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ACHAB: Analysis Code for High Altitude Balloons Chapter 3
SINBAD, NASA’s Balloon Scientific Analysis Model 19 . Although very popular
among the scientific ballooning community, SINBAD is rather old and suffers from
several limitations, especially when it comes to mission planning. For example, it is
not capable of simulating three-dimensional trajectories or it uses a 5-point one-
dimensional atmospheric model.
In the present work we will describe the theoretical bases and the development
process of ACHAB (Analysis Code for High-Altitude Balloons), a new software tool
for the simulation of the dynamical and thermal behavior of high-altitude zero
pressure balloons 26 .
3.1.1 - Overview
The lifting force that allows a balloon to rise derives from Archimedes’ Principle.
Concisely the lift of a balloon can be expressed as:
( ρ − ρ ) ⋅ = M ( 1+
f )
air
gas
Volume gross
where Volume is the volume, ρ air the density of air and ρ gas that of the lifting gas
(helium), M gross the sum of solid masses (balloon envelope and payload), and f the
free lift. Getting off the ground requires a positive free lift (or excess gas) of
generally 10–20% 2,9 .
During its flight, the balloon can be considered as a thermal and dynamical system
that is practically in free evolution inside a complex thermal environment and subject
to atmospheric winds. Therefore careful modeling of the thermal effects is of
fundamental importance for the correct simulation of the vertical speed of the
balloon.
Before delving into the details of the model, it is instructive to give a schematic
overview of how the code works. Figure 3.1 shows an illustrative block diagram
describing the structure of ACHAB.
Buoyancy, which is the most important force to be determined, is related to the other
forces that act on the balloon through a dynamical model. The dynamical model,
though, needs information that can only be given if a geometric model and a
thermal model are introduced. The geometric model gives a geometric description
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