So, back in the middle of last year we were throwing ideas around on how to get to the moon.
One of the first ideas we looked into was the Rockoon. Like the name says it’s a combination of a rocket and a balloon. We thought it would be innovative and cheap.
It wasn’t a bad idea. But after some number crunching we figured out that the whole thing had a terrible catch.
And here is why:
Used in late 40s and 50s, Rockoon was used to transport very light payloads to altitudes that are near the edge of earth’s atmosphere. But to carry a full scale moon rocket that high, the balloon would have to have proportions beyond any economical sense.
1 m³ hydrogene or helium can lift something slightly above 1 kg of mass.
At sea level. But when it goes up, the air-pressure halfves every 5500 m.
Therefore the gas doubles its volume. At an altitude of 49.5 km, the gas will have expanded by a factor of 512.
The diameter of the balloon would be 8 times bigger as it’s been at sea level.
Now let’s take a very optimistic mass of 250 kg for a rocket. For that mass the balloon needs 250 m³ of hydrogene or helium. That results in a diameter for the balloon on the ground of 8 m. Growing to 64 m after the ascend. At first that doesn’t sound too bad. But now let’s see the gain of energy the balloon delivered.
We took 250 kg and brought them up to an altitude of 50 km.
This equivalents to 123 MJ (250 kg * 50000 m * 9.81 m/s²).
The energy density of hydrogene is 11.7 MJ/m³.
This shows that it’s not a bad idea to burn all that hydrogen with a rocket instead.
More interesting is the ratio between the gained potential energy and the energy required to reach the earth orbit. A Mass of 250 kg traveling at 7.9 km/s has a kinetic energy of 7800 MJ ( 1/2 m v² ).
Now let’s think of the energy needed to send a rocket into earth orbit as a 100%.
The energy that the balloon brings into the whole equation is a mere 1.58%.
However, it’s a different story with a balloon. Considering the Rockoon approach to the entire matter greatly complicates things. Increasing the number of variables that can lead to a total failure of the launch.
That’s not even taking into account that the balloon will go where the wind pushes it and not where we want it to go.
One argument for a Rockoon is less air drag. For larger rockets the velocity loss due to air drag can be lower than 3% (1). A small rocket has a higher drag, that is one argument more against small launch vehicles. It makes more sense to modify an air-air-missile and to launch it form an airplane flying at Mach 2.5.
But even that only accounts for less than 1/8 of the needed speed to reach earth orbit. The fact that private people will have a hard time acquiring an air-to-air-missile is a matter for another time.
The conditions are still better than with a balloon, though.
Arne Reiners
A Part-Time Scientist
(1) Space Mission Analysis and Design 3rd Edition Chap. 18.1
_The article image is provided by ICHC
