Summary and Key Points: The X-43D was going to be the fastest air-breathing aircraft ever flown, and it does not exist. NASA and Boeing designed it to hold Mach 15 — roughly 11,000 miles per hour, three miles every second, ninety miles covered in thirty seconds of powered flight. The plan was four years of development beginning in Fiscal Year 2005, followed by three test flights spaced a year apart. What stopped it was heat.
The X-43D Could Have Hit Mach 15
The X-43D could have continued a highly successful NASA program, the X-43A. That program produced one of the most incredible airplanes ever devised.
And because of the X-43A’s success and record-shattering nature, the space agency wanted to up the ante.
So, they devised something much faster: an unmanned, hydrogen-powered scramjet demonstrator capable of reaching Mach 15 (around 11,500 miles per hour).
Sadly, the X-43D never flew.
The Background
Nevertheless, one can only imagine the insights such a program might have yielded. After all, the X-43A “Hyper-X” project was an eight-year-long program that cost $230 million.
The X-43A was a 12-foot-long, five-foot-wide research vehicle.
NASA built three X-43A units. They were all expendable, meaning that NASA did not recover the units after they flew. The first two X-43As could hit Mach 7, and the third X-43A was meant to reach Mach 10 during its flight.
X-43. Image: NASA.
X-43A. Image Credit: Creative Commons.
The X-43A was a small experimental research aircraft designed to flight-demonstrate the technology of airframe-integrated supersonic ramjet or “scramjet” propulsion at hypersonic speeds above Mach 5, or five times the speed of sound. Its scramjet engine is an air-breathing engine in which the airflow through the engine remains supersonic.
Using a Pegasus rocket booster, launched from a B-52B Stratofortress, the X-43A would be propelled up to 95,000 feet. From there, a hydrogen-fueled scramjet would activate for 11 seconds and slam the X-43A into Mach 6.8 (around 5,000 mph).
Later that year, in November, NASA sent the next X-43A to go even faster than the March test.
Indeed, the final X-43A reached Mach 9.6 (7,000 mph) at around 110,000 feet.
NASA’s flight path extended westward across the Naval Air Warfare Center Weapons Division Sea Range off Southern California.
The Mach 10 mission’s completion point was roughly 850 miles offshore.
NASA wasn’t done, though.
And that’s where the X-43D would have come into play.
NASA’s Next Generation Launch Technology program, in conjunction with the Department of Defense’s National Aerospace Initiative, operated under what was then known as the hypersonics roadmap. That visionary roadmap (which ended far too early) led to the X-43D proposal.
The engineers on the project asserted they could squeeze 30 seconds of powered flight at Mach 15 from the X-43D.
90 Miles in 30 Seconds? Across the Atlantic in 18 Minutes?!
The X-43D would have traveled 11,000 miles per hour and covered 3 miles every second. In other words, the X-43D would have traveled 90 miles in just 30 seconds of flight time.
In 18 minutes of flight time, if that could ever have been achieved, the X-43D could have crossed the Atlantic!
The biggest issue facing the engineering team, though, was aerodynamic heating.
NASA logo at the Kennedy Space Center. Image by Harry J. Kazianis for 19FortyFive.
Traveling at those, frankly, insane speeds meant generating a great deal of heat.
That heating risked the engine inlet, leading edges, and other critical parts of the proposed vehicle.
NASA and Boeing then looked at active cooling measures because any conventional structure under those temperatures–even for 30 seconds–would face structural integrity diminishment.
In other words, the X-43D was not just an augmented X-43A. It was basically an entirely new system that needed much more tender, love, and care–meaning higher costs.
Originally, NASA and Boeing intended to take four years to develop the X-43D starting in Fiscal Year 2005. After that, three test flights would have been conducted, with one year separating each of the three test flights of the proposed X-43D.
As happens so often with interesting but complex experimental craft, the X-43D was deemed too complex, too short-lived, and too expensive to justify the investment of US taxpayer dollars.
NASA never built a prototype.
One can extrapolate that the costs would have been onerous for the space agency.
As noted above, the X-43A “Hyper-X” program that preceded the proposed X-43D cost around $230 million.
Moving from small experimental vehicles into the Defense Department’s National Aerospace Initiative roadmap meant that those costs would explode.
In fact, one estimate put the hypersonics roadmap from 2005-2018 at $4 billion for engine ground development, plus nearly $10 billion for flight demonstrations.
Those estimates were rough, too.
The probability was high that the Mach 15 requirement and the strain on conventional structures at those speeds would necessitate additional research and development of innovative airframe structures, tanks, and thermal protection system materials.
So the bean counters feared the investment would balloon into costs ten times the existing annual investment.
Meanwhile, the Air Force funding for near-term small-scale scramjet propulsion and flight demonstrations was only around $100 million.
A Change In Priority (and a Grievous Missed Opportunity)
On top of that, President George W. Bush announced a sweeping new initiative at NASA to refocus the space agency on actual spaceflight with his Vision for Space Exploration. NASA’s priorities shifted away from experimenting with hypersonic flight toward eventually reaching Mars.
Of course, that never came, and many pan the Bush administration’s Vision for Space Exploration as a policy failure. At the time, though, the space agency was compelled to comply with the new directive from the forty-third president.
In just three years, from 2001 to 2004, NASA went from zero to Mach 9.6 for $230 million.
These experiments proved that an air-breathing scramjet could work in hypersonic flight.
X-15. Photo by Harry J. Kazianis for 19FortryFive.com.
Imagine what might have been gleaned, and how that information might have helped the US military’s desperate bid to catch up in the hypersonic weapons race today.
Instead, we are playing catch-up with the Russians, Chinese, Iranians, and even North Koreans in the hypersonic game, even though we were designing and experimenting with hypersonic technologies years before they were using that tech.
About the Author: Brandon J. Weichert
Brandon J. Weichert is the Senior National Security Editor at 19FortyFive.com. He also manages The Weichert Brief on Substack. Weichert also hosts “National Security Talk” on Rumble. He is the author of four bestselling national security books, the most recent of which is A Disaster of Our Own Making: How the West Lost Ukraine (Encounter Books). Follow him via Twitter/X @WeTheBrandon.
