Suppose you and your rocket are in a circular orbit of radius about a stationary planet. You want to change its altitude from to for n > 0. How could you do this? The diagram below shows how you might use a Hohmann transfer orbit.
Suppose you want to increase your altitude. The procedure would be as follows: apply an impulse to your rocket at in a direction tangential to your path; wait until you reach apoapsis at , your point of furthest approach from the planet; apply a second impulse to circularise your orbit. In cases such as this where n > 1, both impulses are delivered parallel to the rocket’s velocity vector. If you wanted to decrease your altitude instead (0 < n < 1), the impulses would be antiparallel, or in the ‘backwards’ direction. Throughout this post, we’ll assume we want to increase the radius of orbit.
We’ll now take a look at why this works. We will quantify the impulse required of the rocket’s engines by finding the fractional change in velocity in terms of . That is, if the rocket’s speed at altitude is , its speed at altitude is .
First we need an expression for . For an object of mass moving in a circle of radius , we know that the centripetal force is related to its tangential velocity by the equation
For an orbit, this force is the mutual gravitation experienced by the rocket and planet, hence
Here, is the gravitational parameter of the planet, the product of its mass and the gravitational constant. The total energy of the rocket-planet system is the sum of its kinetic and potential energy, given by
Hence the system’s energy after the first impulse has been delivered is given by
where the rocket’s velocity has been increased by . Using the expression for above, we can eliminate entirely:
We can equate this with the general expression for the system’s energy quoted in the previous post:
assuming energy is conserved between positions and . Recall that , the system’s angular momentum about the planet, is a constant. The angular momentum of the rocket has a particularly simple form immediately after the first impulse is delivered, since the velocity and position vectors are orthogonal:
When the rocket is at apoapsis or periapsis, the points of furthest and closest approach to the planet respectively, by definition. If the separation of the rocket and planet is at these points, then satisfies the equation
Multiplying through by and rearranging, we can solve a quadratic equation in to get
has two solutions, hence the apoapsis and periapsis are distinct; this means the orbit becomes elliptical after the impulse is delivered. If we take the positive root first, we find
This is the new periapsis. It coincides with the point at which the first impulse is delivered (point ). This seems fairly intuitive: we give the rocket a kick, and its distance from the planet starts to increase. The negative root yields the value of the new apoapsis at as a function of :
This is the point of furthest approach the rocket eventually reaches the opposite end of the orbit. For the Hohmann transfer, we demand that :
Rearranging, the required fractional change in velocity from the first impulse is
So we can get the rocket to reach the height for an instant, but we need a second impulse to keep it there and circularise the orbit. Say this requires an additional change in velocity . Rather than generating new equations, let’s recycle the ones we have just used.
Suppose we are already in a desired circular orbit , and wish to change it back to the elliptical Hohmann transfer orbit with apsides and . We have seen that a circular orbit can be distended into an elliptical one. In this case, we simply swap the roles of and in the equation above:
So if we are in a circular orbit of radius at velocity and change the velocity to , we will be placed on an elliptical path with points of furthest and closest approach and respectively. Now assuming orbits are ‘reversible’, we could supply an impulse in the opposite direction to go from the elliptical orbit to the circular one of radius . That is, when at apoapsis (for n > 1), the required fractional change in velocity is
Note that this is a fraction of the final velocity , once stable orbit at has been achieved. We therefore need to find in terms of . That is quite easy, since
since both orbits are circular. Hence
Let’s piece together an expression for in terms of . Our first change in velocity was given by
The second impulse brings about a change in velocity given by
Hence the total change in velocity is given by
We defined our original quantity in the following way:
the total change in velocity of the rocket, given by the difference between its velocity at and at .
Here is a graph said function again :
We will pick out some of its main features. First, we see that for as we expect: to leave the radius of orbit unchanged, no change in velocity is required. As the radius of orbit goes to , the change in velocity actually becomes unboundedly large. The conservation of angular momentum explains this: as the distance between the planet and the rocket decreases, the speed of the rocket increases. In the limit that the radius becomes infinitesimal, the speed required of the rocket becomes infinite.
Finally, as approaches infinity, the change in velocity required approaches 41.4%. Supplying such an impulse would allow the rocket to escape the planet entirely.
What is interesting is that this figure of 41.4% is not the function’s maximum, far from it in fact. There is a large range of orbits which are, in real terms, ‘more difficult’ to establish than one an infinite distance away. Plotting a log-lin graph of against reveals the shape of the graph:
The most expensive orbit and the required to achieve it can be found using calculus. The derivative of is given by
At its maximum , the derivative is :
Multiplying through by :
The positive root of this equation is to 4 significant figures. Multiplying your radius of orbit by this figure requires a fractional velocity change of 53.63%. This is the hardest you would ever need to work to carry out a Hohmann transfer.
There does exist another type of transfer orbit called a bi-elliptic transfer. There are certain transfers for which this method is ‘cheaper’ (requires a lesser change in velocity) than a Hohmann transfer. We will compare the two methods in another post.