TUTORIAL: CONCEPTS FOR ATMOSPHERIC ENTRY
By José Pablo Luna Sánchez. 2008.
In this tutorial you will learn some basic concepts so you can land on the desired base.
It will not tell you exactly what to do, but to explain you the logic of what is being done, so you decide the right steps to solve a problem.
REQUIREMENTS
You need:
ATMOSPHERIC ENTRY
What is atmospheric entry? It is a process where you collide with atmosphere to make it slow you down.
This is what we call aerobraking, or using air to brake.
There are several ways to use aerobraking, but I will not refer to all of them.
If we imagined a flat Earth or atmospheric entry as seen from the ground it may look like this.
This image exagerated the entry angle so you have an idea of what happens when you come back to base.
The entry angle is usually between 0 and 1.5 degrees.
More than 1.5 degrees would make your vessel to enter too fast into the thicker layers of atmosphere, so it would burn and melt.
As you may notice, when we use atmospheric entry we will have an impact point that is exactly our base.
Vehicles like Space Shuttle and Delta Glider IV are like flying bricks, so it is better to end up over your base when you are at about 15 km above sea level.
When we start the descent, we trade altitude for speed, so it would be a good idea to start with a low orbit.
Since Earth is rotating, so we never pass over the same place twice.
So if we select a place on Earth to land we will have a point of closest pass.
We need to syncronize our pass, so our path passes exactly over the base, so our closest pass takes place exactly over the base.
BASE SYNC MFD
The sole purpose of using this MFD is to adjust our closest pass for a given orbit so our trajectory passes exactly over our base.
We only need to use the TGT button to set the target base.
It has a list of data on near passes.
It has 3 columns but we only need 2:
Notice that the closest of all passes is highlighted in white.
In this example it is 100.32 meters from the base in the second orbit.
100 meters is insignificant, so we could consider that this pass goes over your base.
This is how we understand the display
To change values of distance of near passes, you need to make a normal or antinormal burn.
After near pass is set near to the base, it is time to deorbit (retrograde burn to lower periapsis inside atmosphere and start descent).
I know you are going to ask "where do I perform the base sync burn and deorbit burn?".
Make sync burns when you are 90 degrees from your base.
You can make those burns elsewhere, but you would be wasting fuel.
As the near pass is in the first orbit, and you are in the opposite side of the planet, you may start the deorbit burn.
How much should you burn?
We need to refer to Aerobrake MFDto answer this question.
AEROBRAKE MFD
Using Aerobrake MFD is about setting the proper impact point, right on top of your base.
This is applicable to Earth, but for other planets (like Mars) other considerations might be taken into account.
We need to use TGT button to set the target.
It would be a great idea to use CNT button too, to center your view on the impact point.
Once you have the desired angle of attack (AoA) we could use the AoA button to keep it fixed.
Also, you might like to use the Zoom in Z+ button to have a more precise view of the gap between your base and the impact point.
This a closer view of the impact point and the base at maximum level of Zoom.
Notice that AoA button will make your nose to bounce up and down when you are having atmosphere.
So if you have another MFD that allows to keep a fixed AoA you better use it.
This is how you should understand the display in Aerobrake MFD..
Aerobrake MFD also provides some useful data:
G-Max:
It indicates the maximum G force that you will feel.
It is not a good idea to overstress people of the vessel.
A normal person could withstand 3Gs, a pilot can withstand 6Gs and the best pilots can have small peaks of painful 9Gs.
Land Time:
It estimates how much time it will take you to land.
Land Vel:
It estimates the velocity of arrival at the impact point.
Even if the impact point hits the mark of your base, if you go too fast you either overshoot your base or get crashed.
Overshooting is specially problematic when you land on Mars.
Martian atmosphere does not let you to go retrograde, and it does not generate enough lift to allow you to pilot like a normal plane or drag to have a significantly fast braking.
Mars is quite a headache.
AoA:
Angle of Attack. Angle between the direction of motion and the direction your nose is pointing.
Delta Glider IV has a special display that tells you about temperature.
A good atmospheric entry would have a MaxG of 3 Gs and a maximum temperature that does not exceed 1500C.
Entry angle should not exceed 1.5 degrees for Earth.
Ideally it should not exceed 1 degree.
It means you have your deorbit burn at the lowest possible altitude.
Delta Glider IV has its own autopilot for a constant angle of attack (PRO 104 SPEC 40).
Notice that 40 indicates 40 degrees of AoA, but you may set a different number.
I should point out that with Delta Glider IV it is possible to land softly on Earth, on Cape Cañaveral runway without using your engines.
It takes practice, lots of practice.
Once you master these tools you will enjoy atmospheric entry a lot.
PLANETS WITH NO ATMOSPHERE
For planets and bodies with no atmosphere you can't use atmosphere to brake, so you rely on your engines.
You trade altitude for velocity, so you better watch out or you might hit the ground too hard if your vertical velocity is too high.
With no atmosphere, there is no entry, just a ballistic trajectory.