Delta-v

And I was doing a presentation on China in space and they loved it.

So it was kind of surreal for me, right? So but anyway, so what the expense tells you is that delta V that is refueling.

So delta V is the ability to go from one point in space to the other.

Now, orbital mechanics as such or the inability of the current reality of warfare in space is that we do not have the ability to refuel.

We don't have it as yet.

the ability to collect sunlight in space and beam it down, but they're thinking about beaming it, say, to a spacecraft or a base that they will build, right? So very much what the expanse tells you.

And so

that idea of kind of the Moon being a logistics hub or a depot, the strategic high ground, and then the Delta V coming, you know, trying to get off of Earth, If you can be going and coming from the moon, you don't have that fuel constraint to break out of orbit.

The energy hurdle is staggering.

In astrodynamics, we measure these energy requirements in, uh, terms of delta V or the change in velocity required to perform a maneuver, and the delta V required to cancel Earth's prograde motion, initiate a retrograde trajectory, and execute a 162-degree plane change is just immense.

And this is where we hit the hard limits of traditional chemical rockets.

Oh, yeah.

The tyrant of the rocket equation.

Exactly.

The equation dictates that to achieve a higher delta V, you need to expel more propellant mass, but adding propellant increases the overall mass of the spacecraft.

To push that additional mass, you require even more propellant.

At a certain point, you know, you just have to accept it.

And maybe that's also, uh, a way you stop it, too, is, like, if you have a truly massive asteroid, you require so much delta V to alter that course, you might not care if people know what it is because only an absurdly powerful ship could actually alter its course, and you're not doing that in secrecy, right?

Yeah.

[laughs] That's true.

You know, maybe sometimes you get lucky and the, all the missiles are too far away, and you're able to accelerate, or, you know, maybe...

Like, there's a certain range that once a missile locks on, other than shooting it down, you cannot outrun it.

It just comes down to how much delta V each of you have.

Yeah.

And I was like, well, you can do it if you just crank up the voltage that you're using on the ion thruster high enough.

You run it at such and such a voltage and you get 27,000 seconds of ISP and one kilogram of fuel gives you so much delta V and you can fly all the way to Mars and back and you can do this whole mission.

And I was talking to him about it and he was like, that is the dumbest thing I've ever heard.

And he was right.

We're trying to build the smallest, lightest, most power-dense ion thrusters that have ever existed.

And when you do that, The math works out so that higher ISP is better.

Math works out so that higher power is better and more agility in orbit is better and more Delta V is possible.

Like it enables missions in a category that can't be done yet.

The equation relates four things, the velocity of the exhaust leaving the engine, the mass of the vehicle at the start of a burn, the mass of the vehicle at the end of the burn, and the change in velocity the vehicle achieves.

That change in velocity is usually written as delta V, and it is the true currency of spaceflight.

The relationship is logarithmic, and that is where the difficulty lies.

The velocity gained depends on the natural logarithm of the ratio between starting mass and ending mass.

Most orbital rockets use two or three stages, occasionally four for particular high-energy missions.

There is a mathematical optimum for how velocity should be divided between stages, and it depends on the exhaust velocity and structural efficiency of each.

In general, the ideal division gives each stage a similar share of the total delta V, adjusted for how efficient each one is.

In practice, engineering realities, existing hardware and cost dominate the decision.

And just to go down that rabbit hole, you can always edit this out if you want.

And in that particular case, the delta V, the change in velocity that's required to make it do that maneuver, was literally about half the fuel expenditure for getting it up there in the first place.

So it's extraordinarily unlikely that that's what this was, okay? So you see that making that change.

It literally goes and makes a 30-degree angle and then goes straight again as it fades out.

There's, underway, unrep'ing water.

And there are explosions in several of Galactica's, um, water tanks, blows out water, which would impart a delta V, a ch- delta velocity, a change of velocity, and it would snap the lines and push Galactica away.

And I, I, I didn't see that the first time I read the script, and later on, "Oh, how could I miss this?" And I called them and said, "Well, we just have kind of already filmed that stuff, and the ship has sailed on that one." So it was just an omission, and I did not do that again.

But, um, what was the question originally? [laughs] In, in, in the job-