Send to HN

Liquid-fueled rocket engines operate by flowing fuel and oxidizer into a combustion chamber at high pressure in order to eject mass out of the rocket nozzle at high velocities. While there exist many ways for these propellants to be mixed (called engine cycles), they all have one major technical hurdle: how does a rocket engine get up to operational pressures? Furthermore, how is this done in the zero-normal force environment of orbit?

Starting a liquid rocket engine is a very complex sequence of managing pressures and temperatures throughout all of the valves and pumps in the engine, where the smallest of errors leads to the engine experiencing a RUD (rapid, unscheduled disassembly).

Starting a Solid Rocket Booster/Motor

By far the simplest rocket engine to start are solid rocket boosters, which are widely used in the industry; most notably, NASA’s Space Launch System and Space Transport System (the space shuttle) have used two massive solid rocket boosters. Since solid propellant is fuel and oxidizer mixed into a solid sludge, to light a solid engine a small amount of energy is needed to start combustion.

A render of the NASA Standard Detonator (Credit: Everyday Astronaut)

To start these SRBs, a signal is sent to two completely redundant NSDs to ensure both boosters light properly. The NSDs burst a thin seal, which then ignites a pyrotechnic booster charge. That booster charge then ignites propellant in an igniter initiator, which is what fires down the entire length of the solid rocket motor, lighting the entire surface of the core of the booster simultaneously.

This highlights the entire benefit of solids: they are super simple and super reliable. This comes with disadvantages, such as not being able to shut them down and lower performance than liquid rocket engines.

Igniting Liquid Rocket Engines

Preconditioning The Engine

Before a liquid-fueled rocket engine is able to start, the engine must be prepared for the extremely low temperatures it’s about to experience from the liquid propellants. As discussed here, not only do orbital rocket engines run propellant through the walls of the engine to keep the combustion chamber from melting, but the pumps themselves flow upward of thousands of liters per second of cryogenic propellants, which makes the metals, valves, and bearing brittle and prone to failure. This is especially true for engines that run on RP-1 and liquid oxygen (called kerolox) and especially true for liquid hydrogen and oxygen (called hydrolox), or liquid methane and oxygen (called methalox).

The majority of liquid-fueled rockets are one of the aforementioned propellant combinations. Examples of RP-1 and LOx are SpaceX’s Merlin, the Saturn V’s mighty F1 engine, Rocket Lab’s Rutherford, the Atlas V’s RD-180, the Soyuz’s RD-107 and RD-108 engines, Firefly’s Reaver and Lightning engines, and so on.

Slowly the industry has been moving toward the next-generation rocket propellant: methalox. As previously discussed, CH4 is a good middle ground between keralox and hydrolox, generating a moderate amount of thrust at a moderate specific impulse. The majority of upcoming rockets engines use methalox, including SpaceX’s Raptor, Blue Origins BE-4 (which will fly both on ULA’s Vulcan rocket and Blue Origin’s New Glenn), Relativity’s Aeon engine, Zhuque-2’s TQ-12, and the Archimedes engine on Rocket Lab’s Neutron.

temperature scale, rocket propellants, how to start a rocket engineChill Down

Chill down starts at vastly different times depending on the engine. For instance, the nine Merlin engines on the Falcon 9 begin chill-down at T-7 minutes, whereas the RS-25 on SLS starts chilling down hours before launch. This event is usually paired with a callout on the nets with “engine cool-down” or “engine chill-down.”

Closer to T0, the rocket will transition from the inert gas and will begin to flow some of the propellants through the system at a low flow rate, where it will start to thermal condition the engine to get it to the cryogenic temperatures. This process is surprisingly easy, due to the rocket’s tanks storing propellant at relatively high pressure, usually three-to-six bars. Because of this, some valves can just be opened, and the pressure from the tanks will ensure propellant flows from the tanks through the engine.

Depending on the engine, the rocket, and the ground systems, the propellant that has gone through the engine may be vented into the air. This is especially true with liquid oxygen, which poses no risk when being vented into the atmosphere. However, for CH4 and H2, it will generally be either cooled down again into a liquid or burned in a flare stack, to reduce their impacts on the atmosphere.

Not only is the engine chilled to protect itself from the cold propellant, it’s also done to protect the propellant from the warm engine. If the propellant boils before reaching the impellers in the pumps, it can cause cavitation (small bubbles in the liquid). These bubbles can chip away material from the pumps and damage the blades. SpaceX CEO and CTO Elon Musk talked about this in Everyday Astronaut’s tour of Starbase:

But, not only can these bubbles damage the pumps by chipping at them, but they can also cause the pumps to overspeed, meaning they deliver the incorrect amount of propellant to the combustion chamber. This can cause the engine to burn at stoichiometric conditions, which releases the most heat to the engine, damaging or even destroying it.

This chill-down process is absolutely vital. In fact, SLS’ launch attempt in August of 2022 was scrubbed for this reason; engine 3’s temperature sensor wasn’t showing that the engine had chilled down to the required temperatures, scrubbing the launch for the day. It later turned out that this data was due to a faulty sensor and not the engine not being at operational temperatures.

Exceptions

This all said, there are exceptions to the chill-down process: hypergolics. Hypergolic propellants are those that combust upon contact with each other. The hypergolics that are used in rocketry have a high boiling point, meaning that they can be kept at room temperature. Because of this, the engines do not need to be chilled down.

Obviously, this has its advantages for many intercontinental ballistic missiles and their derived rockets, which generally run on hydrazine. The fuel can be either unsymmetrical dimethylhydrazine, hydrazine, or monomethylhydrazine with the oxidizer being nitrogen tetroxide.

The US’ use of hypergolic-based rockets has been limited. The LR87 on Titan II and the AJ-10 on Delta II’s upper stage were hypergolic. That said, hyperbolic rockets were very popular in the Soviet Union, which can be seen in this article. Additionally, many of China’s launch vehicles run on hypergolics, but they are slowly shifting away from this.

hypergolic fuel, NASA, how to start a rocket engineSpin Up

After the engine has been pre-conditioned for starting, the next goal is to spin up the pumps. To do this, engineers must take into account one of the most fundamental laws of the universe: pressure flows from high pressure to low pressure. In order to ensure that a flame is not sent backwards through the system, which would lead to a catastrophic failure, the pressure upstream an engine must be very high. In fact, for some engines like SpaceX’s Raptor, pressures upstream can approach 1,000 bar!

The simplest engine cycle to spin up is a pressure-fed engine. Because the propellant is already stored at high pressure, simply opening valves will allow propellants to flow into the combustion chamber at the necessary operating pressure.

bi-propellant pressure fed engine, diagram, render, how to start a rocket engine
A diagram of a bi-propellant pressure fed engine. (Credit: Everyday Astronaut)

This said, pressure-fed engines are not enough to get objects into orbit from the Earth’s surface. For this reason, high-power high-pressure engines are needed that have turbopumps. In most engines, the turbine has hundreds of thousands of horsepower from just one turbine. That turbine is of course spun by a gas generator or preburner, which is fed by the pumps. This creates a hard dynamic that the turbine must be spinning to get move propellants to the preburner/gas generator, but the preburner/gas generator must be burning to spin the turbine.

The simplest way of bypassing this, which is done by Rocket Lab’s Rutherford engine, is to use an electric motor to drive the turbine. Then, this is not a problem. But this is not feasible for larger engines, such as the RS-25, which needs 100,000 horsepower to spin its turbine when at full throttle.

What’s more, is the RS-25’s fuel preburner delivers 200 horsepower per kilogram, highlighting why electric motors would not be practical.

Spinning up a preburner/gas generator is generally done by using high-pressure gas to get the pumps spinning. This can either be supplied by an onboard system, like helium stored in COPVs, or by ground service equipment. Spinning up engines with GSE is advantageous since it removes mass and complexity from the rocket.

In either case, high-pressure helium or nitrogen is pumped into the gas generator/ preburner to get the turbine spinning at operational speeds. For a short period of time, the engine’s pumps are powered by what is basically a cold gas thruster, which is very inefficient. Alternatively, some engines use a little solid or hypergolic engine that acts as a gas generator for a small amount of time.

turbine spin up, hydrogen peroxide, how to start a rocket engine
A turbine being spun by decomposed Hydrogen Peroxide. (Credit: Astronomy and Nature TV / Everyday Astronaut)

However, due to the low specific impulse of these systems, you don’t want to run the engine off this for any longer than necessary, so it’s only done until the engine can self-sustain combustion. This means that for engines that require multiple starts (such as engines on upper stages that may have to burn several times to reach a desired orbit or for a propulsive landing), the rocket must carry enough helium to spin up the engines.

Bootstrapping

However, there is another way of starting a rocket engine that doesn’t require a separate source to get the pumps up to speed. This process, called bootstrapping (or tank head / dead head starting) is where the engine carefully lights up using only tank pressure and energy stored in the thermal difference between the propellant and the engine. To do this, the preburner (powering the turbine) will begin to spin up due to hydrogen flowing through the preburner and boiling off. This raises the pressure, starting to spin the turbine. Then a very delicate dance takes place, as some oxygen is let in and the preburner lights. At first, the combustion is very weak, but as the pressure in the preburner rises, the pumps spin faster, providing more fuel to the preburners, causing the pressure to rise, spinning the pumps faster. This is done until the engine is at operational pressure.

A Space Shuttle during launch with its ROFIs igniting excess hydrogen. (Credit: NASA / Everyday Astronaut)

This process is used on the RS-25 (both on the Space Shuttle and on SLS), wherein the RS-25, the high-pressure fuel turbopump must reach 4,600 RPM within 1.25 seconds for the engine to have enough propellant flow for ignition of the main combustion chamber (MCC). Additionally, Relativity has successfully bootstrapped the first stage of their Terran 1 vehicle, and Firefly’s ex-CEO Tom Markusic mentioned in an interview with Everyday Astronaut that firefly was looking into trying to bootstrap Alpha’s second stage.

Deadhead starting is an extremely complex process due to the precise control needed. But this is made even more complex given that when an engine is starting up it is in a transient: the time in between being off and firing at full throttle.

Transients

Transients can best be summed up as the “in-between moments.” In the case of rocket engines, the time between a stationary engine and an engine running at full power. Additionally, any time that the engine changes throttle settings it is in a transient. But, once the engine is in a steady state it is relatively easy to keep running.

These transients make starting an engine very difficult due to the chicken-and-egg scenario previously described. Using bootstrapping the RS-25 as an example: the liquid that flash boils from hitting the engine’s preburner expands as it becomes a gas. This expansion is what starts spinning the turbine. However, this pressure also exerts a back pressure on the rest of the system, slowing down the flow of propellant. Then, as the boil-off decreases, the flow will increase again, where it will boil off, causing the same thing again. In the case of the RS-25, this happened twice every second.

Making things even worse, there are often delays between action and reaction. If these pressure waves are too extreme, the pressure gradient could flow backward, stalling out the startup sequence or blowing up the engine.

Additionally, any time a valve opens the pressure is changed, affecting the flow. The entire engine start-up is full of these feedback loops, making the fact that rocket engines can be started a remarkable feat of engineering.

Ignition Process

Ignition On The Ground

Now that the engine is conditioned for launch, the propellants start flowing to the engine. However, this makes up only two-thirds of the ignition triangle, which consists of fuel, an oxidizing agent, and an ignition source (usually in the form of heat). But, to make things even harder, any slight error in the ignition process can lead to a “hard start,” which is when the propellants combust at the wrong ratio, at the wrong time, or in the wrong place. The worst hard starts can overpressure the engine, causing an energetic detonation, destroying the engine and potentially the vehicle.

Before an engine can be ignited the propellants must be mixed in the combustion chamber. This is done through specially designed injectors, which Everyday Astronaut will do a video/article on in the future. If the propellants are not mixed evenly, the engine can not have stable combustion, causing the engine to explode.

The easiest engines to ignite are hypergolic because, by definition, they ignite on contact. However, this obviously cannot be done for any liquid oxygen engine.

The first and simplest form of engine ignition is what the Soviet Union did with the R7 and what Russia still does with the Soyuz. They put large wooden braces with pyrotechnics on top inside each combustion chamber. Then when these are ignited, the engine ignites. While not elegant but practical, it has one major downside: this cannot be done in space.

TEA-TEB ignition, purdue university, how to start a rocket engine
TEA-TEB coming in contact with oxygen and igniting. (Credit: Purdue University / Everyday Astronaut)
spacex, falcon 9, mvac engine, tea-teb ignitionIgnition In Space

Starting an engine in space has two major additional challenges. First of all, there is no ground support equipment. This means that the mass of all of the ignition system must be on the vehicle. Second, and perhaps more importantly, is that the vehicle is not in an inertial reference frame and is subject to the Earth’s gravitational pull. This means that liquid propellant sits at the bottom of the tanks with only the top of each tank having gasses.

However, when a vehicle is in space (and not firing an engine) it is in an inertial reference frame (and therefore has zero normal force). Because of this, the propellants are just floating around in the tank–not necessarily by the engine inlets. So, to start a rocket engine in space it must first be assured that the propellants is at the bottom of the tanks. To do this, most commonly ullage thrusters are used. This is when either a small solid rocket motor or cold gas thrusters is used to settle the propellants, as solid rocket motors and cold gas thrust can be fired in an inertial reference frame.

For example, the Saturn 1B and Saturn V used ullage thrusters to not only separate the stages but also to settle the propellants. This sequence led to this very iconic imagery of S-IVB ignition:

Using cold gas thrusters is the most common solution as they are easily restartable–which is necessary if a stage is doing more than one burn to reach its desired orbit. In fact, even Falcon 9’s first stage has two propellant settling thrusters, which are used before boost back and reentry burns; they are not used before the landing burn as the stage is slowing down rapidly already from the atmosphere.

Many spacecraft don’t need ullage thrusters due to their use of hypergolics. For example, SpaceX’s Dragon spacecraft uses hyperbolic propellants stored at a high pressure. This way, Dragon can rapidly fire its Draco thrusters for very precise control of the vehicle. The liquid hypergolics are stored in propellant management devices, which are spherical tanks that have a bladder. While not used on Dragon, some cylindrical tanks can use a piston, which pushes propellant forward as it is used.

NASA, Titan II, hot-staging, how to start a rocket engine
A Titan II rocket with its open interstage allowing for hot-staging. (Credit: NASA / Everyday Astronaut)
Roscomos, Soyuz, hot staging, how to strat a rocket engine
A Roscomsos Soyuz rocket with its open interstage allowing for hot-staging. (Credit: Roscosmos / Everyday Astronaut)

Ignition Of The RS-25

To understand everything talked about above, the ignition sequence of the Space Shuttle’s RS-25 engine will be analyzed. This section is partially based on Robert E. Bigg’s Space Shuttle Main Engine The First Ten Years. For this section, knowledge of the RS-25’s fuel-rich closed-cycle engine cycle is needed.

The RS-25 has two preburners. Since it’s fuel rich, both preburners are fuel rich, meaning all of the fuel flows through the preburners with one powering the oxygen pumps and the other powering the fuel pumps. Additionally, there is a small boost pump on the oxygen side that feeds the preburners the higher-pressure liquid oxygen they need to operate.

The engine has pre-valves that connect the tanks to the engines and main fuel valve, which feeds the preburners and the regen cooling channels. The regen system has a separate valve called the chamber coolant valve, which can be throttled to redirect fuel between the MCC and the regen systems.

There are three oxygen valves, one that feeds each of the three combustion chambers (the MCC and the preburners). Finally, there are some recirculation pipes that feed boiling off gaseous propellant either back into the tank or vent it into the atmosphere.

Completing the ignition triangle, the RS-25 has augmented spare igniters (ASIs). There are three sets of these, one in each preburner and one in the MCC. The ASIs have their own fuel and oxygen supply lines and are the first to receive propellant in the system.

While there are obviously many other tubes, pipes, and sensors around the engine, to understand the core concepts of igniting the engine, this simplification is sufficient.

Preconditioning

As noted above, the first step is to purge and thermally condition the engine. The RS-25 goes into the “Start Preparation Phase”; during this phase, the oxygen side of the engine is purged of moisture using nitrogen and the fuel side is purged with helium. Now that the engine is free of moisture, the engine can be thermally conditioned for launch. To do this, the main fuel pre-valves are opened allowing liquid hydrogen to flow through the fuel pumps and into the main fuel valve. Some of this propellant is recirculated so that it either dumps some of the hydrogen overboard or pumps it back into the fuel inlet.

Oxygen fills the oxygen side of the engine by opening the oxygen pre-valve, allowing the cryogenic propellant to flow through the oxygen pumps and the three valves. These valves need to be very precisely controlled during start-up as the main oxygen valve that feeds the MCC, the fuel preburner, and the oxygen preburner. The oxygen is also recirculated and has a bleed valve at the fuel preburner oxygen valve, allowing for some oxygen to be dumped overboard. The propellants are held inside the engine for over an hour to fully condition the engine for startup.

Throughout all of this, the main engine computer is monitoring the pressures and temperatures 50 times per second. Four minutes before ignition, there is a final engine purge with helium downstream of the main fuel valve. Assuming all data looks good to the engine controller, the computer will enter an “engine ready” status.

Ignition Sequence

Three seconds before engine start, the bleed valves for the oxygen and hydrogen lines are closed and the engine waits for the command to start. Once this command is received, the first thing that happens is the main fuel valve is fully opened. This valve takes approximately 2/3rds of a second to fully open.

Throttling And Shutdown

Start-up isn’t the only dynamic situation a rocket engine faces. Once an engine is running in a steady state, the engine should be mostly stable. But what happens when the engine needs to throttle down? This changes a lot based on the engine, but generally, this is done by reducing the flow to the preburner or gas generator. This is usually done with one of the control valves, and often by reducing the flow of oxygen to maintain a fuel-rich state.

The same thing is true for shutting an engine down. It’s another dynamic event, and the general rule is to never let the engine get close to stoichiometric conditions. Since engines usually run fuel-rich, this means first reducing the flow of oxygen and then later reducing the flow of fuel.

Generally, engines are shut down as quickly as possible while avoiding high loads. On the RS-25, the initial oxygen preburner oxygen valve motion is limited to 45% per second. The main oxygen valve also is closed at a particular rate, mostly to ensure there was sufficient backpressure on the turbines so that they wouldn’t accidentally overspeed during the shutdown process.

Raptor

Overall, it is very impressive how many considerations there are to every single input and condition, especially given the fact that many of these lessons can only be learned the hard way. It is this mindset that drives SpaceX to test its Raptor engine so quickly. SpaceX believes that by just getting them on the stand, and not treating them as a one-off golden pony, it can learn more quickly. There are countless lessons they have learned that have shaped safe operations during start-up, throttling, and shutdown. On top of this, due to Raptor being full-flow staged combustion (having a fuel-rich and oxygen-rich preburner) the two preburners are even more intertwined. Changing the speed of one has a very direct impact on the other.

The initial spin-up of Raptor is done using either helium or nitrogen, but then there are torch igniters in the preburners and likely some kind of homogeneous ignition in the MCC. From there, there is a very delicate dance to get them up to operational pressure. This is why SpaceX is firing raptors roughly five times a day to get all the kinks out.

Summary

Starting a rocket engine is very hard. While some are easier than others, it is easy to see why companies can easily come up with engine concepts but very few get into production and operation. When a company shares that it has successfully started up a new engine, it is very applause worthy and it means that it has likely made it through one of the biggest hurdles of development.

Starting a rocket engine can be as simple as sending an electrical signal to an igniter that will begin the ignition of a solid rocket motor, or as complicated as having valves that must adjust their position by 2 degrees within a few milliseconds to avoid an engine RUD. It’s genuinely a miracle that people have figured out how to make rocket engines so reliable.

Read More

By |2023-03-11T06:18:40+00:00March 11th, 2023|Education|0 Comments

Related Posts

Leave A Comment