Liquid fueled rockets propelled the Russians and Americans deep into the space age with the mighty Energiya SL-17 and Saturn V rockets. The high thrust capacities of these rockets enabled our first travels into space. The "giant step for mankind" that took place on July 21, 1969, as Armstrong stepped onto the moon, was made possible by the 8 million pounds of thrust of the Saturn V rocket.
How a Liquid Propellant FunctionsAs with conventional solid fuels rockets, liquid fueled rockets burn a fuel and an oxidizer, however, both in a liquid state.
Two metal tanks hold the fuel and oxidizer respectively. Due to properties of these two liquids, they are typically loaded into their tanks just prior to launch. The separate tanks are necessary, for many liquid fuels burn upon contact. Upon a set launching sequence two valves open, allowing the liquid to flow down the pipe-work. If these valves simply opened allowing the liquid propellants to flow into the combustion chamber, a weak and unstable thrust rate would occur, so either a pressurized gas feed or a turbopump feed is used.
The simpler of the two, the pressurized gas feed, adds a tank of high pressure gas to the propulsion system. The gas, an unreactive, inert, and light gas (such as helium), is held and regulated, under intense pressure, by a valve/regulator.
The second, and often preferred, solution to the fuel transfer problem is a turbopump. A turbopump is the same as regular pump in function and bypasses a gas-pressurized system by sucking out the propellants and accelerating them into the combustion chamber.
The oxidizer and fuel are mixed and ignited inside the combustion chamber and thrust is created.
Oxidizers & FuelsLiquid Oxygen is the most common oxidizer used. Other oxidizers used in liquid propellant rockets includeing: hydrogen peroxide (95%, H2O2), nitric acid (HNO3), and liquid fluorine. Of these choices liquid fluorine, given a control fuel, produces the highest specific impulse (amount of thrust per unit propellant). But due to difficulties in handling this corrosive element, and due to the high temperatures it burns at, liquid fluorine is rarely used in modern liquid fueled rockets. The liquid fuels often used include: liquid hydrogen, liquid ammonia (NH3), hydrazine (N2H4), and kerosene (hydrocarbon).
Advantages/DisadvantagesLiquid propellant rockets are the most powerful (in terms gross thrust) propulsion systems available. They are also among the most variable, that is to say, adjustable given a large array of valves and regulators to control and augment rocket performance.
Unfortunately the last point makes liquid propellant rockets intricate and complex. A real modern liquid bipropellant engine has thousands of piping connections carrying various cooling, fueling, or lubricating fluids. Also the various sub-parts such as the turbopump or regulator consist of a separate vertigo of pipes, wires, control valves, temperature gauges and support struts. Given the many parts, the chance of one integral function failing is large.
As noted before, liquid oxygen is the most commonly used oxidizer, but it too has its drawbacks. To achieve the liquid state of this element, a temperature of -183 degrees Celsius must be obtained--conditions under which oxygen readily evaporates, losing a large sum of oxidizer just while loading. Nitric acid, another powerful oxidizer, contains 76% oxygen, is in its liquid state at STP, and has a high specific gravity--all great advantages. The latter point is a measurement similar to density and as it rises higher so to does the propellant's performance. But, nitric acid is hazardous in handling (mixture with water produces a strong acid) and produces harmful by-products in combustion with a fuel, thus its use is limited.