High-Speed Flight: Hypersonics Is So Hot Right Now
Links to prior posts in this series: High-Speed Flight Webinar Summary, Commercial Supersonic Flight
Today’s post will be the last in my blog series on High-Speed Flight. Rather than starting with the science, I’m going to kick off by summarizing the beliefs I’ve formed about hypersonics over the last six years of my career working in that field.
- Hypersonics is here to stay. Our adversaries have it. The United States must have it too.
- Mission requirements that are clear, definitive, and fixed over a program life cycle are just as important as understanding the physics in order to create real hypersonic systems.
- Hypersonics is still hard. Our physics-based model uncertainties are large, and we can’t replicate steady-state flight conditions in a ground test environment. To close a hypersonic system design, the trade between thermal protection margin and vehicle weight drives a choice between robustness and risk.
- The near-term commercial market for hypersonics isn’t as clear as the defensive applications. Betting on dual-use technologies decreases the market risk for an investor.
Great Power Competition in the 21st Century
As a technical partner, writing about the geopolitical zeitgeist isn’t the norm. However, to properly understand the context for why hypersonics is here to stay, it’s necessary to evaluate the defense landscape. In the 2018 National Defense Strategy the United States turned its focus away from the War on Terror, and towards the reality of a 21st century Great Power Competition. Unlike the Cold War, the United States now has two opposing near-peer competitors, the Chinese Communist Party and the current Russian government. Our adversaries have tested and fielded hypersonic systems, as summarized on pages 12–16 of this briefing on hypersonics for Congress.
Why are hypersonic capabilities critical to continued US security? The hypersonics advantage for military applications is rooted in the combination of speed plus maneuverability. The maneuverability of a hypersonic weapon creates a wide uncertainty band around its trajectory: these things are very hard to shoot down! The United States has demonstrated the ability to intercept ballistic missiles. Defense against hypersonics is an ongoing area of active research.
In January, The New York Times published what I can only describe as a “hit piece” against hypersonics. I won’t link to that article on principle. Instead, for those who’d like to learn more about why hypersonics are critical to our future defense, please read the detailed rebuttal published here. One critical distinction that the rebuttal makes about US hypersonic systems versus those of our adversaries is the type of payload carried. The US Department of Defense is developing only conventional hypersonic systems. A conventional payload means a standard kinetic energy explosive containing metal fragments, while a strategic payload is a euphemism for nuclear. Limiting the scope of US hypersonics payloads is a deliberate choice. Our adversaries have no such reservations.
My time at a national research lab has given me an immense respect for the people inside and outside the US government who are hard at work to keep us safe.
A Brief History of Hypersonics
Imagine being nearly 100 years ahead of your time in predicting a technology — not just its existence, but the mechanisms for how that technology can be realized in practice. In their wonderful book “Coming Home: Reentry and Recovery from Space” (link to free book PDF), NASA historians Roger Launius and Dennis Jenkins briefly cover one of the earliest concepts for a hypersonic spaceplane: the Sänger-Bredt Silverbird. While getting his PhD at the Vienna Polytechnic Institute in 1929, Eugen Sänger conceptualized a reusable rocket-powered space plane that would fly in a phugoid motion (skip-gliding along the top of the atmosphere). He would later refine the design with his mathematician wife Irene Bredt. The design reference trajectory and method of propulsion were both feasible back then (and variations of this solution architecture are used in military hypersonic systems today). What Sänger underestimated were the heat loads.
There exists somewhat of a debate as to the identity of the first vehicle to achieve hypersonic speeds. As aerospace legend John Anderson notes for The Smithsonian, the first object to travel hypersonic (over 5 times faster than the speed of sound) occurred in 1949, via a ballistic trajectory. The first piloted aircraft to achieve hypersonic flight was the X-15, NASA’s rocket-powered test vehicle, whose first flight occurred in 1960. (Fun fact: Neil Armstrong was an X-15 pilot before joining the Apollo program.)
Other notable milestones for hypersonic flight include NASA’s X-43, which was the first scramjet propulsion system to demonstrate net positive thrust. A scramjet, aka a supersonic combustion ramjet, is an air-breathing propulsion technology that enables an order of magnitude higher fuel efficiency than a chemical rocket. (Rockets take BOTH propellants with them — fuel and oxidizer. Jet engines bring along the fuel and combust it with air they intake during flight.) The X-43A flights occurred in 2004, achieving a top speed of Mach 9.6 and 10 seconds of engine operation.
The next major milestone for hypersonic flight was accomplished by the Air Force’s X-51A flight in 2010. The X-51A demonstrated net positive thrust for a hydrocarbon fueled scramjet, burning for over 200 seconds at Mach 5. The important distinctions between hydrogen as a scramjet fuel and conventional hydrocarbon jet fuel are: (1) hydrogen has a much higher flame speed thus is easier to light in a supersonic combustion environment and (2) hydrogen is more energetic. The X-51A achieving sustained net positive thrust with a hydrocarbon scramjet was the hypersonic Wright Brothers’ Flight moment.
Observant readers will note a ~40-year gap between the X-15 and the X-43 programs for successfully demonstrated hypersonics milestones. Hypersonics development in the US was cyclical for decades. Beyond the lack of an external impetus to develop hypersonic capabilities, another reason US funding for hypersonics came in fits and starts was due to some high cost, high profile failures. Depending on who you asked, there exist many documented reasons as to why the National Aero-Space Plane (NASP) failed. (By the time NASP was canceled in 1992 it was 500% over budget and 11 years behind schedule.) Until a few years ago, I had assumed the heat exchanger technology was the main reason NASP failed. In 2019, I attended a fantastic lecture by Dr. Richard Hallion where he shared a story about the programmatic failures of NASP — constantly changing requirements that grew ever more ambitious — aka “death by scope creep”. (A famous example of the terrors of scope creep is hilariously illustrated in this clip about the Bradley Fighting Vehicle in Pentagon Wars.)
That brings us back to the second point made at the beginning of this post: clear, well-defined requirements that don’t change are just as important to success as understanding the relevant fundamental physics. Engineers can’t design a bleeding-edge solution for problems as challenging as those endemic to hypersonics if the requirements are ill-defined and continually changing. This alone drives one reason to remain optimistic about non-traditional commercial hypersonic companies: they are setting their own requirements for the product, rather than the inefficient and painful iterative process with government program managers that happens today.
Flow Physics in the Hypersonic Environment
Going from subsonic to supersonic flight has an immediate noticeable step-change in the flow physics caused by the shock wave. Conversely, when going from supersonic (Mach>1) to hypersonic (Mach>5) speeds, there is a more gradual breakdown of the simplifying assumptions for idealized flow. In the normal shock relations, the post-shock static temperature scales with Mach number cubed. For a freestream temperature of 245 K (atmospheric temperature at an altitude of 60km), the ideal gas post shock temperature increases from ~400K at Mach 2, to ~1,400K at Mach 5, to ~12,000 K at Mach 15. As illustrated in the below computational image, hypersonic flow is non-ideal, with energy dissipation pathways that reduce the near-wall gas temperature closer to ~6,000 K. (The hypersonic flight environment is dominated by tremendous heat loads. This is still hotter than any existing material melting point. Skip ahead to the Table for more on materials.)
As speed increases above Mach 5, the collisions of the gas molecules are no longer elastic (like pool balls bouncing off each other), meaning that the ideal gas assumption becomes invalid. Instead, the gas particles slam into each other with such high energy that they begin to react. (Their kinetic energy is greater than the activation energy for a reaction to occur.) Additionally, in certain flight regimes, the vibrational excitation of the gas molecules decouples that energy mode from the rotational and translational energy, adding further complexity to the physics-based model. The final pathway for energy dissipation in hypersonic flow is through ionization. A large mass fraction of the electrons are stripped at very high Mach numbers (M>10) and altitudes (>60km). This phenomenology creates the plasma responsible for the communications black-out zone during re-entry. The below graph illustrates the regimes for different hypersonic flow physics and flight trajectories for known vehicles.
These changes to the flow physics, known as thermochemical nonequilibrium flow, are a challenge to model accurately. I promised to keep complex equations out of my blog posts, so I won’t include the Navier-Stokes equations here. [For readers who are interested in the mathematics, the Navier-Stokes equations are mixed-type partial differential equations (PDEs). This means that the solutions are weak (non-unique) and dependent on the local flow properties. In the stagnation region of a hypersonic flow, they behave as elliptical PDEs, while beyond the Mach line they behave as hyperbolic PDEs. Getting a converged solution representative of reality is hard. That’s why test data and simplified approximations (aka hand-calcs) are so important — they anchor us to reality.]
A key point to appreciate about the majority of our modern hypersonic computational fluid dynamics (CFD) solvers is that they use a similar numerical approach to solve the equations: second order accurate finite volume implicit Reynolds Averaged Navier Stokes (RANS). This method does a decent job predicting steady-state integrated pressures for the vehicle, with uncertainties around +/-8–10%, which is reasonably within experimental measurement uncertainty bands. It’s the heat flux predictions where they all suck, and they all suck together, to varying degrees of inaccuracy with an enormous range of uncertainties from -50% to +75%.
One of the reasons all hypersonic CFD solvers have such large uncertainties on heat flux predictions is due to the lack of high-quality data in the flight regime. We are all still extrapolating our chemical reaction rates from 1960s shock tube data, because it’s the best data we have. (Tackling this source of epistemic uncertainty head-on, researchers are computing reaction coefficients from first principles statistical atomic collisions and collecting new shock tube data.) The below charts illustrates the disconnect between ground test data we can acquire and the regimes where we actually fly.
CFD practitioners create high-quality converged solutions by paying careful attention to the flow field discretization (aka grid quality and timestep), boundary conditions, initial conditions, and selection of turbulence models appropriate for the flow regime. (For further reading see pp. 16–24 of AIAA-2016–3487.) TL;DR — It’s very easy to create a garbage result using CFD!
Solutions for Surviving High Heat Load Uncertainty
In my last blog post, I shared a graph from Shapiro (aka the compressible flow bible) illustrating how pressure loads on a vehicle scale with velocity squared. What makes hypersonics hard is trying to survive the environment resultant from the heat flux scaling with velocity cubed.
Last time we also learned the importance of the shape of an aerodynamic vehicle — that the form dictates functional performance. Subsonic vehicles have rounded edges. Supersonic ones have sharp leading edges. Hypersonic vehicles go back to rounded (aka blunt) shapes. When designing a hypersonic vehicle, the top engineering priority shifts from reducing drag to surviving the heat loads. Blunt body theory was developed to push the shock wave off from the vehicle, as conventional materials cannot withstand the post-shock environment.
Key takeaways from the above table. (1) Aerospace engineers love titanium because it’s light and strong, but its melting point is low. (2) Tungsten has a much higher melting point, but it’s very heavy and somewhat brittle. (3) Lots of ongoing research into using Silicon Carbide, among other types of Ultra High Temperature Ceramics (UHTCs), as a next-gen material for leading edges and as a coating layer on other parts of the vehicle. The problem with ceramics is their low fracture toughness and lack of tensile strength. (Impact resistance is a problem for all-ceramic thermal protection solutions, with the most tragic example being the loss of Shuttle Columbia.)
There are three types of thermal protection systems (TPS): passive, ablative, and active. Passive thermal systems are reusable; they rely on radiative heat rejection and low thermal conductivity materials (e.g. the Space Shuttle). Ablative thermal systems are not reusable, the heat load transfers into a material that pyrolyzes off during reentry (e.g. every manned reentry capsule to date). Active thermal systems circulate a fluid to reject the heat load — this is where promising technology development is ongoing for reusable hypersonic vehicles. (NASA has an amazing presentation on heat loads and thermal protection design, for the curious reader.)
Synthesizing what we just learned about the hypersonic flow environment (it’s really hot), the ability to predict heat flux (CFD codes all suck by 50% or more), and the solutions available to withstand it — typical TPS designs will have a margin of 3x to ensure survivability of the vehicle in a worst-case scenario. These large TPS margins eat into the weight available to carry passengers and cargo. Hypersonic vehicle designers deal with the large uncertainties on predicted heat loads by allocating a substantial amount of their weight budget to thermal protection systems.
Hype for the Hypersonics Market
It wouldn’t be an EVS blog without me pointing out an amazing example of work funded by NASA (#Patreon4NASA). Today, I want to highlight the high-speed flight commercial market studies that NASA funded, again all available for free on NTRS. [BryceTech/SAIC Summary Presentation and Report. Deloitte/SpaceWorks/NIA Summary Presentation and Report.] If you’re ever daydreaming of better travel while on a long-haul subsonic flight, peruse these reports to understand the dynamics driving a faster future. Both groups did wonderful work analyzing the market, business case, and barriers. The first two charts I’d like to highlight illustrate the time savings by speed class and the customer demand signal for the value of that saved time from surveyed high net-worth individuals.
What stands out from these charts is that the nautical range for city pairs is just as important a characteristic for a high-speed commercial transport as the speed at which a vehicle flies. Supersonic jets hit the demand sweet spot for both Atlantic and Pacific routes. Commercial hypersonic jets have the opportunity to differentiate themselves along the Pacific routes. Based on the above charts, Prime Movers Growth portfolio company Boom has a well-positioned product with their supersonic Overture aircraft.
The work done by Deloitte/SpaceWorks/NIA utilizes a conceptual aircraft design and cost modeling tool developed by SpaceWorks called ROSETTA. The following charts show the Internal Rate of Return for different high-speed aircraft. A notable conclusion from both reports is that the business case for high-speed flight exists without government subsidies.
The BryceTech/SAIC report has a great chart illustrating the high-speed flight market landscape. (Note: this study was done before Prime Movers Lab portfolio company Venus Aerospace went out of stealth, so they do not appear in the chart. Their range and propellant would spot them with the class of vehicles described in Case 5 below.)
One reason NASA ordered these market studies was to solicit outside feedback on what they could do to help remove barriers for the commercial hypersonics aircraft companies. NASA wanted to understand the customer demand for high-speed flight, the pressures on the business case, and any non-technical roadblocks (e.g. regulations). Both reports tested sensitivity to government R&D spend and government purchase of a certain minimum number of hypersonic aircraft. The good news for hypersonic commercial aircraft companies is that neither is required. The better news is that the US government will continue to invest heavily into hypersonic R&D because of the defensive applications.
At the top of this post, I stated that dual-use technologies can be a way to reduce market risk. If the developed technology can also be used for defense applications, the business case for these hypersonic aircraft developers gets much stronger.
Enabling Technologies for Commercial Hypersonics
The most exciting innovations for hypersonics are happening in propulsion and active thermal management systems for reusable hypersonic vehicles. UK company Reaction Engines has the SABRE engine, which uses a novel air-breathing rocket combined cycle. In 2015, the Air Force Research Lab independently validated the SABRE engine’s thermodynamic cycle. (This author loves a good closed-loop regeneratively driven turbine cycle!) Reaction Engines also has heat exchanger technology they validated in 2019 under a full-scale testing program funded by DARPA. Another approach to hypersonic propulsion technology is being taken by Atlanta-based startup Hermeus, which is using a turbine ramjet combined cycle for their propulsion system. While the turbine-based combined cycle (TBCC) is not novel, stable mode transition between turbine engine and ramjet operations at hypersonic speeds will be a major technical breakthrough. (In 2016, DARPA funded the Advanced Full Range Engine (AFRE) program for a hypersonic TBCC demonstrator engine. There is no public information on the result of the AFRE program.) Prime Movers Lab portfolio company Venus Aerospace is working on their own novel approach to propulsion technology and active cooling. I wish I could share more at this time! Like all Prime Movers Lab companies, they are leveraging a recent scientific breakthrough for their key technology.
Earlier in this article, the flight demonstration of scramjet combustors was hailed as hypersonics’ Kitty Hawk moment. Yet none of the startups covered in the prior paragraph are focused on scramjets for their propulsion solution. This is because current scramjet designs are constrained in size and operational envelope due to the combustion dynamics at supersonic speeds. The challenge to overcome is that supersonic combustion dynamics is mixing limited. To improve scalability and operability, clever engineers are working on solutions that improve mixing efficiency while maintaining reusability and without substantially increasing pressure losses. Two promising research approaches to watch come from UT Arlington and FGC Plasma.
What about breakthroughs in materials? For systems that are not reusable, the most exciting innovation in heat shield materials comes from NASA’s 3-D carbon fiber woven HEEET material, which will enable the capsules that return humans safely from Mars. For passive thermal management systems, there is research being done on additive manufacturing for Ultra High-Temperature Ceramic structures [Ref1, Ref2, Ref3, Ref4], to enable hybrid material architectures that provide more robust properties.
These new hypersonic technologies, combined with modern multi-disciplinary engineering design and optimization tools, are enabling humans to push the envelope of what’s possible for high-speed flight.
Prime Movers Lab invests in breakthrough scientific startups founded by Prime Movers, the inventors who transform billions of lives. We invest in companies reinventing energy, transportation, infrastructure, manufacturing, human augmentation, and agriculture.
Sign up here if you are not already subscribed to our blog.
