NASA prepares to launch supersonic flight BOLT II

The BOLT II supersonic flight test will kick off Monday night from NASA’s Wallops Test Flight Facility in Virginia.

Supersonic vehicles, which can fly much faster than passenger jets, would allow passengers to get from Sydney to Los Angeles, for example, in just a few hours.

They can also provide more flexible options for launching payloads into space than conventional rockets, and their speed and maneuverability mean they also have a wide range of applications. potential military tactics.

Russia and China have claimed to have operational hypersonic missiles, but supersonic passenger aviation remains a dream rather than a reality.

However, several types of hypersonic vehicles already exist, including rockets, planetary vehicles such as SpaceX’s Dragon capsule, and intercontinental ballistic missiles.

What is supersonic flight?

Supersonic flight is faster than supersonic flight — the latter term, by definition, means faster than the speed of sound.

To break the sound barrier — and get past “Mach 1” — you need to go about 1,235 kilometers per hour or one kilometer faster in less than 3 seconds. Mach 2 is twice as fast, etc

There is no well-defined Mach number that marks the line between supersonic and supersonic flight. But Mach 5 is often considered by aerospace engineers to be where supersonic speeds begin.

Supersonic travel causes some additional problems not encountered at more pedestrian speeds. The most important of these is the fact that the airflow over the vehicle causes the external friction of the vehicle to greatly exceed 1,000 degrees Celsius.

Like all aircraft, flying depends on not having too much mass on board. So specialized materials, either high-temperature ceramic or “abrasive” materials that burn out during flight, are required outside the vehicle to insulate the device against this heat and remain light enough. to fly.

Supersonic engines, known as scramjets (supersonic combustion jets), need to ignite fuel in a supersonic airflow, which is tricky.

Another nagging problem is that supersonic flight is difficult to model accurately, because of the interplay of various physical effects emitted at extremely high speeds.

So if you want to understand everything together, you have to do actual flight tests such as today’s launch. But these are expensive and technically demanding.

Boundary test

One of the most difficult hypersonic problems is predicting something called the “boundary layer transition position”.

As the plane flies through the air, a thin layer of air forms around its surface and is dragged along with the vehicle.

This “boundary layer” is important, as most of the warming happens here, along with a significant portion of the drag trying to slow the car down.

As this boundary layer grows along the length of the vehicle, it will eventually “transition” from a smooth “layer” flow near the front edge to an even more intense “turbulent” flow downstream. .

Although we understand what leads to this “boundary layer transition”, we cannot predict it perfectly, especially at supersonic speeds.

The point is that accurately predicting the transition position of the boundary layer is crucial for the design of hypersonic vehicles. In most cases, turbulent flow is bad. It significantly increases both heating and drag.

Uncertainty about where the flow will be turbulent is a big problem, as heating uncertainty and great drag make some vehicle designs inefficient or completely unfeasible.

BOLT II: a new test

BOLT (short for Boundary Layer Transition) is a $6 million supersonic test flight launched in June 2021 from the Esrange Space Center in northern Sweden to study boundary layer transitions.

But it failed to reach supersonic speed, following problems with the rocket’s launch mechanism.

BOLT II (this time for Boundary Layer Transition and Turbility) is the next scheduled flight in the program, with similar funding but a larger means of ensuring that more current disturbances can be studied. run.

Both the BOLT and BOLT II vehicles feature complex geometries, with concave surfaces to represent a true hypersonic vehicle. The aim is to generate complex real-world data that engineers and scientists can use to improve transition prediction models on hypersonic vehicles.

A separate test was performed on each side of the vehicle, with one side “smooth” and the other “rugged”. The flow length along the car is 1 meter, slightly larger than the original LOCK car.

BOLT II will be launched into underground orbit by a two-stage navigation rocket. During the ascent, it is scheduled to reach Mach 6, where an ascent test will take place. It will travel through space and then re-enter the atmosphere, before performing a descent experiment at Mach 5.5.

The BOLT II is a fully autonomous vehicle and it has more than 400 sensors and onboard devices to collect data on the flow environment during testing.

Assuming that BOLT II’s projected trajectory will be the same as that of the original BOLT flight, BOLT II will reach a maximum altitude of about 281 km. The entire mission will be finished in less than 10 minutes after launch.

Air Force Scientific Research Office

Where to go from here?

To develop the hypersonic vehicles of the future, we need to properly understand how to predict the boundary layer transitions on the actual vehicle geometry and what small effects of turbulent flow on hypersonic vehicles are. how. Data from the BOLT II flight test will help with that.

The launch will be streamed live on NASA’s Wallops Youtube channel, so we’ll know right away whether the flight was a success or not. Assuming so, in the coming years we will see more scientific papers published on breakthroughs from this important experiment.

The ability to accurately predict the propagation of the supersonic boundary layer will bring us closer to supersonic passenger flight someday. NASA’s planned NASP supersonic spaceplane was canceled in the 1990s, in part due to the inability to accurately predict its transit position. Hopefully, we can get over that soon.

However, there are still many problems. Air-breathing supersonic engines are still in their infancy; the materials used to shield hypersonic vehicles are very expensive; and the design of hypersonic vehicles is still very complex. Companies like Australia’s Hyperpersonix, which aims to use an air-breathing supersonic vehicle to launch small payloads into orbit, hope to bring us closer to making the dream of super-flying super-flight a reality. sound into reality.

Chris James is an ARC DECRA Fellow at the Hypersonics Centre, School of Mechanical and Mining Engineering, University of Queensland