Liquid-fueled rocket ullage motors

Hi all,
Why do liquid-fueled rockets use ullage motors to ensure the fuel/oxidizer is pumped to the engines as a liquid instead of any gas that's in the fuel/oxidizer tanks? Because i have read that liquid-fueled rockets can't suck any gas into the engine.Why must the engines not allow any gas-form fuel/oxidizer to be injected into the combustion chamber?

a reply to: Conspiracyskeptic

Because a gas can vary in density, while a liquid does not. Density variations can cause combustion instability, which at the energy levels found in rockets is a Very Bad Thing. Combustion instability has many causes, and minimizing or eliminating it is a major problem for rocket designers.

Search for "combustion instability" on both Google & YouTube. You'll find many interesting links.

a reply to: Conspiracyskeptic

it causes an explosion...

Baby makes a boom-boom.
Astronauts have some serious cajones.
They basically strap their ass to a huge explosion.

originally posted by: Conspiracyskeptic
Hi all,
Why do liquid-fueled rockets use ullage motors to ensure the fuel/oxidizer is pumped to the engines as a liquid instead of any gas that's in the fuel/oxidizer tanks? Because i have read that liquid-fueled rockets can't suck any gas into the engine.Why must the engines not allow any gas-form fuel/oxidizer to be injected into the combustion chamber?

Here's the deal. The reason for firing thrusters in the first place is to impart a known amount of velocity to the spacecraft. Since the engineer who designed the spacecraft knows what kind of fuel and oxidizer is being used, he/she knows what the exhaust velocity of the rocket thruster is supposed to be. Using the rocket equation, it is easy to calculate how much velocity will be produced by a thruster burn of a given duration. So when they want to impart --let's say--3.5 kilometers per second of velocity to the spacecraft, they know they should have the thruster valves open for (again, I'm just making up representative values) let's say 300 seconds.

The idea is that when they open up the thruster valves, they want the fuel and oxidizer liquids to start flowing immediately. That way, they know if the valves are opened for a specified length of time, they will get a corresponding specific amount of velocity increase.

Now imagine that a spacecraft had let's say a 100 liter cylindrical propellant tank that only had 50 liters of liquid propellant in it, and the spacecraft was in zero-g. Unless they took some special precautions, the propellant would float around somewhere in the middle of the tank in an amorphous blob. In that condition, if you opened up the thruster valves, you wouldn't know if gas or liquid was going to flow into the thrust chambers. As Saint Exupery pointed out, the results (in terms of thrust) would be highly unpredictable. You might get no thrust; you might get full thrust, you might get hiccups. So you would have no way of even knowing how much velocity you were going to pick up. Depending on the circumstances, that could be a mission-ending problem.

One way to solve that problem is with ullage thrusters. They impart a very, very small acceleration to the spacecraft so that the blob of propellant is not floating out in the middle of the fuel tank, but is snuggled right up against the thruster inlet, located at the back of the tank. Another way that is sometimes used to solve the same problem is a small wire mesh cage adjacent to the inlet. The cage is made out of a material whose surface attracts the propellant; the propellant will "wet" the surface and capillary action will keep the blob of propellant next to the thruster inlet. In either of these cases, as soon as the main thruster starts firing, the inertial forces will keep the blob of fuel pressed against the inlet. So these solutions are only needed in the initial start up.

A third way of solving this problem is to use a flexible diaphragm between the fuel and the tank wall. The diaphragm provides an impermeable boundary between the liquid phase and the vapor phase of the propellant.

In all cases, the point is to make sure that as soon as the thruster valve opens, liquid propellant is flowing into the chamber.