The “Impossible” Tech Behind SpaceX’s New Engine

Followers of the Church of Elon will no doubt already be aware of SpaceX’s latest technical triumph: the test firing of the first full-scale Raptor engine.

Followers of the Church of Elon will no doubt already be aware of SpaceX’s latest technical triumph: the test firing of the first full-scale Raptor engine. Of course, it was hardly a secret. As he often does, Elon has been “leaking” behind the scenes information, pictures, and even video of the event on his Twitter account. Combined with the relative transparency of SpaceX to begin with, this gives us an exceptionally clear look at how literal rocket science is performed at the Hawthorne, California based company.

This openness has been a key part of SpaceX’s popularity on the Internet (that, and the big rockets), but its been especially illuminating in regards to the Raptor. The technology behind this next generation engine, known as “full-flow staged combustion” has for decades been considered all but impossible by the traditional aerospace players. Despite extensive research into the technology by the Soviet Union and the United States, no engine utilizing this complex combustion system has even been flown. Yet, just six years after Elon announced SpaceX was designing the Raptor, they’ve completed their first flight-ready engine.

The full-flow staged combustion engine is often considered the “Holy Grail” of rocketry, as it promises to extract the most possible energy from its liquid propellants. In a field where every ounce is important, being able to squeeze even a few percent more thrust out of the vehicle is worth fighting for. Especially if, like SpaceX, you’re planning on putting these new full-flow engines into the world’s largest operational booster rocket and spacecraft.

But what makes full-flow staged combustion more efficient, and why has it been so difficult to build an engine that utilizes it? To understand that, we’ll need to first take a closer look at more traditional rocket engines, and the design paradigms which have defined them since the very beginning.

The Open Cycle Engine: Wasteful by Design

Many of the best known rockets to have ever flown have used engines based on what is known as the gas-generator cycle, including the Saturn V, the Soyuz, the Delta IV, and even the Falcon 9. In fact, outside of the Space Shuttle, you could probably argue that nearly every milestone in the history of spaceflight was made with a gas-generator cycle engine. It’s a technology that dates back to the V-2 rocket, and is one of the key breakthroughs that made liquid-fueled rockets possible. But despite its incredible success, the technology is not without its faults.

Merlin engine with black preburner exhaust

Put simply, the gas-generator produces the gasses that spin a turbine, which in turn drives the propellant pumps. In some engines, the gas-generator operates on a different principle than the engine itself and has its own separate fuel supply. For example, the turbine of the V-2 rocket was spun with steam created by the chemical reaction between hydrogen peroxide and warm sodium permanganate.

The downside of this method is that the secondary fuel system for the gas generator adds additional weight and complexity, the last thing you want on a rocket engine. The more modern approach, used in engines such as the Falcon 9’s Merlin, is to power the turbine by burning a relatively small amount of the same fuel and oxidizer as the primary engine. In this case, the gas generator is usually referred to as a preburner.

But if the preburner was given the same fuel and oxidizer as the engine itself, and at the same ratio, it would essentially just be a smaller rocket. The exhaust would be far too hot to run through a turbine. To get around this, the preburner is run on a fuel-rich mixture which leads to incomplete combustion and lower exhaust temperature.

The defining characteristic of what’s known as an open cycle engine is that the exhaust from the preburner gets dumped overboard as a waste product. In some rockets this unburned fuel can be seen as a black streak alongside of the otherwise bright exhaust plume. It’s never been a secret that there were performance gains to be had by closing the cycle, that is, capturing the preburner exhaust and feeding it into the engine’s combustion chamber. But the sooty exhaust produced from the unburned kerosene is unsuitable for recirculating through the engine. It ended up being easier to simply build larger rockets than try to capture this lost fuel.

Finding the Right Mix

In the 1950’s, Soviet scientists came up with something of a compromise. Instead of using a fuel-rich mixture in the preburner which produced an exhaust that couldn’t be safely recirculated into the engine, they experimented with running the preburner oxygen-rich. Unfortunately this idea solved one problem while creating another, as there was no metal that could survive the incredibly hot oxygen-rich gas produced by the preburner. In fact, American scientists had deemed it impossible, and believed claims that their Soviet counterparts were working on the concept to be Cold War propaganda.

Eventually the Soviets mastered the metallurgy required to build the turbine and developed several engines that operated on the oxygen-rich preburner concept. The exhaust was piped into the engine’s combustion chamber and recovered at least some of the propellants which would otherwise have been dumped overboard. The modern day Russian RD-180 engine, currently in use by the American Atlas V, is a continuation of this technology.

American engineers went in the opposite direction. They believed that a fuel-rich mixture in the preburner was possible and could be done with existing metal alloys, so long as hydrogen was used as the fuel instead of kerosene. This ultimately lead to the development of the Space Shuttle Main Engine, which to date remains the most efficient liquid fuel rocket engine ever flown. While the Space Shuttle has long since retired, a variation of the engine itself will go on to power the Space Launch System. It will be the most powerful rocket NASA has ever built and is slated to begin missions in 2020.

Closing the Cycle

Either approach, whether it recaptures the oxidizer or fuel rich preburner exhaust, is clearly an improvement over dumping everything overboard. But neither is an ideal solution as there’s still potentially combustible products being wasted. Essentially the Soviet Union and the United States had both solved different halves of the same problem; to truly close the cycle, the fuel-rich and oxidizer-rich preburner designs would need to be utilized simultaneously in the same engine.

The logical end result is the full-flow staged combustion engine. Such an engine has two independent pumps for oxidizer and fuel, spun by two turbines powered by their own dedicated preburners. But unlike the gas-generator cycle which burns just a small amount of fuel and oxidizer in the preburner, in a full-flow staged combustion engine, all of the fuel and oxidizer is run through their respective preburners. In other words, the fuel is burned twice: once at a lower efficiency in the preburners to produce energy for spinning the turbines, and again at maximum efficiency in the combustion chamber to produce thrust. While an exceptionally difficult engine to design and test, it completely eliminates the waste of the gas-generator cycle.

But increased fuel efficiency isn’t the only advantage to this design. For example, the seals separating the turbine and the pump are less critical, as there’s no concern of contaminating the pumped liquid; ultimately they’re going to the same place. In addition, the fact that the fuel and oxidizer enter the combustion chamber as gasses further improves engine efficiency over conventional designs which spray them in as liquids.

The individual turbines on a full-flow staged combustion engine also tend to run cooler and at lower pressure than the single turbine used in a conventional rocket engine. This puts less stress on the turbine impellers, and should allow them to run for longer before they need to be inspected and replaced. Given SpaceX’s focus on reusability, this is likely seen just as important as the increased fuel efficiency.

The Wait is Almost Over

SpaceX Starship Prototype

While it will be the first one to fly, the SpaceX Raptor isn’t actually the first full-flow staged combustion engine to be built. The RD-270 was completed in 1967 by the Soviet Union as part of their program to reach the Moon, and performed several static test burns. But after the United States landed on the Moon in 1969, effectively winning the Space Race, the engine (and the rocket it was meant to power) was canceled. Less complex, and ultimately cheaper, engines were used for the remainder of the Soviet space program.

In the 1990’s, the United States Government expressed an interest in the development of a domestic full-flow staged combustion engine. While there was promising turbine and preburner development by contractors Rocketdyne and Aerojet, neither the Air Force nor NASA chose to provide the funding that would have been required to complete the entire engine.

So when will Raptor finally become the first full-flow staged combustion engine to leave Earth? If you believe Twitter, very soon. Elon Musk claims a prototype of the Raptor-powered Starship will be making low-altitude “hops” around their test facility located near Brownsville, Texas within the next couple of months. Of course a lot can happen between now and then, so it’s entirely possible that date may slip. But even still, it seems like 2019 will finally be the year that the “Holy Grail” of rocket engines finally takes to the air. Here’s hoping its been worth the wait.

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SpaceX test fires Raptor engine for first time

Engineers at SpaceX have carried out a full-power test of a methane-fuelled full-flow staged combustion cycle engine for the first time. The Raptor …

Engineers at SpaceX have carried out a full-power test of a methane-fuelled full-flow staged combustion cycle engine for the first time.

The Raptor engine, which is fuelled by methane and liquid oxygen, instead of the hydrogen and liquid oxygen mix used in most rockets, is panned to be used by the company’s Super Heavy and Starship reusable rocket and spacecraft system which will transport crews and cargo to the Moon and Mars.

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Elon Musk announced the milestone test results for the Raptor in a series of tweets posted from February 7:

Raptor reached 268.9 bar today, exceeding prior record held by the awesome Russian RD-180. Great work by @SpaceX engine/test team! pic.twitter.com/yPrvO0JhyY

— Elon Musk (@elonmusk) February 11, 2019

The first test of the Raptor engine was conducted in September 2016. Three of the engines are built into its Starship Hopper prototype vehicle, which is being developed in Texas.

As well as being fuelled by methane, the Raptor is a full-flow staged combustion cycle engine.

FFSC engines have a twin-shaft staged combustion cycle that uses both oxidizer-rich and fuel-rich preburners, enabling the full flow of both propellants through the turbines

There have been two earlier attempts at creating a FFSC rocket motor. The Soviet RD-270 engine developed during the 1960s, mentioned by Musk in his tweet, and the Integrated Powerhead Demonstrator engine which was developed by Aerojet and Rocketdyne in the late 1990s and early 2000s.

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SpaceX Mars launch: Elon Musk vows ‘sell your house and fly to Mars’ for THIS ticket price

The South African genius behind SpaceX, Tesla and Paypal, aims to put astronauts on the surface of Mars by the year 2024. In a recent spate of …

In a direct reply to the SPEXcast science podcast Twitter account, teased future Mars mission launches will cost between £77,830 and £390,100 ($100,000 and $500,000).

Included in the ticket price, of course, is a free ride back to Earth should the astronauts decided not to stay behind.

In the Twitter exchange, SPEXcast asked: “What are the estimated costs for tickets to Moon/Mars accounting for reusability?”

Mr Musk answered: “Very dependant on volume, but I’m confident moving to Mars (return ticket is free) will one day cost less than $500k and maybe even below $100k.

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SpaceX’s Mars Raptor engine claims rocketry record on path to lift-off levels of power

How does one lift 100 metric tons of spacefaring gear into low-Earth orbit and onward to Mars? For SpaceX and its ambitious CEO, that means …

Overnight, continued test-firing of the Raptor engine saw it reach 268.9 bar, surpassing the chamber pressure record set by the Russian-built RD-180, which today powers the Atlas V rockets for United Launch Alliance.

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New SpaceX Raptor Engine Beats the Chamber Pressure of Russia’s RD-180 Engine, According …

2019 has been shaping up to be an interesting year for SpaceX and its founder, Elon Musk. After completing work on the miniaturized version of the …

2019 has been shaping up to be an interesting year for SpaceX and its founder, Elon Musk. After completing work on the miniaturized version of the Starship (Starship Alpha or “Starship hopper”) over the holidays, SpaceX moved ahead with the test-firing of its new Raptor engine in late January/early February. In accordance with Musk’s vision, these engines will give the Starship the necessary thrust to reach the Moon and Mars.

The test-firing took place at SpaceX’s Rocket Development and Test Facility, located just outside of McGregor, Texas. As Musk recently tweeted, the tests went very well, achieving the thrust necessary for both the Starship and its first-stage booster, the Super-Heavy. Musk also claimed that the engine broke the previous record for combustion chamber pressure, which was established by the Russian RD-180.

The RD-180 was the product of the Soviet-era Energia rocket program, which sought to create a super-heavy launch vehicle that would take the reusable Buran spacecraft (Russia’s version of the Space Shuttle) into orbit. While the program was discontinued, the engine survived and was even imported to the US, where it became part of Lockheed Martin’s Atlas III rocket and United Launch Alliance’s Atlas V.

Raptor reached 268.9 bar today, exceeding prior record held by the awesome Russian RD-180. Great work by @SpaceX engine/test team! pic.twitter.com/yPrvO0JhyY

— Elon Musk (@elonmusk) February 11, 2019

For decades, the RD-180 and its predecessor (the RD-170) held the record for combustion chamber pressure, ranking in at 26.7 MPa (3,870 psi) and 24.52 MPa (3,556 psi), respectively. However, three days after Musk initially tweeted about the Raptor’s performance results, he declared that the chamber pressure had “reached 268.9 bar” – which works out to 26.89 MPa (3,900 psi), thus establishing a new record by about a 1% margin.

Musk was asked if this performance had anything to do with SpaceX’s decision to use a combination of cryogenically-supercooled liquid methane and liquid oxygen (LOX) to power the engines, which will reportedly be able to give them a 10-20% boost over conventional engines. To this, Musk replied in the negative, stating that the methane and LOX fuel were barely kept below liquid temperature for the test-firing.

When kept at cryogenic temperature, Musk anticipates that the engine will be able to achieve its target chamber pressure of 300 bar (4350 psi), but went on to state that “only 250 bar is needed for nominal operation of Starship/Super Heavy.” If this proves to be true, the Raptor will have exceeded the previous performance record by a margin of 11%.

Artist’s impression of the prototype Starship, known as Starship Alpha (or Starship Hopper). Credit: SpaceX

Granted, these results are all preliminary and the Raptor has yet to perform as part of an integrated system. And with almost 20 years of experience under its belt, reaching chamber pressures of up to ~257.5 bar (3735 psi) regularly aboard the Atlas V rocket, the RD-180 still has the edge. However, with test flights of the Starship Alpha expected to begin next year, we won’t have to wait long to see if the Raptor can truly outperform the RD-180.

If all goes well, SpaceX hopes to being construction on a full-scale prototype of the Starship in late 2019 or early 2020. Based on the tentative dates Musk has provided in the past, it is hoped that the Starship will be ready to conduct its first cargo flights as early as 2022, its first lunar passenger flight by 2023, and its first crewed flight to Mars by 2024 (which is to be followed by the construction of Mars Base Alpha by 2028).

Further Reading: Teslerati, Twitter

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