A Quick Word on Dip Tubes

by Richard Garcia, Director of Research, Reaction Research Society

I’ve had the opportunity to talk a lot with many amateur rocketeers getting into building liquid rockets. Because many commercial pressure vessels only come with one port, you often have to choose between making or modifying a pressure vessel. I always see a third option, dip tubes, being overlooked.

I would not recommend using a dip tube on a flight vehicle, but they are quick to put together for a test stand. A dip tube can be made from two parts: a tee fitting and a modified compression fitting.

Compression fittings do not normally have enough room to send a tube all the way through them. Most compression fittings purposefully have a stop to bottom out the tubing at a specific depth within the fitting. Usually these compression fittings have enough meat on them to drill through and open their internal diameter to fit the tube straight through them.

Cross-section of a compression tube fitting, before and after drilling
Typical compression fitting before and after modification to allow the tubing to fully penetrate the fitting. The stop feature is common which makes tubing joints repeatable and accurate. This internal diameter is opened up drilling to allow the tube to pass completely through. Note that the compression fittings shown have a tapered pipe connection (NPT, typically). Other pipe or tubing connections are possible depending on the fitting manufacturer.

Putting this modified compression fitting along the straight path of the tee fitting will allow you to pass a tube through both fittings and into the tank itself. The tee connects on to the tank’s port and the tube goes down to just above the bottom of the tank (if connecting at the tank’s top). The branch of the tee feeds the tank ullage space with pressurant gas and the tube picks up fluid from the bottom of the tank as long as the liquid line is above the tube opening.

Dip tubes on top and bottom of tanks
Dip tubes shown in both top and bottom mounted configurations. Note that the fluid passes from the annular opening on the tank bottom mounting configuration. Note that the pressurant gas flows through the annular gap in the tank top mounting configuration. The tee fitting shown has tapered pipe fittings (NPT, typically).
Illustration of a dip tube mounted on top of a liquid filled tank
Illustration of a top mounted dip tube on a liquid filled tank. Pressurant gas is supplied at the branch connection of the tee fitting and flows into the ullage space through the annular flow path. The liquid is driven up the central tube and out of the tank if the gas pressure is sufficient to overcome the liquid head and tube friction.

This can be flipped upside down, pressurizing through the tube and flowing the liquids out of the tee branch connection. It is always important to consider the amount of flow area in the tubing you are using as well as the annular flow area created by this combination of fitting as it is likely to be the area of minimum restriction to flow.

illustration of dip tube used in pressurized expulsion of liquid from a tank
Illustration of a dip tube mounted on the bottom of a liquid filled tank. Pressurant gas flows up to the top into the ullage space of the tank expelling the liquid from the annular flow path in the tee fitting and out of the branch connection.

Also, instead of a tubing connection, one could put a thermocouple through the compression fitting of the proper size to make an internal fluid temperature measurement in the tank either immersed in the liquid below or measuring the ullage gas temperature in the top of the tank. This is a convenient way of mounting a temperature sensor.

P.S. Here are two kinds of tanks with ports on each end that are readily available. Try looking for automobile air tanks like this one from Speedway Motors:


Or a gas sample cylinder such as this one from Swagelok


For questions, contact Richard at