TWR and Terminal Velocity

[Northstar1989]

You mean decreases, right? The drag coefficient tends to go up until Mach 1 (rising rapidly when it hits the drag divergence mach number), and then it falls pretty sharply.

No, I meant to say increases. Read what I said carefully- I said *drag* increases, not the drag coefficient. The drag coefficient decreases, but actual total levels of drag continue to increase. Drag approximately forms an "S-curve" (gradual increase --> rapid increase around Mach 1 --> gradual decrease)

Regards,

Northstar

[LostOblivion]

For real rockets, and FAR if you build plausible rockets, terminal velocity doesn't really matter, as it tends to be quite high to begin with and then race away as you go up, so this is (ironically) more relevant for the borked stock aero model.

[wumpus]

There's nothing magical about a TWR of 2 that keeps you at terminal velocity (TV).

Huh? Terminal velocity is the speed an object reaches when it is subjected to 1g (typically by freefall) in the atmosphere. When you fire off a rocket straight up with a TWR of 2.0, you lose 1g to gravity and accelerate at 1.0g. It isn't that far off to say that terminal velocity is defined by a TWR of 2. This will cause you to asymptotically approach terminal velocity. If you have a TWR>2 you will actually hit terminal velocity, but will need to throttle down a bit once you hit it to conserve fuel (assuming you have the liquid rockets to throttle). The only magic here is having terminal velocity be the path of least resistance.

One thing I haven't seen mentioned much is that KER will give you two numbers (per stage) for TWR. When using SRBs, I've found that a TWR closer to 1.8 works better (in stock, .90 and before) as it lets the rocket last longer in "the sweet spot" of ~100% atmospheric efficiency. For single-liquid stage + SRBs, I often find it hard to keep the throttle near 100% (a better explanation is that price-effective configurations don't always get close to 100% atmospheric efficiency. Don't sweat it too much.)

Quick question: from memory, gravity at LEO is something like .95g. Is gravity at KEO similar? Judging from the wierd density Kerwin has I would say not, but the size of the atmosphere might be scaled to make it so.

[Red Iron Crown]

Huh? Terminal velocity is the speed an object reaches when it is subjected to 1g (typically by freefall) in the atmosphere. When you fire off a rocket straight up with a TWR of 2.0, you lose 1g to gravity and accelerate at 1.0g. It isn't that far off to say that terminal velocity is defined by a TWR of 2. This will cause you to asymptotically approach terminal velocity. If you have a TWR>2 you will actually hit terminal velocity, but will need to throttle down a bit once you hit it to conserve fuel (assuming you have the liquid rockets to throttle). The only magic here is having terminal velocity be the path of least resistance.

I'd explain, but I already did so in the post you quoted. It's as if you only read the first line.

One thing I haven't seen mentioned much is that KER will give you two numbers (per stage) for TWR. When using SRBs, I've found that a TWR closer to 1.8 works better (in stock, .90 and before) as it lets the rocket last longer in "the sweet spot" of ~100% atmospheric efficiency. For single-liquid stage + SRBs, I often find it hard to keep the throttle near 100% (a better explanation is that price-effective configurations don't always get close to 100% atmospheric efficiency. Don't sweat it too much.)

The reason you're not seeing much mention of it is that KER did not have that feature when most of this thread was written, last May.

Quick question: from memory, gravity at LEO is something like .95g. Is gravity at KEO similar? Judging from the wierd density Kerwin has I would say not, but the size of the atmosphere might be scaled to make it so.

Kerbin gravity falls off more quickly with altitude than Earth gravity does.

[GoSlash27]

I'm not sure why this thread got necro'd, but the original premise is flawed.

As a ship tilts prograde, terminal velocity increases at a slower and slower rate. Accordingly, you need less and less thrust to maintain terminal velocity.

The whole "terminal velocity rises exponentially" thing only applies when you're climbing vertically, which is a fairly small part of the launch profile.

Best,

-Slashy

[Northstar1989]

I'm not sure why this thread got necro'd, but the original premise is flawed.

As a ship tilts prograde, terminal velocity increases at a slower and slower rate. Accordingly, you need less and less thrust to maintain terminal velocity.

The whole "terminal velocity rises exponentially" thing only applies when you're climbing vertically, which is a fairly small part of the launch profile.

Best,

-Slashy

Not sure why this was necroed, either, but all this is obsolete anyways. With the new drag-model coming out it 1.0, terminal velocity *should* be so high on the launchpad with a large and streamlined rocket that it's virtually impossible to catch up to. Thus, your main aerodynamic concern becomes maintaining stability and a dynamic pressure that is manageable (shape-based drag will now make the Angle of Attack and dynamic pressure on a rocket a lot more of a concern...)

Regards,

Northstar