Fuel loses because of fighting gravity during ascent

[kiwiak]

Lets say we are launching fromm airless planet to eliminate termminal velocity issues etc.

Its most fuel efficient to launch witch highest thrust possible, right?

Im not sure i understand it.

But i heard that you have keep your twr as high as possible, to avoid loses of "fighting witch gravity".

So, how much fuel do i loose when i launch witch twr of 2 instead of dor example 3? What calculations woudl i have to maketo find out?

[Taki117]

No seriously, it describes how to do a proper gravity turn, and how efficient Gravity Turns are.

Now, Youw ant to keep your velocity below something known as Terminal Velocity. Terminal Velocity is the point at which your dV losses due to drag equal those due to gravity. Now, logically you would want to have a really high TWR to punch through the atmosphere as quickly as possible, but this is actually inefficient as you will more than likely exceed terminal velocity for most of the flight and waste fuel trying to go faster. If you have a lower TWR (Thrust to weight ratio) then you will not accelerate as fast and spend more time fighting gravity to increase your speed.

[breakthrough]

[thread=39812]Tavert[/thread] does all the math in a thread on the tutorial board. The TL;DR of it is more TWR is better, but it has decreasing returns so much above 2 won't make as big of a difference.

[maccollo]

Lets say we are launching fromm airless planet

On an airless planet you want to go sideways right away. Don't gravity turn, just go sideways and give minimal vertical thrust to counter gravity. As you build up speed you will be able to reduce the angle to the horizon and increase your efficiency.

Higher thrust means you can angle more towards the horizon faster, but that also means you have more rocket, and more mass, and less deltaV, so it's a balancing act.

I think the amount of fuel you waste burning vertically can be calculated with a cosine function, so if you aim 20 degrees above the horizon you are still 93% efficient, yet you're still using 34% of your thrust to fight gravity.

Edited February 4, 2014 by maccollo

[Streetwind]

The main tradeoff to "moar thrust = better" is the fact that increasing thrust usually comes in tandem with decreasing Isp. That means you'll be burning less fuel to fight gravity but you'll also be burning more fuel because your engine is less efficient.

[SuperBigD60]

I think ideally you want to point prograde or as close to it as possible, so the trick is to somehow be pointing prograde the whole way until you've made orbit.

Turning sideways very early is a decent way to accomplish this because burning radially while you're not moving as fast means it takes less dV to change direction (similar to why you want to do inclination burns at high altitudes).

burning in any direction other than prograde incurs steering losses, but that obviously has to be balanced against gravity losses from burning vertically in a gravity well as well as actually having to turn after taking off.

As for optimization, I usually just turn early and hope for the best. But you are definitely correct to say that higher thrust will reduces losses to gravity.

[kiwiak]

Can you actually explain what are gravity loses?

When i enter orbit, there are no gravity loses anymore, arent they? But im still in gravity well of body.

Ahh i wish i payed attenion on my science class.

[Khatharr]

The idea is to get into orbit as quickly as possible, so that you don't have to keep spending fuel to prevent yourself from lithobraking.

If you point straight up into the air and burn just enough to keep you from falling, then you're spending all of your fuel on fighting gravity and come back down in the same spot. If you angle 5 degrees to one side and keep yourself just above the ground, you'll still use all your fuel, but you'll build up a lot of horizontal speed as well, so you got more practical effect from your thrust.

In terms of orbit, if you shoot straight up to 100km and then start burning sideways, you still have to keep burning away from the ground a bit in order to keep from falling back to the ground before you get to orbital speed. If you think about how much force it takes to pop up to 100km vs how much force it takes to go from zero to 2km/s, you'll see that you need to generate a lot more horizontal speed than vertical.

Thus the most efficient launch would be to shoot almost straight sideways, since that's going to start pushing you toward orbital insertion right away. You still have to fight gravity on the way up, though, so you angle slightly so that you're climbing a bit. The optimal trajectory would be one that puts you at your desired orbital height just as you reach the necessary orbital speed. The problem on planets with atmosphere is that you also have to fight against air friction, and shooting straight sideways would keep you in atmo a lot longer, which would waste a lot more fuel. To counter this, you want to pop up above the low atmosphere, then start leaning gradually but aggressively toward your insertion angle.

Gravity losses are all that fuel you spend on fighting against gravity when you're entering or exiting an orbit. That's fuel that you won't get to spend on transfers once you get into space.

Once you're in orbit you don't have to fire the engines in order to stay aloft, so there are no gravity losses. You're technically still falling, but you're moving sideways so fast that you continually 'miss' the planet on the way down, and just continue around it in a circle.

[kiwiak]

Gravity losses are all that fuel you spend on fighting against gravity when you're entering or exiting an orbit. That's fuel that you won't get to spend on transfers once you get into space.

Once you're in orbit you don't have to fire the engines in order to stay aloft, so there are no gravity losses. You're technically still falling, but you're moving sideways so fast that you continually 'miss' the planet on the way down, and just continue around it in a circle.

So, what i understood is that TWR matters mostly when we are do circuralization burn, on top of the curve, so we dont have to to fight gravity long to stay on desired altitude. Until we get to desired altitude, it does not matter what twr is, until its as close to terminal velocity as possible (to avoid fighting atmosphere/spending more time in atmosphere than unnesessary), right?

And on airless worlds, twr during ascent can be very low, at least until we have to circuralize.

Also, can someone explain steering loses?

Edited February 5, 2014 by kiwiak

[Taki117]

So, what i understood is that TWR matters mostly when we are do circuralization burn, on top of the curve, so we dont have to to fight gravity long to stay on desired altitude. Until we get to desired altitude, it does not matter what twr is, until its as close to terminal velocity as possible (to avoid fighting atmosphere/spending more time in atmosphere than unnesessary), right?

And on airless worlds, twr during ascent can be very low, at least until we have to circuralize.

Also, can someone explain steering loses?

TWR is important during ascent up to the point you finish your circularization burn. on an airless world your TWR MUST be higher than 1 for the local body to get anywhere. Steering losses are the losses that you accrue when you are not thrusting along your prograde vector.

[EtherDragon]

The main issue with "fighting gravity" is this:

During your ascent as long as your Periapsis is below the surface of the planet some of your thrust is used to simply counteract gravity attempting to pull you back down.

Try this experiment to see what I mean - Start of on the Mun.

1. Take your typical launch angle (say 30 degrees above horizon) and ascend to a 20km apoapsis at 100% throttle. Once at Apoapsis, circularize to a 20x20km orbit, then record your remaining fuel.

2. Do the same test, but try it at 50% throttle.

3. Repeat again at 25% throttle.

As you lower your throttle (and thus TWR) you will see that as your ascent takes longer it actually ends up costing more fuel. The reason it took more fuel is you spent more time "climbing" against gravity on your way to the Apoapsis.

To further illustrate this - try a test where you just barely lift off (throttle so that TWR just above 1.0) and sort of hover away... you will find that you can't reach orbit - every second you spend hovering is a bunch of fuel used doing nothing but overcoming gravity. So the less time you spend doing so, the better.

Lastly - you correctly identified that Gravity is still pulling you, even when you are in orbit. But once you reach stable orbit, your momentum - the tendancy for your craft to go "that way" - is strong enough to counteract gravity's force trying to pull you down - and you stay up there forever (until you change it by applying more thrust).

Edited February 5, 2014 by EtherDragon

[DrMonte]

This gives some ideas for Tylo mission - you can launch from some higher mountain directly sideways (even down to gain speed faster)

Note that this literally applies for landing as well. There is a video on most efficient landing somewhere

[Kasuha]

When you launch on an airless planet, you can look at the matter completely from the orbital mechanics point of view. As soon as your landing gear stops touching the surface, you're in orbit. A very elliptical orbit, where you're almost at the apoapsis and the periapsis is buried deep underground near the center of the planet.

What's the most effective and least effective thing you can do near apoapsis?

Most effective is to raise your periapsis.

Least effective is to raise the apoapsis you're at.

And that's the matter. You need to raise your apoapsis somewhat. You don't want to fall back on the ground and you don't want to collide with some hill nearby. So you need to raise your apoapsis somewhat, to avoid terrain. But what you want to do the most is to pull your periapsis out of the ground and get to orbit.

The most effective approach is "apoapsis sailing", where you push your apoapsis right in front of your ship by slight above-prograde pitch and put all the rest of your thrust into pushing the periapsis out of the ground, i.e. acquiring orbital speed.

[Soda Popinski]

On an airless planet you want to go sideways right away. Don't gravity turn, just go sideways and give minimal vertical thrust to counter gravity. As you build up speed you will be able to reduce the angle to the horizon and increase your efficiency.

Within reason. I've done that and smacked into mountains because I didn't get my altitude high enough to clear them, thought I was "in orbit."

Can you actually explain what are gravity loses?

When i enter orbit, there are no gravity loses anymore, arent they? But im still in gravity well of body.

Funny, I don't recall orbital mechanics in my physics classes, at least not to this amount of detail.

Anyway, gravity loss is loss of energy (eg fuel) that isn't kept as potential energy. Launch a rocket and maintain a thrust to weight ratio of 1.0. This means you are hovering. ALL your fuel is going to gravity loss, as you aren't gaining altitude or getting into orbit (potential energy). You will eventually run out of fuel and end up exactly where you started off. Not very efficient.

So the less time you spend not in orbit, the less you lose to gravity. But of course, if you go too fast, you lose to wind resistance. Hence the balance at terminal velocity.

[Pecan]

When you launch on an airless planet, you can look at the matter completely from the orbital mechanics point of view. As soon as your landing gear stops touching the surface, you're in orbit. A very elliptical orbit, where you're almost at the apoapsis and the periapsis is buried deep underground near the center of the planet.

What's the most effective and least effective thing you can do near apoapsis?

Most effective is to raise your periapsis.

Least effective is to raise the apoapsis you're at.

And that's the matter. You need to raise your apoapsis somewhat. You don't want to fall back on the ground and you don't want to collide with some hill nearby. So you need to raise your apoapsis somewhat, to avoid terrain. But what you want to do the most is to pull your periapsis out of the ground and get to orbit.

The most effective approach is "apoapsis sailing", where you push your apoapsis right in front of your ship by slight above-prograde pitch and put all the rest of your thrust into pushing the periapsis out of the ground, i.e. acquiring orbital speed.

Wow!

Do you know, I think that's the simplest, clearest, explanation I've read anywhere. The whole (vacuum) launch summed-up in only a handful of sentences - well done :-)