Goddard’s liquid fuel rocket design was extremely revolutionary, but was also much more complex than his solid fuel design, and thus took many tweaks and changes to perfect. The basic design included 3 compartments: one that stored the liquid fuel, one for the oxidizer, and finally a combustion chamber where combustion would take place. Rather than being stored directly within the combustion chamber as in solid fuel engines, fuel and oxidizer could be stored in separate tanks and pumped into the combustion chamber at any rate, allowing full control over the rate of the combustion reaction itself.
Therefore, unlike solid fuel rockets, liquid fuel rockets are "throttleable": they can be slowed down or even stopped mid-flight in order to preserve fuel and ensure passenger safety. Following combustion, exhaust would then be pushed out of the back of the rocket to create thrust in the same way as in solid fuel rockets. Goddard completed and flew the first successful liquid-fuel rocket in 1929, but had design patents for his liquid fuel engine as early as 1919. Goddard worked with an extremely small team – just him and a couple colleagues – making this revolutionary invention all the more impressive.
Illustration by Nasa https://www1.grc.nasa.gov/wp-content/uploads/lrockth-1.jpg
Understanding Question: Based on the above description and diagram, and the following video explaining how liquid propulsion works, can you think of some engineering challenges that Goddard might have faced during the development of liquid propulsion?
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Throughout the process of developing this engine, Goddard constantly had to face significant engineering challenges. A prime example was how to deal with the extremely high temperatures generated in combustion. Goddard solved this problem in two ways. First of all, he used ceramic coating on all of this pipes. This is because ceramic heats up much slower than metal. Your are likely familiar with this fact: think about how much hotter metal gets than stone when baking in the sun on a hot day. Additionally, he created a revolutionary system that used the fuel itself to cool down the engine. Specifically, Goddard took advantage of the fact that he used liquid fuels that were stored at extremely low temperatures. Instead of pumping fuel directly into the combustion chamber, Goddard diverted a small portion of the fuel into a chamber that surrounded the engine. As the combustion reaction took place, that extra fuel would soak up a lot of the excess heat, preventing the engine from exploding or melting. Goddard received yet another patent for this system in 1914, which included the illustration depicted below.
Image 1: Goddard, Robert (1914), Apparatus for Pumping Low Temperature Liquids, US 1860891, USPTO, https://patents.google.com/patent/US1860891A/en?oq=US+1860891
Image 2: Brittanica . (1924). Robert Goddard. Encyclopedia Brittanica. Retrieved 2025, from https://www.britannica.com/science/space-exploration/History-of-space-exploration.
Similar engineering decisions had to be made for every single problem he encountered in his development process. Goddard had to design each compartment to perfection, choose the best and most efficient fuel source, and even design exactly how narrow or wide the nozzle of the rocket should be. These considerations were not just specific to the rocket, but also to the environment the rocket wold operate in. For example, engines designed to fire near sea level, such as booster rockets, have smaller nozzles to create a stronger exhaust stream, whereas engines designed to fire in space have wider nozzles, as they are more efficient when less immediate thrust is necessary.
Image 1: Brown, M. (1964). 15 DEGREE CONICAL NOZZLE - TITAN TRANSTAGE CONTOUR ENGINE - APOL. NASA Image Gallery. NASA. Retrieved 2025, from https://images.nasa.gov/details/GRC-1964-C-69633.
Image 2: NASA. (1992). STS-45 Atlantis, OV-104, lifts off from KSC Launch Complex (LC) Pad. NASA Image Gallery. Retrieved 2025, from https://images.nasa.gov/details/s45-s-055.
