Rockets have gone mostly unchanged since their introduction in the 20th century. From Apollo 11 in 1969 to the SpaceX Falcon Heavy missions, which began in 2018, both were propelled by rockets with bell-shaped nozzles. So this design is not only tried and true, but stout enough to sling spacecraft outside of Earth’s orbit. But what if there was a better way?

Those behind the aerospike rocket engine certainly believe so. This relatively innovative concept promises to capitalize on the shortcomings of the early bell-shaped nozzle rockets, which were inefficient, expensive, and heavy. Engineers have been toying with the aerospike engine concept since the 1950s, but interest picked back up in the early 2000s with NASA’s Project X-33. In fact, just this year the German military recently awarded a contract to Polaris, a small startup testing out a new linear aerospike rocket engine.

A Quick Primer on Rocketry

Before we get started, let’s talk about how conventional rockets function—and how the aerospike can bring us to the next level. If it wasn’t already apparent, all rockets need to use a nozzle of some type to accelerate hot exhaust to produce thrust. The nozzle itself is nothing more than a specially shaped tube, which hot gasses can flow through.

All rocketry functions through Newton’s third law of motion:

For every action, there is an equal and opposite reaction

Why Are Rocket Nozzles Bell-Shaped?

space shuttle discovery
NASA’s Space Shuttle Discovery.
Bjarte Rettedal//Getty Images

Pictured above are conventional bell-shaped rocket nozzles—also known as a convergent-divergent nozzles—on the space shuttle Discovery. Inherent in its name, the nozzle converges down to a pinch point and proceeds to diverge and expand towards the exit. The size of the convergent point (also known as the throat of the nozzle) can be altered to tune the amount of thrust the rocket produces; this process is critically important, as this design produces varying levels of performance at different altitudes.

This means that the size of the nozzle’s throat needs to be chosen to produce optimal performance during the burn cycle as the spacecraft climbs. “Basically, you pick the best operating altitude…and then you realize when you get to high altitude, your efficiency is going to decrease as you’re not gaining all the momentum you could,” says Stephen Whitmore, a mechanical and aerospace engineering professor at Utah State University. It’s a notable disadvantage of the bell-nozzle design that forces engineers to make a calculated compromise with the size of the nozzle’s throat section.

The bell nozzle is actually more efficient in space compared to being near the Earth’s surface. This is because the air pressure within our atmosphere inhibits the thrust generated by any given rocket—meaning they produce more thrust in space than on Earth.

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How Aerospike Engines Work

a closeup of one of the cesaroni technology, inc constructed aerospike nozzles used in the dryden aerospike rocket test
A closeup of one of the aerospike nozzles used in the Dryden Aerospike Rocket Test.
NASA/Carla Thomas

The aerospike engine—specifically the conical aerospike—looks quite similar to a conventional rocket and functions using largely the same principle: exchanging thermal energy for kinetic energy. You’ll see in the cross-section above that the aerospike uses a, well, spike-shaped section that fits inside where the divergent part of a bell nozzle would be. “It replaces this fixed boundary with a free boundary…instead of the bell shape being an external boundary, it’s an internal boundary which you push against,” says Whitmore.

You may have also seen a linear aerospike engine, which is essentially a conical aerospike that’s been unrolled and flattened out. This configuration was used for Project X-33, as it was better suited to the flat shape of the spacecraft. “The linear aerospike for the X-33 was driven by basically what the base of the x-33 looked like,” says Whitmore.

stennis space center successfully performs 90 second firing of the linear aerospike srx 2200 flight engine set to test electro mechanical activator technology for the sli program
A test of the twin Linear Aerospike XRS-2200 engine for the X-33 program is performed at NASA’s Sternis Space Center, Mississippi, on August 6, 2001.

Plain and simple, it’s essentially a bell nozzle that’s been turned upside down and inside out. This means the aerospike nozzle can not only be packaged smaller than a bell nozzle but can compensate for altitude much more effectively. This produces a considerable reduction in performance drop off while the rocket is catapulted to the outer edges of the earth’s atmosphere.

So by now, you might be curious why we keep mentioning the importance of dropping weight and increasing performance for space exploration. While weight reduction is often labeled as a way to go faster, space exploration is all about carrying a heavier payload further. To offer some perspective, Whitmore mentioned that every pound of payload currently costs roughly $10,000 to launch into space.

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What About Thrust Vectoring?

Along with sufficient gains in efficiency, the aerospike engine also promises advantages when it comes to control.

Bell-nozzle rockets use mechanical systems called gimbals to produce differential thrust—used to control the direction of any given spacecraft. This involves a system of hydraulic actuators that adjust the direction the rocket nozzle is pointing. Along with not offering a ton of control, this system adds weight, which takes away efficiency.

a rocket engine gimbal test
A closeup of a J-2X rocket engine during a gimbal test at Stennis Space Center, 2013.

While gimbals are active systems for controlling differential thrust, an aerospike engine use a passive process called pressure injection.

Passive differential thrust means that the rocket’s plume can be manipulated without needing any of the hefty gimbals seen on bell-nozzle rockets. Instead of changing the direction of the nozzle itself, the aerospike can inject pressure through a series of ports and passages within the spike itself to manipulate where the power goes. Along with simplifying the process, subtracting the weight of the gimbals means the rockets can be much more efficient—lighter spacecraft use less fuel to go a given distance.

Watch the video below to see a gimbal test in action on a J-2X engine—and turn your volume down!

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How We Got Here

Contrary to what you might think, the aerospike has actually been in the design phase for quite a while. The Rocketdyne Propulsion & Power unit of Boeing laid the foundations in the 1960s and 70s for the same aerospike we’re writing about today.

NASA’s Project X-33 is what really got the ball rolling in the late 90s and early 2000s. Partnering with Lockheed Martin Aeronautics Co., in Palmdale California, they took the original idea and brought it up to date with new technologies and materials.

It’s no accident that the aerospike engine checks most of the boxes that NASA was looking for in its plans to build a reusable launch vehicle (RLV), which needs a propulsion system that’s lightweight and efficient, but stout enough to sling it into space. It also needs to be affordable enough to operate, with good reliability and short turnaround times between flights.

Validation tests began in 1998 not long after the working group was put together. Proceedings took place at NASA’s Stennis Space Center in Mississippi, where the aerospike was first fired up in the spring of 2000. In all, the team conducted 14 hot fire tests—fancy NASA speak for firing up the engine.

The Aerospike Isn’t Perfect—Yet

With all of the promised advantages of the aerospike, you might be wondering why NASA hasn’t already ditched the bell nozzle. Spoiler alert: the aerospike still needs some work before it’s ready to be crammed in the back of a spacecraft with passengers onboard.

Whitmore noted that the new design creates much higher temperatures in the throat of the nozzle—the existing materials used in bell nozzles would simply just melt. “This increased heating level will require that the operational nozzles will use throat inserts fabricated using more exotic materials such as refractory metals, pyrolytic graphite, or Boron Nitride—a material used to insulate nuclear reactor chambers,” says Whitmore.

While the aerospike does come with a whole host of benefits, Whitmore cautioned that they aren’t the magic bullet that many promised them to be. He reckons that aerospikes likely wouldn’t be replacing bell-nozzle rockets for another 20 years or so. It’s an idea that’s bounced around the aerospace engineering industry for quite some time, but could actually come to fruition given how far material sciences have advanced in such a short period of time.

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Matt Crisara

Matt Crisara is a native Austinite who has an unbridled passion for cars and motorsports, both foreign and domestic, and as the Autos Editor for Popular Mechanics, he writes the majority of automotive coverage across digital and print. He was previously a contributing writer for Motor1 following internships at Circuit Of The Americas F1 Track and Speed City, an Austin radio broadcaster focused on the world of motor racing. He earned a bachelor’s degree from the University of Arizona School of Journalism, where he raced mountain bikes with the University Club Team. When he isn’t working, he enjoys sim-racing, FPV drones, and the great outdoors.