P R E S S U R E F E D R O C K E T S
Regulated vs. Blowdown
Pressure feeding is perhaps the easiest and simplest way to supply a liquid rocket motor with propellants. This involves pressurizing a liquid container with an often inert gas. This gas occupies a space above the liquid level of the vessel, and is known as “ullage” (pronounced Oh-ledge). The gas is a compressible fluid, and will want to expand if it’s allowed to. We use this to our advantage by letting the ullage expand and push the incompressible liquid propellant out of the nearest exit, which usually happens to be downwards towards the motor (though not always). In a liquid rocket, this exit is the main valve, and is opened when we want to allow propellants to flow into the combustion chamber through the injector. The more pressure bearing on the liquid propellant, the faster the tank will drain, feeding propellant into the injector. The rate in which it drains is generally a function of Area and Differential Pressure. A higher flow area through the main propellant lines and injector orifices allow propellant to flow at a slower velocity, which reduces pressure losses from friction and other hydraulic effects. A higher differential pressure provides more pushing power to get the fluid where it wants to go, and helps to overcome the frictional losses from doing so at speed.
When comparing two pressure fed systems, Regulated and Blowdown, we are mostly concerned with the effects on differential pressure.
Blowdown is a condition where the ullage gasses apply pressure to the liquid propellant purely by its own expansion. As a result of the ideal gas law PV = nRT, simplified to PV = T, and then again to P = 1/V with a constant temperature, the pressure of the vessel is inversely proportional to its compressible volume. If a tank is 1/3 ullage, and 2/3 liquid propellant, then the ullage gas will expand to its greatest compressible volume by pushing out the liquid. The ullage will expand to 3 times its original size, and the pressure will drop to 1/3 of its original pressure. The flow rate of the liquid propellant also decreases over the course of its drainage as differential pressure decreases, and consequently, the thrust of its unchanging rocket motor will also decrease. This ideal gas relationship assumes much, but in practice, you will always see temperature affect the volume of liquid in a vessel, and the pressure of a gas, as temperature directly affects the density of all things. Keep that in mind when you leave your rocket in the heat of the summer sun.
Regulated pressure feed systems draw additional gas from a regulated high pressure bottle, often a composite overwrapped pressure vessel, or COPV. This bypasses the pressure drop due to expansion by supplying more nitrogen gas to fill up the increasing volume. By doing so, the pressure and flow rate of the liquid propellant remains more or less constant, allowing for constant thrust in an unchanging motor. This constant flow rate is particularly important for maintaining adequate cooling in regenerative cooling jackets. Because it does not rely on large initial ullage volumes, the same amount of propellant requires less tankage, and the storage of required gas in a much smaller and stronger COPV can be very mass and space efficient. Some ullage is still necessary however, and a tank filled with almost entirely liquid can find itself at the mercy of the thermal expansion of the liquid or the continuous boiling of cryogens.
When comparing the two systems, as with many things in rocketry, there is no distinct “better” system, but there are different things that they excel with that may better suit your specific design criteria and mission requirements.
A regulated pressure system lets you get more propellant into your tanks, which can be important when tank sizing or manufacturing constraints are an issue.
A regulated pressure system requires more hardware, such as regulators and high flow check valves, which adds to cost, complexity, and leak sources.
High pressure gas systems, such as those required to supply regulated pressure, are more prone to leaking. Leaks at high pressures can sometimes be unsustainable or difficult to remedy. Helium systems are especially prone to leaking in much the same was as hydrogen gas, since He and H2 molecules permeate elastomeric seals at a higher rate than heavier gasses, assuming these seals were sound to begin with.
Composite Overwrapped Pressure Vessels (COPV) can only be filled and pressurized slowly or under active water cooling. High fill rates heat up the tank due to compression heating, which softens and weakens the composite material, leading to failure well below its nominal proof pressure. This slow fill rate can add many minutes to the pressurization time, or make filling unsustainable with higher leak rates. Pre-pressurization, whereby a secondary system pressurizes the run tanks separately from the COPV, is a common but optional mitigation technique.
Blowdown systems don’t flow pressurant through the top, so it can get away with simpler manifolding. A regulated pressure system must be able to sustain high flow all the way from the bottle. A gas diffuser section is mandatory to avoid injecting gas directly into a liquid volume, and check valves must allow for high flow and checked regularly for backflow as a result of ordinary use.
Blowdown systems can be pressurized at different levels if necessary, but a regulated pressure system would require two regulators to do so.
High flow regulators for regulated pressure systems should typically be dome loaded and non-venting. Dome loading allows for the regulator to be remotely opened and closed at set pressures or else the tank pressures are at the mercy of the high pressure side. This adds electrical and plumbing complexity to a system.
Regulated pressure systems add mass and volume to a system, which reduces the benefit gained from smaller run tanks and higher feed pressures.
A blowdown system provides the same initial feed pressure and thrust as a regulated pressure system, so takeoff TWR is equal. The thrust trails off when this TWR is less important, and when lowering atmospheric pressure reduces overexpansion effects.
A blowdown system requires additional tankage relative to its propellant mass, which can add considerable length to a rocket system. A regulated pressure system adds volume relative to flow requirements, so a higher impulse rocket is better able to write off the extra volume, making it suitable for larger rockets.
Regulated pressure systems can also be controlled electronically using actuated valves to supply pressurant. This can be done with variable flow needle valves or with short bursts in what is known as bang-bang pressurization. This provides room for throttling motor thrust from the pressurant side, so long as power or data isn’t lost during an on state, which could rapidly over-pressurize a tank to failure.
And much more.
The influence on performance of a blowdown motor is a little nuanced, but can be examined in RPA using throttled performance. It is important to note the sea level performance, as “optimum expansion” is a shifting and relative term. An eventual efficiency loss of 20% is not ideal, but this is at the end of what appears to be a 3rd order polynomial curve, indicating an average efficiency loss of closer to 8% at constant altitude. This is not always prohibitive when you consider the efficiency gains from other sources.