Power without control is nothing

[Keymaster89]

I want to ask couple things that i didn't manage to find answers...

1) is the fuel consumption and the throttle directly proportional? i mean: if i throttle 100% i consume X fuel/sec, if i throttle 50% i consume exactly X/2 fuel/sec?

2) as the first one, is the trust also directly proportional to the throttle?

3) i hear often about fight atmosferic drag at high speed, but i don't understand it... so far, i set full throttle and keep going until orbit by the law of "shortest burns the better" but i have the bad suspect i'm wasting hundreds of dV for nothing... can someone explain this concept so i can correct my flight profile accordingly? i have no problem reaching orbit, even with 80-90 t payloads, but if i can save 100 or more units of fuel for my stations, i will be more careful for sure.

[blizzy78]

1) is the fuel consumption and the throttle directly proportional? i mean: if i throttle 100% i consume X fuel/sec, if i throttle 50% i consume exactly X/2 fuel/sec?

2) as the first one, is the trust also directly proportional to the throttle?

Yes and yes. In KSP, the Isp of an engine is only related to atmospheric pressure, so with that being constant (in space, for example), fuel consumption and thrust are linearly proportional to throttle.

3) i hear often about fight atmosferic drag at high speed, but i don't understand it...

The keyword is terminal velocity. At lower altitude, the atmosphere is thick enough so that you actually waste fuel by going faster than the terminal velocity. At higher altitudes, the atmosphere gets thinner, and terminal velocity goes way up.

[Geschosskopf]

3) i hear often about fight atmosferic drag at high speed, but i don't understand it... so far, i set full throttle and keep going until orbit by the law of "shortest burns the better" but i have the bad suspect i'm wasting hundreds of dV for nothing... can someone explain this concept so i can correct my flight profile accordingly? i have no problem reaching orbit, even with 80-90 t payloads, but if i can save 100 or more units of fuel for my stations, i will be more careful for sure.

At Kerbin, the atmosphere has a huge discontinuity at 10km. Below 10km, the air is thick and really doesn't change at all with altitude. But as soon as you cross 10km, the air suddenly loses about 1/3 of its density and falls off logarithmically from there. To observe this, build a jet-powered rocket in the VAB, just a probe core, an intake, a liquid-only tank, and a turbojet, and go straight up with this. Open the resources tab and look at the line for IntakeAir. It will stay at it's starting value until you cross 10km, then fall off rapidly.

So, as Blizzy says, going fast below 10km wastes fuel because of the air resistance. In general, you don't want to exceed about 150m/s below about 5km and don't exceed 170m/s until you pass 10km. This is a function of TWR, however. If your TWR isn't much above 1.0, then you probably won't have to worry about this because you won't get very fast until above 10km anyway. But if you've got lots of TWR, you'll have to throttle back slightly to keep your speed below the above numbers. If you use a lot of SRBs, then it's possible to throttle the liquid engine all the way to idle while the SRBs are burning. SRBs will often make you go too fast while too low, but this isn't really a problem because you're not burning any fuel you care about.

[meeper]

If you use a lot of SRBs, then it's possible to throttle the liquid engine all the way to idle while the SRBs are burning. SRBs will often make you go too fast while too low, but this isn't really a problem because you're not burning any fuel you care about.

Addendum to this: If you find this happening, you can always reduce the maximum allowable thrust that an SRB produces in the VAB.

[Red Iron Crown]

At Kerbin, the atmosphere has a huge discontinuity at 10km. Below 10km, the air is thick and really doesn't change at all with altitude. But as soon as you cross 10km, the air suddenly loses about 1/3 of its density and falls off logarithmically from there. To observe this, build a jet-powered rocket in the VAB, just a probe core, an intake, a liquid-only tank, and a turbojet, and go straight up with this. Open the resources tab and look at the line for IntakeAir. It will stay at it's starting value until you cross 10km, then fall off rapidly.

I thought KSP's atmospheric model used proper scale heights. The wiki certainly indicates that it does, and it seems to be borne out by the fact that terminal velocity changes continuously as altitude increases.

[Poynting]

You are trying to balance a tradeoff against drag and burn time. Longer burns are more inefficient when you are trying to gain altitude due to gravitational losses. However as your velocity increases drag increases as velocity squared so you begin to loosing delta v to drag losses. Terminal velocity is the optimum speed to travel whilst minimising both of these simultaneously.

Yes you only need to throttle back if you get above the terminal velocity for that height. With a low thrust to weight ratio this doesnt always happen and 100% throttle launches are possible. You should actually try to engineer your TWR match your acent velocity profile so that 100% throttle roughly follows terminal velocity. This will minimise your ship weight (TWR too high - ship has too many engines and therefore weighs more than it needs to) and gravitational losses (TWR too low -> longer than optimal burn). Obviously this all gets more complicated as you add many stages...

Yes kerbin has a smooth exponetial atmosphere. There is no discontinuity at 10km its just an illusion of the sharply dropping pressure and air density.

[Claw]

I thought KSP's atmospheric model used proper scale heights. The wiki certainly indicates that it does, and it seems to be borne out by the fact that terminal velocity changes continuously as altitude increases.

I think so. I think intake air is a bad example because the intakes are saturated at low altitude.

At 10km it seems like a dramatic change in the atmosphere, but that's because of the logrithmic dropoff.

And terminal velocity is about 260 m/s at 10km. Because of the simplified drag model of stock KSP, if you fly at terminal velocity you can save a lot of fuel. The wiki page below will give you terminal velocity at several points in Kerbin's atmosphere. I can post the equation if anyone wants it.

http://wiki.kerbalspaceprogram.com/wiki/Kerbin

Keymaster, I'm not sure if we've answered your #3 question of "why." Let us know if you want more.

EDIT: Also realize that flying at terminal velocity might not be the best ascent profile for a particular craft. So if your ship breaks up trying to do this in conjunction with, say a gravity turn, then don't do it.

Edited February 14, 2014 by Claw

[thescientist]

At Kerbin, the atmosphere has a huge discontinuity at 10km. Below 10km, the air is thick and really doesn't change at all with altitude. But as soon as you cross 10km, the air suddenly loses about 1/3 of its density and falls off logarithmically from there. To observe this, build a jet-powered rocket in the VAB, just a probe core, an intake, a liquid-only tank, and a turbojet, and go straight up with this. Open the resources tab and look at the line for IntakeAir. It will stay at it's starting value until you cross 10km, then fall off rapidly.

So, as Blizzy says, going fast below 10km wastes fuel because of the air resistance. In general, you don't want to exceed about 150m/s below about 5km and don't exceed 170m/s until you pass 10km.

...Unless are you playing with Ferram Aerospace Research. Maybe you know that KSP hasn't a realistical drag model that makes you ships very unaerodynamics at low altitudes, and this mod furnish a more accurate one that allow you to go faster and save Delta V (=fuel).

Anyway, I think you can go a little faster also in stock KSP than speeds listed above. Remember that every second spent in vertical ascent means fuel wasted to counteract gravity. The goal is to find a speed that don't waste too much energy both in drag and gravity. In stock KSP I used to keep speed...

>150 m/s if >3000 m

>250 m/s if >6500 m

>400 m/s if >10000m

The you can throttle up and go for the gravity turn.

[wilt57]

...Unless are you playing with Ferram Aerospace Research. Maybe you know that KSP hasn't a realistical drag model that makes you ships very unaerodynamics at low altitudes, and this mod furnish a more accurate one that allow you to go faster and save Delta V (=fuel).

Anyway, I think you can go a little faster also in stock KSP than speeds listed above. Remember that every second spent in vertical ascent means fuel wasted to counteract gravity. The goal is to find a speed that don't waste too much energy both in drag and gravity. In stock KSP I used to keep speed...

>150 m/s if >3000 m

>250 m/s if >6500 m

>400 m/s if >10000m

The you can throttle up and go for the gravity turn.

But you are still want to travel at terminal velocity. Remember, terminal velocity is where the drag acceleration equals the gravity acceleration. In stock KSP, all things have the same drag coefficient, except nose cones and parachutes, but their mass is usually a very small percentage of the total ship's mass. In the real world, aerodynamics play a more important role. Their purpose in this case is to increase the value of terminal velocity for any given altitude, because the ship has a lower drag acceleration.

In KSP we associate terminal velocity as a constant relative to the planet with the atmosphere we are trying to escape. In the real world terminal velocities vary depending upon the aerodynamics of the ships, in conjunction with the attributes of the planet with the atmosphere. I've never played with FAR, but what it should do is raise terminal velocities of ships as they are designed more aerodynamically. At this rate, you are requiring less delta-v because you can lower your losses to gravity. But, you should still want to limit yourself to the terminal velocity of your specific ship.

[Geschosskopf]

I thought KSP's atmospheric model used proper scale heights. The wiki certainly indicates that it does, and it seems to be borne out by the fact that terminal velocity changes continuously as altitude increases.

Yes, I've heard this all my life, and if you use MJ and turn on "limit to terminal velocity", you'll see your velocity increase with altitude below 10km. HOWEVER, in actual practice, I do think Kerbin's atmosphere has constant thickness up to 10km, but then suddenly drops down to the decay curve it should have been on from the get-go. MJ can be explained by having the calculated terminal velocities hardcoded into it, as it probably has no way of determining terminal velocity from its own observations during operation.

Seriously, it's not just IntakeAir that's constant from sea level up to 10km, then suddenly drops. It also affects airplane handling. Take off and go into a gentle climb at as constant a velocity both horizontal and vertical as you make it. Once you get this set while you're near the ground, the plane will fly hands-off maintaining this climb all the way up to 10km. But as soon as you cross that line, you lose a bunch of lift and the plane reacts to this. I suppose I could also test parachutes by tweaking them to fully open at 11-12km and see if the capsule suddenly slows down as it crosses 10km.

But this is all off-topic here. The bottom line is, if you have a big enough TWR, you don't want to go full throttle until you're above about 10km or you waste fuel.

[Red Iron Crown]

Yes, I've heard this all my life, and if you use MJ and turn on "limit to terminal velocity", you'll see your velocity increase with altitude below 10km. HOWEVER, in actual practice, I do think Kerbin's atmosphere has constant thickness up to 10km, but then suddenly drops down to the decay curve it should have been on from the get-go. MJ can be explained by having the calculated terminal velocities hardcoded into it, as it probably has no way of determining terminal velocity from its own observations during operation.

I don't know how MJ calculates it, I'm just going by the information on the wiki. Why quote a scale height for each body if it's not implemented? And why give a table of terminal velocities that vary by height below 10km? I can't see the advantage in having a logarithmic scale atmosphere above 10km but a linear one below. If anything, I would expect either all logarithmic or a stepwise approximation, not both.

Seriously, it's not just IntakeAir that's constant from sea level up to 10km, then suddenly drops. It also affects airplane handling. Take off and go into a gentle climb at as constant a velocity both horizontal and vertical as you make it. Once you get this set while you're near the ground, the plane will fly hands-off maintaining this climb all the way up to 10km. But as soon as you cross that line, you lose a bunch of lift and the plane reacts to this. I suppose I could also test parachutes by tweaking them to fully open at 11-12km and see if the capsule suddenly slows down as it crosses 10km.

Frankly, the aerodynamic model in KSP is bad enough that I don't think aircraft experiments demonstrate much of anything. I'd be interested in your parachute results, though.

[wilt57]

Deployed parachute or not, any object falling in the atmosphere will be traveling at terminal velocity. The only thing to watch is whether it has a drastic change in velocity when it passes through 10 km.

[EtherDragon]

Deployed parachute or not, any object falling in the atmosphere will be traveling at terminal velocity. The only thing to watch is whether it has a drastic change in velocity when it passes through 10 km.

This is a really good point as it brings in something that is often subtly overlooked - Terminal Velocity is different based on the drag profile of your craft. It's usually not radically different, but different. Ships with chutes deployed have much lower terminal velocities due to the drag of the chute.

How this pertains to OPs question? Well - only in so much as you can make an educated guess as to you terminal velocity during ascent. The most efficient ascent profile climbs at terminal velocity, and here is why:

As you ascend, you are overcoming gravity. The longer it takes to do so, the more fuel is "wasted" as a portion of your thrust is used overcoming gravity.

As you go faster, your drag increases. The faster you go, the more drag, and you end up "wasting" fuel as a portion of your thrust is used to push against the atmosphere.

Both of these items are competing goals - but there is an optimal configuration - that point where your speed is high enough to minimize time to orbit, but your speed is low enough to minimize drag. It so happens that this balance point is at terminal velocity.

But, that's not the whole story... More engines means higher dry mass, which means lower total Delta-V. So rocket designs also need to balance thrust-to-weight ratios against bringing useless empty mass. Given this factor, sometimes the optimal configuration isn't one that peaks at terminal velocity, but something lower - because the total Delta-V is higher due to less empty mass. <- That can really only be sussed out through test launches though.

Note: Configurations that are capable of going faster than terminal are always sub-optimal. You are simply carrying extra mass (in the form of powerful engines) that go unused fully, which will always reduce your total Delta-V - a double whammy: dV lost due to going faster than terminal and dV lost by having extra mass.

[numerobis]

Yes, I've heard this all my life, and if you use MJ and turn on "limit to terminal velocity", you'll see your velocity increase with altitude below 10km. HOWEVER, in actual practice, I do think Kerbin's atmosphere has constant thickness up to 10km, but then suddenly drops down to the decay curve it should have been on from the get-go.

Right-click on an engine. Notice the Isp improve as you climb? That's because air pressure is falling.

Right-click on an intake. Notice the airflow drop as you climb? That's because air pressure is falling.

Build a plane that takes off from the runway with 0 pitch at 50 m/s. Fly it with 0 pitch at 5km at 50 m/s. Notice how you aren't maintaining altitude? That's because air pressure is falling.

Build a rocket with two stages that each start with the same TWR. Fly at full throttle. Drop the first stage at 5km. Notice how your speed is higher at 6km than it was at 1km? That's because air pressure is falling.

Any other examples?

MJ can be explained by having the calculated terminal velocities hardcoded into it, as it probably has no way of determining terminal velocity from its own observations during operation.

MechJeb's terminal velocity calculator looks up the air density and drag values from the game, as you can ascertain from looking at its source code.

[numerobis]

For the OP: in vertical flight, drag losses grow with the square of your speed whereas gravity losses shrink linearly with your speed. To minimize your total losses, where the losses are equal is where it's optimal to be flying (in vertical flight; in horizontal flight it's not so clear). That is, where gravity = drag is where you want to be -- and that's the definition of terminal velocity. It takes TWR of about 2 to maintain that in the atmosphere, because it takes TWR of 1 to defeat gravity, and we just said you're losing as much to drag, so there goes another TWR (but drag falls fast with altitude so you need a tiny bit more to stay at terminal velocity). If you have no drag, then infinite throttle is what you want. Analyzing the vertical portion of flight is called the Goddard problem, and this "fly at terminal velocity" advice holds IRL, not just in KSP.

The fact that you can't have infinite throttle brings up another point: you have to add engines to get higher thrust, which costs you mass. In order to minimize mass (or, equivalently, maximize payload fraction) it turns out that with the stock KSP engines you want to reduce your thrust so that your TWR is around 1.5-1.8 throughout the lower parts of the flight, about to 30 or 40km, and even lower TWR above there. Then you fly full throttle the whole time, because even at full throttle you're well below terminal velocity.

[wilt57]

But, that's not the whole story... More engines means higher dry mass, which means lower total Delta-V. So rocket designs also need to balance thrust-to-weight ratios against bringing useless empty mass. Given this factor, sometimes the optimal configuration isn't one that peaks at terminal velocity, but something lower - because the total Delta-V is higher due to less empty mass. <- That can really only be sussed out through test launches though.

Note: Configurations that are capable of going faster than terminal are always sub-optimal. You are simply carrying extra mass (in the form of powerful engines) that go unused fully, which will always reduce your total Delta-V - a double whammy: dV lost due to going faster than terminal and dV lost by having extra mass.

Exactly. It turns out that the 4,500 number is pretty spot-on for fuel efficient ascents.

Remember OP, delta-v is not fuel usage, it's merely change in velocity. We can get to orbit by flying at terminal velocity as long as we can, and in doing so we can reduce our losses to gravity so much that we may be able to get to orbit using 4,300 delta-v. Maybe even less. Problem here, as EtherDragon has mentioned, is that we are carrying massive amounts of thrust high into the gravity well in order to keep up with the ever-increasing speed of terminal velocity. More thrust generally comes at the expense of more dead-engine mass, lower specific impulse, or a combination of both. These require us to pack more fuel to re-achieve our delta-v goal.

By using less thrust and smaller, more efficient engines we are capable of getting a given tonnage of payload to orbit using less fuel, even though we "used" more delta-v. The trick is finding that middle zone that gives us enough TWR and delta-v while using the least tonnage in our lifter. Payload fraction is more important than delta-v usage. As payload mass is constant, the only way to increase payload fraction is to reduce lifter mass. The majority of lifter mass is fuel. Of course, that only matters when costs become important.

[Geschosskopf]

I don't know how MJ calculates it, I'm just going by the information on the wiki. Why quote a scale height for each body if it's not implemented? And why give a table of terminal velocities that vary by height below 10km? I can't see the advantage in having a logarithmic scale atmosphere above 10km but a linear one below. If anything, I would expect either all logarithmic or a stepwise approximation, not both.

Well, for starters, I don't think the wiki is maintained by Squad but by a player.

But regardless of that, this sort of thing isn't a lack in implementation, it's a case of a bug. In something like this, you have a bunch of different modules, subroutines, and whatnot each contributing a little bit to the final result. And if one of them drops the ball, you get weird outcomes. Because no single module/subroutine/whatever does the entire job, it's very easy for the game's left hand not to know what its right hand is doing. Anyway, I'm sure Squad's intent was to do as shown in the wiki, but due to some bug, that's not exactly how it works.

[Keymaster89]

thanks for all the answers guys!

Ok, if i understand correctly:

the terminal velocity is a speed limit caused by massive drag with air, so trying to apply more velocity is pointless because it will be rapidly countered by this force.

the terminal velocity is reached easily with a TWR of 2 in the first 10Km in the atmosphere, so have TWR >> 2 commonly cause fuel waste, beyond that point the limit grow fast, so i don't have to care about until i have huge TWR.

the goal of a correct ascent profile is get a speed as close as possible to the terminal velocity without get over it.

In short: if i keep accelerate at a sustained rateo at full throttle, it means that i haven't reached terminal velocity, and i'm doing this right, otherwise i should slow down until i start to decelerate. Correct?

[Taki117]

thanks for all the answers guys!

Ok, if i understand correctly:

the terminal velocity is a speed limit caused by massive drag with air, so trying to apply more velocity is pointless because it will be rapidly countered by this force.

the terminal velocity is reached easily with a TWR of 2 in the first 10Km in the atmosphere, so have TWR >> 2 commonly cause fuel waste, beyond that point the limit grow fast, so i don't have to care about until i have huge TWR.

the goal of a correct ascent profile is get a speed as close as possible to the terminal velocity without get over it.

In short: if i keep accelerate at a sustained rateo at full throttle, it means that i haven't reached terminal velocity, and i'm doing this right, otherwise i should slow down until i start to decelerate. Correct?

Not quite. If you slow down and you begin decelerating then you are losing dV fighting gravity (because at that point gravity has taken over) you want to decrease your throttle to a point where you stop accelerating.

[StrandedonEarth]

The "Kerbal Engineer Redux" mod lists the current terminal velocity under the "surface" tab while launching. This makes it easier to keep your velocity a few m/s under terminal velocity

[SSSPutnik]

Isn't there a barometer science part?

[Claw]

the terminal velocity is a speed limit caused by massive drag with air, so trying to apply more velocity is pointless because it will be rapidly countered by this force.

Yes

the terminal velocity is reached easily with a TWR of 2 in the first 10Km in the atmosphere, so have TWR >> 2 commonly cause fuel waste, beyond that point the limit grow fast, so i don't have to care about until i have huge TWR.

"easy" is a relative term, but yeah, this is a good starting point.

the goal of a correct ascent profile is get a speed as close as possible to the terminal velocity without get over it.

For the most part. Gravity turns also play a big role. Something else for you to look into if you haven't already.

In short: if i keep accelerate at a sustained rateo at full throttle, it means that i haven't reached terminal velocity, and i'm doing this right, otherwise i should slow down until i start to decelerate. Correct?

Not quite. Like Taki was saying... When you keep it at full throttle with a TWR of 2.0, you WILL (and you want to) keep accelerating. The rate of acceleration will keep you roughly on track to maintaining terminal velocity during your ascent.

I'm not exactly sure what you mean by "i should slow down until i start to decelerate." But if you find yourself going faster than terminal velocity, then you would want to throttle back a bit. How much depends on the TWR, but you don't want to slam the throttle to idle. Just work it back. Eventually you'll get the hang of it.

Remember that terminal velocity is higher as you climb. So you WANT to accelerate as you climb out.

[thescientist]

*ignore this post*

Edited February 15, 2014 by thescientist
I haven't seen the post above

[Alchemist]

Atmosphere density is proper. The strange effect about 10 km mark (or slightly higher) when you stop having too much drag loss is only because the higher you are the faster the terminal velocity increases with altitude. And you suddenly find yourself in the situation when the terminal velocity grows faster than you can accelerate (and with you gaining more speed it grows even faster). Then you start the gravity turn increasing your lateral velocity while sustaining the vertical only at the point when you climb fast enough to not approach the terminal velocity with your total speed increase.