I have read in many references that Delta V is fixed or constant for example ( Delta V to LEO = 10km/s). They did not mention the payload mass or the Propellant mass, and also they did not mention that its approximation results. From my reading, I expect that Delta is not depening on also the Mission distance. Does anyone have a clear answer for me ?!!
Another way of saying delta V to LEO = 10 km/s is this:
- To be in orbit, a thing needs to move horizontally at a speed of at least 7.8 km/s
- To get to orbit, the rocket delivering the thing will have to get up to that speed, and get out of the atmosphere
- While it does that, the drag from gravity and air resistance make it have to exert as much force as if it was speeding up to 10 km/s, not 7.8 km/s
Whatever kind of rocket you are using, whatever the payload, you have to calculate whether the thrust of the engine will be hard enough, long enough, to get the payload up to that final speed and in position to stay in orbit.
To do that, you use the Tsiolkovsky rocket equation.
Thinking about mission distance is better done in terms of thinking about how much gravity you have to overcome to get where you want to go. Once you are in space, there is no friction of any kind* to slow you down, so you will keep going at the speed you had when you arrived in space, and your course will only be affected by gravity.
But let's take the example of LEO. After a thing has gotten to orbit, usually it still isn't in the orbit it wants. So, its engine has to fire again for a while to move it into the right orbit. It may need to do that twice. And what it really needs to do is change its speed by the right amount, at the right time, to end up in the right orbit. To calculate what needs doing, that's the first thing you need to know, and from that you figure out how much fuel the engine being used needs in order to do it.
*Okay, actually in LEO there is still a teeny weeny bit of air, and over time it slows things down. So, for instance, the ISS needs to be boosted occasionally to keep it at the right altitude.
The simple theoretical delta V to achieve a particular orbit is constant, but in practice (or on more detailed analysis) Delta V is not constant for a number of reasons.
For launches from the surface of a moon or planet delta V will be greater than the theoretical value because:
A rocket will not be able to achieve orbit instantly, it will need to accelerate for some minutes and during this time it will lose energy to gravity increasing delta V.
If an atmosphere is present as on Earth this will also provide resistance slowing the rocket and increasing the required delta V depending on the aerodynamics of the rocket.
If the launch is from a rotating body, delta V will also depend on the launch site and the direction of launch. Equatorial launch sites will require less delta V if launching in the direction of rotation (prograde) and much more if launching in the opposite direction (retrograde). Polar launch sites will require an intermediate delta V.
Delta V calculations for moving from one planet or moon orbit to another also suffer complications:
It depends on the planetary alignment at the time of departure. Some departure dates require more delta V than others and this may also vary year to year. This is further complicated if the planet or moon is not itself orbiting in the same plane as the rocket.