Calculate change in velocity per second?

[Talfryn]

I've been playing KSP since it was a baby, and we were still thrilled by orbits and circularizing them... however, I've since become one of the mechjeb era. I want to change this, so I've been attempting to get to the various moons and planets mainly mechjeb free.

Getting on to the actual problem, I keep using the calculators to figure out phase and ejection angle, but the first wave of probes I'm sending are utilizing old MMI Ion engines (Because I find them nifty, and yeah. Spess probe.) so their thrust is pretty horrific, making ejection angles neigh impossible to get right.

So, using my head, I figured out if I could take the orbital period of a given circular orbit around a planet, divide by 360 to get seconds per degree, and then use my knowledge of the delta v per second of my craft, and the delta v requirements of the injection, I could figure out how many degrees before and after the suggested ejection angle I needed to burn for.

At this point I made a sad faced, dug around Wikipedia, and realized I didn't have the google fu to find the requisite calculation.

Somebody please help.

[Talfryn]

Or I could just be semi intelligent and realize that Newton's second law has me covered.

Somebody please reply to either confirm this or tell me I'm a bloody idiot.

[vexx32]

You don't need to calculate it exactly. Just do a quicksave (F5), look at the Map and note how far apart Kerbin and the planet you want to get to are (phase angle), and start your burn. Then, note down by how much you missed the target, and adjust your angle before burning accordingly after quick-loading your previous save (hold F9).

[Talfryn]

I know it's not needed, I just (unfortunately) go by the policy that I want things to not die too often, or miss tragically too often, so since I'm forgoing mechjeb for some things I want to keep the accuracy of those parts nice and good.

[Kosmo-not]

Since the mass of your rocket is decreasing as you are thrusting, you're going to need some calculus to figure out your burn time.

[Talfryn]

Heh, Kosmo-not, one, you're impressive, two it was done for an Ion Engine probe that wouldn't be losing mass, which was part of my issue because the thrust is so low.

[untitled]

If you want to find your final velocity, simply use the rocket equation:

http://ed-thelen.org/rocket-eq.html

[Talfryn]

The original need was to find acceleration per second rather than total delta v.

[untitled]

The original need was to find acceleration per second rather than total delta v.

We can easily derive that.

After t seconds, the mass of the rocket is m(t) = m0 - vt, where v is the volume/mass of fuel ejected after time t. (you can find this with the rocket motor specs)

At time t, a(t) = F / m(t), where F is the thrust of the rocket.

Plug in values for F, v, and t, and you have your acceleration at time t.

[felixar90]

Correct me if i'm wrong, but IRL, spaceships can't directly know their actual velocity right?

They have accelerometers so they know they acceleration, and use other means to find they velocity, like radar altimeter + knowledge of the gravitational parameter of the body.

It would make more sense to have a display of the current acceleration rather than the current velocity. But well KSP is not meant to be hyper realistic.

But I guess there would be nothing wrong with creating a new part + plugin that can just tell you your current acceleration...

... If fact, it would be cool if you really needed those parts... Like, once outside of the atmosphere, you can't tell your current altitude or velocity unless your ship embarks a radar altimeter and an accelerometer (if the command pod isn't).

[phoenix_ca]

Correct me if i'm wrong, but IRL, spaceships can't directly know their actual velocity right?

They have accelerometers so they know they acceleration, and use other means to find they velocity, like radar altimeter + knowledge of the gravitational parameter of the body.

It would make more sense to have a display of the current acceleration rather than the current velocity. But well KSP is not meant to be hyper realistic.

But I guess there would be nothing wrong with creating a new part + plugin that can just tell you your current acceleration...

... If fact, it would be cool if you really needed those parts... Like, once outside of the atmosphere, you can't tell your current altitude or velocity unless your ship embarks a radar altimeter and an accelerometer (if the command pod isn't).

I respectfully disagree, as that kind of hyper-realism would (a) make KSP less viable as a commercial product and ( it would hurt my brain to have to do so much math in real-time. Think about it; the Apollo missions didn't send on person out in a pod who had to do the whole thing themselves. They send three, all highly-skilled and -trained, who could do this kind of stuff in their heads, but they weren't alone. They had a simple (by our standards) guidance computer, and a swarm of intellectuals at mission control supporting them, doing calculations and checking figures so the astronauts could concentrate on what action they needed to do, not the complexities of the math that got them to the point where that particular action was necessary. In short, the work was spread-around to a lot of very bright people, so no one's brains melted from overload.

A lot of problems with "realism" are solved when you make the simple (logical) abstraction that a command pod likely contains a guidance computer of some sort, life support, and a radio link to a team of Kerbals back on Kerbin ready to help.

[untitled]

Correct me if i'm wrong, but IRL, spaceships can't directly know their actual velocity right?

They have accelerometers so they know they acceleration, and use other means to find they velocity, like radar altimeter + knowledge of the gravitational parameter of the body.

It would make more sense to have a display of the current acceleration rather than the current velocity. But well KSP is not meant to be hyper realistic.

But I guess there would be nothing wrong with creating a new part + plugin that can just tell you your current acceleration...

... If fact, it would be cool if you really needed those parts... Like, once outside of the atmosphere, you can't tell your current altitude or velocity unless your ship embarks a radar altimeter and an accelerometer (if the command pod isn't).

There are also pitot tubes that measure wind speed relative to your craft.

[PakledHostage]

Correct me if i'm wrong, but IRL, spaceships can't directly know their actual velocity right?

They have accelerometers so they know they acceleration, and use other means to find they velocity, like radar altimeter + knowledge of the gravitational parameter of the body.

IMUs are pretty accurate... They accumulate error in the longer term, but this can be corrected with things like radar, celestial navigation, etc. Data from an IMU can be used to calculate velocity in any reference frame you like.

[felixar90]

I respectfully disagree, as that kind of hyper-realism would (a) make KSP less viable as a commercial product and ( it would hurt my brain to have to do so much math in real-time. Think about it; the Apollo missions didn't send on person out in a pod who had to do the whole thing themselves. They send three, all highly-skilled and -trained, who could do this kind of stuff in their heads, but they weren't alone. They had a simple (by our standards) guidance computer, and a swarm of intellectuals at mission control supporting them, doing calculations and checking figures so the astronauts could concentrate on what action they needed to do, not the complexities of the math that got them to the point where that particular action was necessary. In short, the work was spread-around to a lot of very bright people, so no one's brains melted from overload.

A lot of problems with "realism" are solved when you make the simple (logical) abstraction that a command pod likely contains a guidance computer of some sort, life support, and a radio link to a team of Kerbals back on Kerbin ready to help.

I was just saying that if velocity is available to you, you acceleration should be available too... Linear & angular acceleration can be obtained very easily. The craft's computer then calculate the velocity. (not you)

(For Apollo I think every burn time was calculated in advance. They even had the burns time for a free return readily available, this is how they were able to bring back Apollo XIII using the LEM's engine)

There are also pitot tubes that measure wind speed relative to your craft.

I know about pitot tubes, that's why I was talking about outside the atmosphere.

IMUs are pretty accurate... They accumulate error in the longer term, but this can be corrected with things like radar, celestial navigation, etc. Data from an IMU can be used to calculate velocity in any reference frame you like.

Oh, cool that was interesting, thanks.