Orbital Tips

[CoolVikingCo]

For some reason, I just can't get into orbit without burning more than half of my total fuel. Can somebody please give me some advice? I'd appreciate it

[AlexinTokyo]

Best advice: Look in the tutorials subforum for tutorials on how to get to orbit.

If you're into videos, I'd recommend Scott Manley, or pebble_garden's 'Orbital Mechanics 101' (It's a sticky thread there).

If you're not into videos, there are also written tutorials that explain it very well.

As a basic summary, the most important thing is that getting to orbit is not about going 'up' as much as it is about going 'sideways' very fast.

In stock aerodynamics, basically go vertically up to (about) 10,000 m, then make a turn to the east (90° heading) at an angle of about 45° above the horizon. Once your apoapsis reaches your target altitude, cut the engines and start to coast. In map mode, create a maneuver node on your apoapsis, and drag the prograde handle until the predicted periapsis is above 70,000 m. Execute that burn, and congratulations, you're in orbit.

Note that this is not the most efficient ascent, even in pure stock. There are refinements to make. However, it's a pretty good starting point, to get used to what will get you to orbit and what won't.

ETA: Also, getting to orbit really is about half-way to anywhere, so if you really are only burning half your fuel, you're not doing that badly at all

[cicatrix]

This wholly depends of how much fuel you had in the beginning. I suppose you have no Kerbal Engineer/MechJeb or similar tools that would indicate the total delta V of your craft.

You can calculate it yourself basing on ship's mass / engine IsP, but it's boring and time-consuming.

In stock aero you need ~4300 m/s dV to get to orbit (~3600 with FAR) provided you make *ideal* ascent. Normally I'd have about ~4500 m/s dV for your lifter vehicle.

Your initial goal is to fight gravity / atmospheric drag, this means you should start off vertically up to a point the atmosphere gets thinner. Your thrust to weight ratio should be >1 but not by too much since too big TWR will result in greater drag losses. Powerful thrust is not ultimately a good thing because such engine has less IsP (burns more fuel and provides extra thrust you don't need). Your first stage TWR should be around 1.2-1.3 (1.5 at the most) and your final stage TWR can be 1 or even slightly less since you burn horizontally with it.

Then you need to turn your vehicle in the direction of your desired orbit and increase your horizontal velocity.

Once your apoapsis (the highest point of your trajectory) hits the desired altitude you stop burning and coast towards it (you might want to make additional short burns if you lose speed due to drag). Once you're at apoapsis turn your vehicle prograde and burn until your orbit is circular.

Non-modded, you will usually need to turn your rocket at 45 degree towards horizon at approx 10 km and gradually 'lower' your rocket parallel to the surface until you're out of the atmosphere.

Edited February 2, 2015 by cicatrix

[Panzerbeard]

Well, it would help if we could see some screenshots of your rocket, and if you told us what you're currently doing when you launch it

Generally speaking, though, getting to orbit will use up a lot of fuel; punching through that thick atmosphere and getting up to orbital velocity takes a hell of a lot of delta-v. As the saying goes, once you're in orbit you're halfway to anywhere. You can do a complete mun landing + return mission, even go interplanetary with less fuel than it takes to get into orbit from KSC.

[Taki117]

First off, while the above is very good advice, I'd like to know what you are currently doing. How are you getting to orbit, then we can refine that (And point out what you're doing right or wrong) so that you don't have to unlearn everything, just your bad habits.

[KerBlam]

Seems to me if you're getting to orbit with half your fuel you're doing ok.

[eonmoon]

for me to get to LKO ( low kerbal orbit) I use 5k delta v, as stated, burn strait up to 10km, then angle to 45 deg, then at about 30 k angle down to about 15 to 10 degrees while you apoapsis is at 50k, then once your orbit is at +75ish km you stop your burn, coast to your desired burn point and burn to orbit for about 1k delta v, ofcorse there are other ways, much much more effective ways, but this works for me.

[nekogod]

I've got a very boring video I made for another thread that shows me building a simple 2 stage rocket that goes to orbit and back. It's pure stock bar KER and DR (which leads to a hilarious chute failure).

It starts with 5.4kms delta v and makes orbit with over 700ms delta v left so around 4.7kms to orbit.

Not perfect, but as you can see in the video there is nothing fancy or complex about the ascent path, pretty much any craft with similar TWR's following a similar path will get similar results.

Edited February 2, 2015 by nekogod

[EtherDragon]

Burning more than half your fuel is actually pretty typical. It takes a considerable amount of energy to go from stopped (on the ground) to orbiting at altitude.

You'll learn about this over time, but this is one of the reasons that rockets have multiple stages.

Reaching Orbit is all about Isaac Newton, an extremely tall mountain, and a really big canon.

Isaac hypothesized and demonstrated mathematically that if a tall enough mountain existed such that a man could wheel a powerful canon to the top, a sufficiently powerful charge in the canon would send the canon ball completely around the Earth.

Launching your rocket is quite similar. First thing you do is launch from the launch pad to build up enough momentum to make a "mountain" (of momentum, instead of earth) and coast to the top. Once at the top, the rocket continues firing sideways to build up enough velocity to circumnavigate the whole earth.

Edited February 2, 2015 by EtherDragon

[Sigma88]

Burning more than half your fuel is actually pretty typical. It takes a considerable amount of energy to go from stopped (on the ground) to orbiting at altitude.

You'll learn about this over time, but this is one of the reasons that rockets have multiple stages.

Reaching Orbit is all about Isaac Newton, an extremely tall mountain, and a really big canon.

Isaac hypothesized and demonstrated mathematically that if a tall enough mountain existed such that a man could wheel a powerful canon to the top, a sufficiently powerful charge in the canon would send the canon ball completely around the Earth.

Launching your rocket is quite similar. First thing you do is launch from the launch pad to build up enough momentum to make a "mountain" (of momentum, instead of earth) and coast to the top. Once at the top, the rocket continues firing sideways to build up enough velocity to circumnavigate the whole earth.

So this is not possible on earth (no such mountains) but would be possible on mars?

[eonmoon]

actualy we have plans for a rail gun to lauch stuff into orbit that would be placed in the ocean at the equator, unfortunately no humans or live creatures ( 100G+ launch would splat the living thing)

[wibou]

So this is not possible on earth (no such mountains) but would be possible on mars?

That's actually quite an interesting question. Considering that the tallest summit in the solar system is on Mars (Olympus Mons), that Mars has a very thin atmosphere (and that said atmosphere is probably close to 0 on top of Olympus) and that the martian gravity is about 1/3 of Earth's gravity (and probably somewhat less on top of Olympus)... what kind of force would you need to achieve orbit if your starting point is the very summit?

I don't have the math required to give an answer but I am very curious about this...

[AlexinTokyo]

Well, the orbital speed can be calculated (let's assume a nice circular orbit):

μ = 48,828 km³s^-2 : Standard gravitational parameter for Mars (from Wikipedia)

r = 3,389.5 km + 21.229 km = 3,410.729 km : datum (mean) radius of Mars plus datum altitude of Olympus Mons' summit (also Wikipedia)

O_v = √(μ(2/r - 1/a)) = √(μ/r) : a is the orbit's semi-major axis and is equal to r for circular orbits

This works out to be 3.784 km/s, so still pretty fast

ETA: That means a tank gun with KE ammo (up to ~1.7 km/s muzzle velocity) couldn't do it, but a light-gas gun (wow, up to ~8.5 km/s!!) could, in spades.

Edited February 3, 2015 by AlexinTokyo

[Kryxal]

It would be on the order of 3 km/s (escape velocity is just over 5 km/s likely at datum point so somewhat less (but not a great deal) up there, orbital velocity is *.7071), not likely to be too practical. To accelerate an object that much in one second would take 1.5 km of distance, and over 300 g.

[CoolVikingCo]

OK guys, thanks for the quick answers!

[soulsource]

The fact that you use a lot of fuel during launch is perfectly normal, it's part of the physics behind rockets.

If a maneuver requires a certain change in velocity (ÃŽâ€v), the mass that the ship has before the maneuver has a fixed proportionality to the mass the craft has afterwards:

Given that a launch from Kerbin takes approximately 4550 m/s (in stock KSP) and a typical engine has a vacuum Isp of 370 s, this means, that the mass of your rocket on the ground has to be about 3.5 times higher than the mass it has when reaching orbit - the difference has to be fuel of course. This further means, that if you have zero payload, and weightless tanks and engines, you will use at least 7/9 of your fuel (the resource - not the bar displayed in the staging sequence, which I think is proportional to the remaining ÃŽâ€v of the current stage) just to reach orbit.

The rocket equation is unforgiving when it comes to ÃŽâ€v. There's a good reason why we didn't do any manned return missions to Mars yet in real life... They would probably not work out without orbital construction, or at least without orbital refueling...

[Sigma88]

Well, the orbital speed can be calculated (let's assume a nice circular orbit):

μ = 48,828 km³s^-2 : Standard gravitational parameter for Mars (from Wikipedia)

r = 3,389.5 km + 21.229 km = 3,410.729 km : datum (mean) radius of Mars plus datum altitude of Olympus Mons' summit (also Wikipedia)

O_v = √(μ(2/r - 1/a)) = √(μ/r) : a is the orbit's semi-major axis and is equal to r for circular orbits

This works out to be 3.784 km/s, so still pretty fast

ETA: That means a tank gun with KE ammo (up to ~1.7 km/s muzzle velocity) couldn't do it, but a light-gas gun (wow, up to ~8.5 km/s!!) could, in spades.

are those velocities calculated keeping in mind you're shooting inside earth's atmosphere? or maybe it's just something you can ignore since it's too low to see the difference?

[AlexinTokyo]

I'm not sure what you're asking, Sigma.

If you're referring to the Mars orbital velocity calculations I did, they're all using Martian values with the assumption that the Martian atmosphere is negligible at the summit of Olympus Mons (I believe this is a safe assumption, at least to a first-degree approximation, but I'm not an areologist).

If you're referring to the muzzle velocities, they were copied directly from the values given on the linked pages. For the tank gun, I assume this is at normal atmospheric pressure, and for the light-gas gun at it's normal operating pressure, whatever that is. These would presumably increase somewhat in a hard vacuum, but I can't find any links to indicate what effect air pressure / density has on muzzle velocity (plenty on ballistic effects after firing, but not on muzzle velocity).

[Specialist290]

EDIT: Nevermind, I'm having a bad day with reading comprehension today.

Edited February 4, 2015 by Specialist290
Didn't quite catch they were discussing the math for IRL Mars, not something in-game.