Is SpaceX's Raptor engine the king of rocket engines?

Name: Is SpaceX's Raptor engine the king of rocket engines?
Uploaded: 2019-05-25T18:59:59.000Z
Duration: 1 h 38 min 2 s

Introduction

In this section, Tim Dodd introduces the video and explains that he is at SpaceX's launch facility in Boca Chica, Texas to check out the Raptor engine. He also provides some context about the complexity of rocket engines and how he will compare the Raptor engine to other engines.

Introducing the Raptor Engine

Tim Dodd introduces himself as the Everyday Astronaut and explains that he is at SpaceX's launch facility in Boca Chica, Texas to check out the Raptor engine.

The Raptor engine is a methane-powered full flow staged combustion cycle engine that has never been used on a rocket before.

Tim Dodd compares the Raptor engine to other engines including SpaceX's current workhorse, the Merlin engine, as well as the RS-25, F-1, RD-180, and Blue Origin's BE-4.

Why Methane?

In this section, Tim Dodd discusses why SpaceX chose liquid methane as a propellant for their rockets.

Characteristics of Methane

Liquid methane has never been used on an orbital class rocket before.

Tim Dodd discusses some characteristics of methane and why it may be a good choice for rocket propulsion.

What Makes The Raptor Engine Special?

In this section, Tim Dodd explains what makes the Raptor engine special compared to other rocket engines.

The King of Rocket Engines?

While not necessarily being "the best" at anything specific (powerful or efficient), Tim Dodd explains that there are many things that the Raptor engine does really well.

By the end of the video, Tim Dodd hopes to provide enough context for viewers to understand why the Raptor engine is special and how it compares to other rockets.

Understanding Rocket Engine Cycles

In this section, Tim Dodd provides a brief physics lesson on rocket engines and explains different types of pressure-fed rocket engines.

Pressure-Fed Rocket Engines

Rockets are essentially just propellant with some skin around it to keep it in place and a thing on the back that can throw said propellant really fast.

The easiest way to do this is by storing all the propellant in tanks under high pressure then putting a valve on one end of the tank and a propelling nozzle that accelerates the propellant into workable thrust. This is called a pressure-fed rocket engine.

There are different types of pressure-fed rocket engines including cold gas, monoprop, and bipropellant pressure fed engines.

Limitations of Pressure-Fed Rocket Engines

In this section, Tim Dodd discusses some limitations of pressure-fed rocket engines.

COPVs

One big limiting factor of pressure-fed rocket engines is that they can never be higher pressure than the propellant tanks.

In order to store propellant under high pressure, tanks need to be strong and therefore thicker and heavier. Composite overwrapped pressure vessels (COPVs), for example, are capable of storing gases at almost 10000 PSI or 700 bar but are still heavy.

Enthalpy and Rocket Engine Cycles

In this section, we learn about enthalpy and how it relates to rocket engines. We also explore the concept of turbo pumps and the three most common types of liquid-fueled rocket engine cycles.

Enthalpy and Rocket Engine Efficiency

Enthalpy is the relationship between volume pressure and temperature.

Higher pressure and temperature inside the combustion chamber equals higher efficiency and more mass shoved through the rocket engine equals more thrust.

Turbo Pumps and Staged Combustion Cycle

Turbo pumps are used to move hundreds of liters of fuel per second into the combustion chamber with a really high-powered pump.

A tiny rocket engine can be aimed right at a turbine to spin it up really fast, exchanging some of the rocket propellant's chemical energy for kinetic energy which could then be used to spin these powerful pumps. This is known as turbo pumps in staged combustion cycle.

There are limiting factors such as how high pressure always wants to go to low pressure, heat has that habit of melting stuff, etc., while trying to squeeze every bit of power out of your engine.

Types of Liquid-Fueled Rocket Engine Cycles

The three most common types when putting Raptor into context are gas generator cycle, partial flow staged combustion cycle, full flow staged combustion cycle.

Gas generator cycle or open cycle is one of the most common types used on orbital rockets but less efficient since fuel and oxidizer used to spin the pumps is basically wasted.

The spent propellant from preburner in open-cycle system is simply dumped overboard and does not contribute any significant thrust making it less efficient.

Turbo pumps need to be a higher pressure than the chamber pressure, and this means the inlets leading to the preburner is actually the highest pressure point in the entire rocket engine.

Rocket Propulsion: Closed Cycle Engines

In this section, we learn about the closed cycle engine and how it increases engine efficiency by using what would normally be lost exhausts and connects it to the combustion chamber to help increase pressure and also increase efficiency.

The Problem with Fuel Rich Preburners

Running at the perfect fuel and oxidizer ratio is the most efficient but produces a crazy amount of heat.

Running an RP-1 engine fuel rich means unburned fuel appears as dark clouds of soot which can block injectors or even do damage to the turbine itself.

If you want to create a closed cycle engine with RP-1, running the preburner oxygen-rich is necessary.

The Soviet Solution

The Soviets made a special alloy that can withstand the crazy conditions of an oxygen-rich preburner.

With an oxygen-rich cycle, all of the oxygen actually goes through the preburner and just enough fuel goes to give the turbine enough energy to spin pumps fast enough for combustion chamber power.

Now hot gaseous oxygen is forced into the combustion chamber where it meets liquid fuel. They meet and go boom, resulting in a nice clean and efficient burn without really wasting any propellant.

United States' Solution

The United States pursued a closed loop cycle but they went with a fuel-rich preburner instead of an oxygen-rich one.

The RS-25 Engine

In this section, the speaker discusses the development of the RS-25 engine and its unique features.

Dual Preburner Fuel Rich RS-25

RP-1 and LOX are similar in density and ratios, so they can be run on a single shaft using a single preburner.

The large difference between hydrogen and oxygen pumps required two separate preburners.

Two separate shafts created a new problem: high pressure hot gaseous hydrogen was put next to the liquid oxygen pump. A leak could start a fire in the LOX pump.

Engineers had to create an elaborate seal called a purge seal to keep hot hydrogen from sneaking out. It is pressurized by helium so that it's the highest point of pressure.

Full Flow Staged Combustion Cycle

The full flow staged combustion cycle combines fuel-rich and oxygen-rich preburners.

The fuel-rich preburner powers the fuel pump, while the oxygen-rich preburner powers the LOX pump.

SpaceX developed their own super alloys in-house called SX500 capable of over 800 bar of hot oxygen-rich gas.

Full flow likely wouldn't work with RP-1 due to coking problems with a fuel-rich preburner but other fuels are still valid to use this design.

Advantages of Full Flow Staged Combustion Cycle

Both fuel and oxidizer arrive in the combustion chamber as hot gas, resulting in better combustion and hotter temperatures.

There's less need for a complex sealing system, making it easier to reuse the engine with little to no refurbishment between flights.

The inherent increase in mass flow means turbines can run cooler and at lower pressures because the ratio of fuel and oxidizer needed to spin the turbo pumps is much lower.

Full Flow Staged Combustion Cycle Engine

This section discusses the full flow staged combustion cycle engine and its development history.

Development History

The full flow staged combustion cycle engine has only been demonstrated by three engines.

The Soviet Union developed an engine called the RD-270 in the 60s, which never flew.

Aerojet and Rocketdyne worked on an integrated powerhead demonstrator called the Integrated Powerhead Demonstrator in the early 2000s, which also never made it past the test stand.

SpaceX's Raptor engine is only the third attempt at making this type of engine and is hoping to be the first full flow staged combustion cycle engine to reach orbit.

Advantages

Fuel to oxidizer ratios will be so fuel-rich and oxygen-rich that temperatures at turbines will be much lower, resulting in longer lifespans for turbo pump assembly.

More combustion happens in the combustion chamber and less in preburner.

Highest pressures of any chamber pressure ever achieved.

Liquid Methane vs RP-1 and Hydrogen

This section discusses why SpaceX chose liquid methane for their Raptor engine over RP-1 or hydrogen.

Propellant Density

Having a denser fuel means tanks are smaller and lighter for a given mass of fuel.

RP-1 is 11 times more dense than hydrogen, which is only 70 grams per liter.

Methylox is right in the middle at 422 grams per liter.

Oxidizer to Fuel Ratio

Rocket engineers have to take into account the mass of the fuel and the corresponding weight of the tanks so they don't actually burn propellant.

Rocket Fuel Comparison

In this section, the speaker compares different rocket fuels based on their density, fuel efficiency, specific impulse, and boiling point.

Density Comparison

Hydrogen burns at 6 grams of oxygen to 1 gram of hydrogen and methane burns at 3.7 grams of oxygen to one gram of methane.

Burning LOX and RP-1 at a 2.7 to one ratio for every liter of LOX you'd need a little over half a liter of RP-1.

Engineers have found that it pays to burn LOX and hydrogen at a 6 to 1 ratio for a good compromise.

You need 0.73 liters of methane for every liter of LOX.

Specific Impulse Comparison

Specific impulse is measured in seconds and determines how efficient an engine is.

The longer an engine can push with 9.8 newtons of force while sipping on fuel, the higher its specific impulse.

An ideal RP-1 powered engine could achieve about 370 seconds, an ideal hydrogen-powered engine could get 532 seconds, and a methane-powered engine is right in the middle with 459 seconds.

Thermal Considerations

A fuel that burns cooler potentially makes for a longer lifespan for the engine.

The higher the boiling point, the easier it is to store the fuel without insulation on tanks to keep propellant from boiling off.

RB1 has a very high boiling point even higher than water at 490 Kelvin, hydrogen is near absolute zero at a crazy cold 20 Kelvin, and methane is between the two at 3550 Kelvin.

The speaker compares different rocket fuels based on their density, fuel efficiency, specific impulse, and boiling point. Hydrogen burns at 6 grams of oxygen to 1 gram of hydrogen and methane burns at 3.7 grams of oxygen to one gram of methane. Burning LOX and RP-1 at a 2.7 to one ratio for every liter of LOX you'd need a little over half a liter of RP-1. Engineers have found that it pays to burn LOX and hydrogen at a 6 to 1 ratio for a good compromise. You need 0.73 liters of methane for every liter of LOX. Specific impulse determines how efficient an engine is, with an ideal RP-1 powered engine achieving about 370 seconds, an ideal hydrogen-powered engine getting 532 seconds, and a methane-powered engine in the middle with 459 seconds. A fuel that burns cooler potentially makes for a longer lifespan for the engine while higher boiling points make it easier to store fuel without insulation on tanks to keep propellant from boiling off.

LOX and Hydrogen's Temperatures

In this section, the speaker discusses how the temperatures of LOX and hydrogen can affect each other during combustion.

Combustion Byproducts

RP-1 is the only fuel that pollutes with unburned carbons left in the atmosphere alongside water vapor.

Hydrogen produces only water vapor while methane produces carbon dioxide and water vapor.

Methane as a Fuel for SpaceX

This section explains why SpaceX sees methane as an important or even necessary part of their future plans.

Advantages of Methane

Methane is fairly dense, making rocket sizes reasonable.

It burns clean and efficiently, making it highly reusable.

It burns relatively cool, expanding engine lifespan.

It is cheap and easy to produce and can be easily reproduced on Mars.

Comparing Engine Metrics

This section compares different engines based on their fuel type and cycles.

Engines by Fuel Type and Cycle

RP-1

Open cycle Merlin engine powers Falcon 9 and Falcon Heavy rockets by SpaceX

Oxygen rich closed cycle RD-180 powers Atlas 5 rocket by NPO Energomash

Open cycle F-1 powers Saturn 5 rocket by Rocketdyne

Methane

Full flow staged combustion cycle Raptor engine will power Starship and Super Heavy booster by SpaceX

Closed cycle oxygen rich BE4 engine will power New Glenn rocket by Blue Origin

and ULA's upcoming Vulcan rocket

by ULA

Other Fuels

Closed cycle fuel rich RS25 engine powered space shuttle by Aerojet Rocketdyne

The cost of fuels was not discussed in detail due to varying prices that are hard to pin down.

Rocket Engine Comparison

In this video, the presenter compares various rocket engines based on their thrust output, thrust-to-weight ratio, and specific impulse. The engines compared include Merlin, RS-25, Raptor, BE-4, RD-180, and F-1.

Thrust Output at Sea Level

The Merlin produces 0.84 meganewtons of thrust.

The RS-25 produces 1.86 meganewtons.

The Raptor currently is at 2 meganewtons.

The BE-4 is hoping to hit 2.4 meganewtons.

The RD-180 produces 3.83 meganewtons.

The F-1 is still the king out of these at 6.77 meganewtons.

Thrust-to-weight Ratio

Space shuttle's RS-25 has a ratio of 73 to 1.

Merlin leads with an astonishing ratio of 198 to

Specific Impulse

F1 engine has a specific impulse of 263 to 304 seconds

Merlin engine has a specific impulse of282 to311 seconds

RD180 engine has a specific impulse of311 seconds to338 seconds

BE4 engine has a specific impulse around310to340seconds

Raptor engine has a specific impulse between330and350seconds

RS25 engine has a specific impulse between366and452seconds

Chamber Pressure

F-1 had 70 bar in this chamber pressure.

Merlin engine at 97 bar

BE-4 will be around 135-ish bar.

Engine Cost and Reusability

In this section, the speaker discusses the cost and reusability of various rocket engines.

Cost of Engines

The RS-25 is the most expensive engine at over $50 million per engine.

The F-1 costs about $30 million per engine.

The RD-180 costs $25 million per engine.

The BE-4 costs around $8 million per engine.

The Raptor is estimated to cost around $2 million per engine.

Reusability of Engines

Only the RD-180 and F-1 were not reusable or never reused.

The RS-25 was reused up to 19 times with refurbishment.

Merlin hopes to see up to 10 flights without major refurbishment.

BE4 aims for reuse up to 25 times.

Raptor hopes to see up to 50 flights.

Thrust-to-Dollar Ratio

The speaker introduces an interesting concept called thrust-to-dollar ratio, which compares how much an engine costs relative to its thrust output.

Dollar-to-Kilonewton Ratio

In this section, the speaker discusses dollar-to-kilonewton ratios for various rocket engines.

The RS-25 has the highest dollar-to-kilonewton ratio at $26,881 per kilonewton of thrust.

The RD-180 is $6527 per kilonewton.

The F-1 is $4431 per kilonewton.

The BE-4 is $3333 per kilonewton.

The Merlin engine is $1170 per kilonewton.

Raptor has a ratio of around $1000 per kilonewton.

Reusing SpaceX's Merlin Engine

In this section, the speaker discusses how SpaceX reuses its Merlin engines.

The speaker explains that each engine goes from California to Texas for testing before being integrated onto the rocket. Afterward, it returns to Texas for another full duration static fire before finally being shipped to the launch pad. By this point, each engine has already undergone multiple full duration burns and can be reused without major refurbishment.

Thrust-to-Dollar Ratio

In this section, the speaker discusses how Elon Musk hopes to improve the thrust-to-dollar ratio of SpaceX's Raptor engine.

Elon Musk tweeted in February 2019 that he hopes to make the Raptor more efficient in terms of its thrust-to-dollar ratio. This metric compares how much an engine costs relative to its thrust output and could make a big difference in future space missions.

The Cost and Reliability of Rocket Engines

In this section, the video discusses the cost and reliability of rocket engines. It compares different engines based on their cost per kilonewton per flight and their reliability based on the number of operational flights they have had.

Engine Cost Comparison

Blue Origin's BE-4 engine has a potential cost of $133 per kilonewton over 25 flights, making it about as cheap to operate as SpaceX's Merlin engine at $117 per kilonewton per flight.

If SpaceX's Raptor engine lives up to its hype, it could bring the cost down to $20 per kilonewton per flight.

Operational Flights and Reliability

The F-1 engine was used on 17 flights, while the Merlin engine is at 71 flights and catching up quickly to the RD-180 which is at 79 flights.

The RS-25 saw 135 flights, making it the most flown engine.

The space shuttle main engine is over 99.5% reliable, while the Merlin is at 99.9% reliable with only one early failure in its career.

The RD-180 and F-1 are technically considered 100% reliable but with some caveats.

Design Considerations for SpaceX's Raptor Engine

To achieve rapid and full reusability, SpaceX needs an engine that runs clean with low maintenance requirements.

A methane-fueled full flow staged combustion cycle engine sounds like a good fit for these requirements.

For interplanetary trips, methane makes sense because it can be produced on Mars where it has a boiling point that makes it usable on long-duration trips.

Is the Raptor Engine Really the King of Rocket Engines?

In this section, Tim Dodd discusses whether or not the Raptor engine is truly the best rocket engine available.

The Goldilocks Engine

The Raptor engine is not necessarily the most efficient, powerful, or cheapest engine available.

However, it does everything it needs to do very well and is a perfect fit for interplanetary spaceships.

Despite its complexity, SpaceX is developing this engine at a rapid pace and it will only get better from here on out.

The Future of the Raptor Engine

It remains to be seen if the Raptor engine will be the king of other applications besides fulfilling SpaceX's goals for their starship vehicle.

Ultimately, that decision lies with rocket scientists and engineers who make those decisions every day.

Conclusion and Call to Action

Tim Dodd asks viewers if they think it's worth all the hassle to develop such a complex engine like the Raptor.

He thanks his Patreon supporters for helping him create this video and invites viewers to join his exclusive Discord channel and subreddit by becoming a Patreon member.

He also promotes his web store where viewers can purchase space-themed merchandise.