H-O Racing's "Rocket Box"

Some theory

All carburetors use Bernoulli's principle which states that as the air velocity increases, pressure falls. This lower presssure pulls fuel into the airstream, mixing the air and fuel together and then discharging that mix into the engine's intake manifold. The increase in air speed comes mainly from constricting the cross-sectional area, a section called the venturii. Rocket exhaust nozzles work on the same principle.

Quadrajet carburetor

Quadrajet carburetors have a pair of fixed booster-style venturiis in the front (primary side) and a pair of variable size simple venturiis in the rear (secondary side). Rocket exhaust venturiis are much simpler than carburetors since there are no booster rings, throttle plates and such. BUT, there is another difference that is significant. The expansion section (downstream from the venturii) of a carburetor is much shorter and a different shape than the expansion section of a rocket nozzle. It was Craig's opinion that these differences caused the carburetor discharge to be less efficient than a rocket nozzle discharge. Craig was also of the opinion that it was possible to partly compensate for this difference with what turned out to be called the "Rocket Box".

Expansion Area Ratio

In theory, the only important parameter in rocket nozzle design is the expansion area ratio (the ratio of exit area to throat area). Fixing all other variables (primarily the chamber pressure, i.e., atmospheric pressure for a carburetor), there exists only one such ratio that optimizes overall performance for a given ambient pressure. The problem is simpler for a carburetor that runs a drag race at only one altitude and sees a small range of pressure changes during the day. Conversely, a rocket typically travels over a wide range of altitude pressures, i.e., sea level to outer space. Below is an illustration of under-, optimum- and over-expanded Expansion Area ratios in rocket exhaust nozzles.

Bell nozzle

To gain higher performance and shorter length, the bell-shaped nozzle was developed. It employs a fast-expansion (radial-flow) section in the initial divergent region, which leads to a uniform, axially directed flow at the nozzle exit. The wall contour is changed gradually enough to prevent oblique shocks.

A 15-degree half-angle conical nozzle is commonly used as a standard to specify bell nozzles. If the length of an bell nozzle is 80% of that of an equivalent 15-degree half-angle conical nozzle, the performance is the same. Bell nozzle lengths beyond approximately 80% are not significantly better.

One convenient way of designing a near optimum thrust bell nozzle contour uses this parabolic approximation: