efficiency.

Welp that was a lot of text, hope you can use something from it Cheers!

(i hope i't was bearable for native english users, sorry for any mistakes)

[EasyAce]

You want to go slow nearer the ground as air resistance increases with the square of the velocity. Gravity near the surface barely drops off at all: astronauts on the ISS weigh almost the same as they do on Earth. They're "weightless" because they're in orbit, I.e. free fall. The way to do less work against gravity is by turning east after takeoff, but that has the balanced with the longer time spent in thicker air as a result, which is why we generally go straight up for the first ten thousand metres.

I learned this in physicsclass 10 years ago

Weight = Mass x gravity.

Since a person in orbit has a centripetal force that equals the earth gravity - it means he has no force that moves him towards the ground = no gravity.

Weight = Mass x 0 = 0.

A person in space with no force pulling on him has no weight.

If he had no speed relatively to the ground, i.e. no centripetal force, and was standing on a fixed plateau - the earth gravity would pull him against it and he would have weight. What the gravity would be... - is math

[ComradeGoat]

And the way you maximise that centripetal effect countering gravity is to go *sideways* fast, I,e, an orbit. Trying to thrust as fast as possible straight up is pointless; you aren't doing anything useful to nullify the effect of gravity, but instead using fuel to push air around. Unless your rocket is horrendously underpowered, the force you want to work to minimise early in the flight is air resistance.

[Brabbit1987]

I learned this in physicsclass 10 years ago

Weight = Mass x gravity.

Since a person in orbit has a centripetal force that equals the earth gravity - it means he has no force that moves him towards the ground = no gravity.

Weight = Mass x 0 = 0.

A person in space with no force pulling on him has no weight.

If he had no speed relatively to the ground, i.e. no centripetal force, and was standing on a fixed plateau - the earth gravity would pull him against it and he would have weight. What the gravity would be... - is math

First of all, just because you have a centripetal force that equals the earths gravity, doesn't mean the earths gravity disappears. It is still there pulling on you. Weightlessness is caused because you are still technically falling back to earth but you keep missing. "No gravity" is a huge misconception and simply isn't true.

Edit: Keep in mind, I am talking about orbits here.

Edited May 13, 2013 by Brabbit1987

[foamyesque]

Kerbin's so small, though, that there actually is a major reduction in gravity by the time you're in orbit; I'm currently staring at a 6m/s^2 local gravity at 165km.

[EasyAce]

Hey, don't missunderstand me

I never wrote that the gravity dissapeared. I responded to ComradeGoat who wrote that an astronaut had almost the same weight in space as he/she has on ground. Which is wrong. To have weight you need to have a force pulling you towards something that pushes in the opposite direction. An astronaut is in free fall and has no weight - only mass and speed. He is affected by gravity at any time.

Any way, I agree that TWR lower than 1.8, with an avereage of 2.0 is better than starting of with 2.2++.

Physics and KSP is fun!

[tuguley]

From experience, a TWR of 1.85-1.95 on the launch pad is in the sweet spot for lift-off. 2 or higher eventually results in a thrust which is too high because weight reduces as fuel is burnt.

From then on you can reduce the TWR of each stage and aim for around 1.2-1.4TWR during your gravity turn.

No real math backing these numbers. They just work for the hundreds of payloads I've put to orbit.

[PakledHostage]

A couple of threads for reference:

Closette's Mini-challenge: max altitude with this supplied spacecraft

My Optimal Ascent Profile for this spacecraft

Both challenges are old enough that the configurations used will no longer work the same way if built using the game’s current parts, but the findings are still valid. To my knowledge, Kerbin's mass, the drag model and Kerbin's atmospheric properties haven't changed since those experiments were undertaken.

Many forum members made exceptional contributions to these two challenges. Indeed, if I remember correctly, some of the analysis and programming that was done by TheDuck, et al in Closette’s challenge was even incorporated into MechJeb’s ascent autopilot.

[Qumefox]

I personally usually start my gravity turn almost right off the pad. Not hugely, just a couple of degrees to the east, and if you time it right, gravity will have pulled your prograde to around 45 degrees shortly after you pass 10km. This lets you just follow the prograde vector to orbit with very little off axis (and thus, wasted) thrusting needed. The long burns deep in the gravity well of kerbin also maximise the oberth effect benefits. You can actually shave quite a bit of delta-V off a launch by doing a proper gravity turn like this and doing most of your burning low, as opposed to burning straight up to 10km then turning.. The higher you are, the less benefit you get from the oberth effect.

[foamyesque]

Pure altitude doesn't generate an Oberth effect, as far as I am aware. It's a velocity phenomenon.

[Reavermyst]

Be aware that getting at TWR of 3 or more can be dangerous for your rocket! XD I decided to get silly and use bobcat's Jool V rockets to launch things...... 15 TWR.... The rocket went up, pilot stayed grounded xD like something out of those old cartoons!

[Qumefox]

Pure altitude doesn't generate an Oberth effect, as far as I am aware. It's a velocity phenomenon.

It's related to both velocity and gravity. The deeper you are in the gravity well, the more pronounced the effect will be for a given velocity.