WTF is... Terminal Velocity?

[cy-one]

Hey guys,

I stumbled over this term several times, including MJ-Ascent Guide. I also looked into wikipedia, but I don't really get what it means. What is the advantage of limiting the speed to Terminal Velocity? What is it, simply explained?

cheers, cy

Edited July 31, 2013 by cy-one

[Lheim]

Terminal velocity is the natural velocity that a falling object will reach when gravity acceleration downward is balanced by drag from the atmosphere. Imagine a skydiver. If he spreads himself out horizontally, he increases his drag, and the rate he's falling at will slow a bit. If he balls himself up, or dives down head first, he'll fall faster. When he deploys his parachute, that's just massively increasing his drag and slowing his descent.

I'm not sure how it works in real life, but in KSP if you're moving at much faster than that in a thick atmosphere you're wasting energy fighting the air.

[Skorpychan]

Generally, terminal velocity is the limit of how fast you can fall in an atmosphere, due to drag.

Empty space has no such speed limit.

[cy-one]

So, limiting my speed to terminal velocity will result in a more fuel-efficient ascent?

I'll have to fiddle a little bit around with that.. Because I would guess that while I need less fuel on one side, I need more fuel because I will stay in atmosphere longer, because I'm ascending slower... Mhm, thank you guys

[UbioZur]

Terminal Velocity is the speed at which your rocket don't accelerate anymore and so you are just wasting fuel.

Without going too much into details (that I do not know), the faster you go, the more you try to push the air in front of you which create more drag.

At a certain velocity (for a given atmospheric pressure, so air density), your rocket is no more accelerating but actually just creating more drag.

It's like trying to push something, and the more you push the harder it gets to push it. so it's more efficient to find the sweet spot between pushing it at a good rate without making it too difficult.

There are the value on the wiki (the table with altitude and speed): http://wiki.kerbalspaceprogram.com/wiki/Kerbin

I personally just try to have about 100m/s for 5000km, 200m/s for 10000km, it's a bit lower than the optimized value but they are easier to understand.

[Xeldrak]

Allright, if you let an object fall from a skyscraper, it will not accelerate all the way through, but at some point the force of gravity will be canceled out by drag. So, at some point it's speed will be constant. Think of a feather, it hits its terminal velocity almost the moment you let it go.

But I guess you heard it somewhere around here in the context of "rockets". So, if you start a rocket and it accelerates beyond it's terminal velocity, you will basically burn lots of fuel just to fight the drag. You can save fuel by sticking to your terminal velocity and waiting for thinner atmosphere. Herem, drag is obviously less of a factor, so your terminal velocity will be higher.

[balu0]

Terminal velocity is the speed where you will stop accelerating even if you apply more thrust because the drag will be soo high. That is why the MJ autopilot will throttle down to save you fuel, you could not accelerate more anyway. (Terminal Velocity gets higher as you go up, less air less drag)

[Lheim]

Shame you can't find it online, but mythbusters did a great episode where they took a look at the Point Break skydiving scene. 'Airplane Hour'. Very instructive.

They actually had an experienced skydiver do a head first dive and he managed to catch up and then pass a pair of people falling horizontally after a significant lead.

Edited July 31, 2013 by Lheim

[matzusdog]

Incidentally, a falling cat has a terminal velocity of around 60 mph, which is why cats can survive falls from a great height. A humans terminal velocity is over 100 mph, which usually results in a terminal impact.

[tavert]

Terminal velocity is the speed where you will stop accelerating even if you apply more thrust because the drag will be soo high. That is why the MJ autopilot will throttle down to save you fuel, you could not accelerate more anyway. (Terminal Velocity gets higher as you go up, less air less drag)

Several people seem to have this impression, which is incorrect. Terminal velocity is defined in the absence of forces other than gravity and drag, so rockets or aircraft are capable of exceeding terminal velocity by applying thrust. Terminal velocity is the equilibrium speed of a free-falling object, which will be falling downwards. For a rocket ascending upwards, terminal velocity is also the speed at which the acceleration due to gravity and the acceleration due to drag are equal, however both drag and gravity will be acting in the same direction (down), as opposed to acting in opposite directions and cancelling each other out for the free-falling object.

If the density of the atmosphere didn't change as you ascended and if your rocket's mass didn't change as you burn fuel, then you would need a thrust-to-weight ratio of exactly 2 to maintain terminal velocity. Lower than 2 and your full-throttle equilibrium velocity (at which drag plus gravity equals thrust) would be lower than terminal velocity, higher than 2 and your full-throttle equilibrium velocity would be higher than terminal velocity. For the vertical-ascent portion of a launch, you use the least fuel by ascending at terminal velocity. This is still true when the atmospheric density changes with altitude and when your mass changes with fuel use, but the critical TWR is not exactly equal to 2 because of these effects.

[PrivateFlip]

Several people seem to have this impression, which is incorrect. Terminal velocity is defined in the absence of forces other than gravity and drag, so rockets or aircraft are capable of exceeding terminal velocity by applying thrust. Terminal velocity is the equilibrium speed of a free-falling object, which will be falling downwards. For a rocket ascending upwards, terminal velocity is also the speed at which the acceleration due to gravity and the acceleration due to drag are equal, however both drag and gravity will be acting in the same direction (down), as opposed to acting in opposite directions and cancelling each other out for the free-falling object.

If the density of the atmosphere didn't change as you ascended and if your rocket's mass didn't change as you burn fuel, then you would need a thrust-to-weight ratio of exactly 2 to maintain terminal velocity. Lower than 2 and your full-throttle equilibrium velocity (at which drag plus gravity equals thrust) would be lower than terminal velocity, higher than 2 and your full-throttle equilibrium velocity would be higher than terminal velocity. For the vertical-ascent portion of a launch, you use the least fuel by ascending at terminal velocity. This is still true when the atmospheric density changes with altitude and when your mass changes with fuel use, but the critical TWR is not exactly equal to 2 because of these effects.

Aha! Knew what terminal velocity was but never understood what the link was with rockets going up. I imagined the terminal velocity just happened to coincide with what is considered a nice balance between speed and drag.

So flying exactly at terminal velocity is the most fuel efficient for every rocket, period?

[Toshogu]

Terminal Velocity is the fastest an object can fall toward the surface of a planet.

i.e. a feather has a slower terminal velocity compared to a bowling bowl. a bowling bowl will have a slower terminal velocity compared to an object of the same mass, but with an aerodynamic shape. That same feather on earth will have a different terminal velocity on the moon, and because there is no air on the moon have the same terminal velocity as the bowling bowl.

Terminal Velocity = maximum speed **** can fall on a given planet

[tomf]

The energy lost to drag is proportional to your velocity squared.

At terminal velocity the gravity drag and the air drag are equal. Any faster and the energy lost to air drag climbs rapidly and the energy lost to gravity falls slowly, and slower and the energy lost to gravity climbs quickly instead.

[Synapse]

http://forum.kerbalspaceprogram.com/showthread.php/43734-Optimal-Ascent-Path Just posted this

[Stochasty]

So flying exactly at terminal velocity is the most fuel efficient for every rocket, period?

For a vertical ascent, if the density of air were constant, flying at terminal velocity is the most fuel efficient method. For air density exponentially decreasing with altitude (such as for KSP), it also happens to be the case that terminal velocity is the most efficient for vertical ascent (although in this case you will need a TWR somewhat higher than 2 to maintain the correct speed).

For non-vertical ascent, or for air density profiles that differ from exponential, the problem is much more difficult to solve, and it is not obvious that terminal velocity is the correct speed for maximum efficiency; however, typically, during the non-vertical portion of your ascent you should already be clear of the thick part of the atmosphere and so drag losses should be less important. Thus, "always terminal velocity" is a good rule of thumb to follow.

[lammatt]

for a freefailing object

when mg = buoyancy force

it experience a zero net external force and hence no acceleration

and that particular velocity at which the buoyancy force cancels the weight is the terminal velocity

while the buoyancy depends on density and the interfacial area and contact angle and other parameters... the terminal velocity is different at different altitude.

[kiwiak]

SOme graph showing drag forces/velocity on cerrtain altitude (lets say 1000m) woudl really help me understand this.

[lajoswinkler]

Incidentally, a falling cat has a terminal velocity of around 60 mph, which is why cats can survive falls from a great height. A humans terminal velocity is over 100 mph, which usually results in a terminal impact.

Unfortunatelly that's a myth. I don't know the terminal velocity of cats, but they do not survive falling from great heights. 10 metres will cause damage to their feet. Broken tendons, at least. Damage increases at a steep exponential rate as you increase the height. Dislocated joints and fractures appear.

The fact they can twist rapidly to ensure they don't fall on their spine gives them an advantage at low heights, till around 5 metres.

They're cats, not tiny fluffy squirrels.

SOme graph showing drag forces/velocity on cerrtain altitude (lets say 1000m) woudl really help me understand this.

You don't need graphs and tables.

Just pay attention to the accelerometer next to the navigation ball while you're launching. Make sure it stays in the green area (1-2 G). That's all you need to do. If it climbs to 2 G, lower the throttle. If it falls below 1 G, your vertical speed falls down below zero (you're essentially starting to fall down). If it's exactly 1 G, you are either hovering or climbing at a constant speed.

[kiwiak]

You don't need graphs and tables.

Just pay attention to the accelerometer next to the navigation ball while you're launching. Make sure it stays in the green area (1-2 G). That's all you need to do. If it climbs to 2 G, lower the throttle. If it falls below 1 G, your vertical speed falls down below zero (you're essentially starting to fall down). If it's exactly 1 G, you are either hovering or climbing at a constant speed.

thanks for advice.

but aside from just flying more officiently, i also want to understand everything. and i thought such graph woudl help me.

[Johnno]

Please do not resurrect months old threads. The graphs you're asking for are already linked from earlier posts in the thread, if you're interested then read the posts and click the links.

Thread closed.