How to make scramjets to work

Recently, as a passion project, I’ve been playing around with one of aerospace engineering’s most fun puzzles: how to make scramjets work. And I think I did it. But before we pop the champagne, let's explore what scramjets are, why they're awesome, and why they did not go mainstream for decades.

What Are Scramjets?

Scramjets (supersonic combustion ramjets) are air-breathing engines capable of hypersonic speeds, starting at Mach 6. Unlike conventional jet engines that rely on compressors, scramjets do not have any moving parts, and incoming air is compressed simply by the geometry of the inlet. They are super simple devices capable of moving vehicles of the air (opens in a new tab) at absurd speeds.

Scramjets are awesome because they can unlock things like:

• Rapid global travel: Think SF to Tokyo within 1.5 hours vs. 11 hours.
• Cost-effective space access
• Real-world Darkstar

And so on.

Why Scramjets Aren't Everywhere

The big boss problem in this story is the flameholding problem - an issue best described as trying to light a matchstick in a hurricane (The name of the author escapes me, unfortunately, so I couldn't make the proper reference. I believe it was a Stanford professor).

Here's the basic description of the problem: the time between air entering through the inlet and exiting through the nozzle (known as residence time) is measured in milliseconds, and within that timeframe, we need to:

a) Inject the fuel.
b) Mix it with the air.
c) Ignite and combust it.
d) Expand the compressed air to generate thrust.
e) Maintain the laminar flow of the air.

Pure madness. We'd need God himself to carefully mix the air and fuel atoms together in the allotted time.

In addition, there are challenges with heat management and cooling, materials, and testing limitations. All of these are challenging yet ultimately solvable. The "big boss" problem here is flameholding. We need a solution that helps us avoid the usual pitfalls (shock-induced quenching, sensitivity to flow uniformity, etc.) - a solution that could be the key to unlocking hypersonic flight.

So What Do We Do?

Being a good engineer is less about solving problems at face value and more about asking the right questions that stem from fundamental truths rooted in physics (yes, that overused first-principle thinking).

Reframing the Problem

Instead of asking, "How do we solve the flameholding problem?" we can ask, "How else can we add enthalpy to compressed air?" After all, the exhaust velocity in a scramjet is determined by the energy added to the airflow, and nowhere is it written that energy must come from combustion of carbon-based fuels.

Don't get me wrong, combustibles are cool. I also read the Ignition! book by John D. Clark and have a soft spot for the decorated community of, as Isaac Asimov eloquently put it:

Now it is clear that anyone working with rocket fuels is outstandingly mad. I don't mean garden-variety crazy or a merely raving lunatic. I mean a record-shattering exponent of far-out insanity.

But we need to get the job done, so combustibles are a no-go for now.

Could we bypass combustion altogether? Can we add heat or enthalpy into the compressed air another way, without requiring carbon-based fuels? The answer is yes.

So, What If We Bypass Combustion?

The idea is very simple: instead of injecting fuel into a combustion chamber, we place a heat exchanger (or, lets say, an array of tungsten rods) into the airflow path where the combustion chamber used to be. Because tungsten has a melting point of about 3422 °C (roughly 3700 K), it can tolerate some of the extreme temperatures we'd encounter at hypersonic speeds.

But that raises the question: Where does the energy come from to heat those rods, and in turn, heat the air? Let us consider:

1. Using Li-ion batteries
2. Harnessing an external energy source (more on this later)

...and see what the math suggests.

A Quick Thermodynamic Refresher

Before diving into possible energy sources, we need a sense of how much enthalpy (energy) must be added to the airflow. This is what we will be optimizing for and using as a benchmark for assessing potential energy sources.

Let's pick:

• Initial speed: Mach 6
• Target speed: Mach 8
• Altitude: ~30 km (where the ambient temperature $T_{\infty} \approx 226.5\,\text{K}$)

Using standard compressible-flow formulas, the total (stagnation) temperature at Mach $M$ is:

where $\gamma \approx 1.4$ for air.

1. At Mach 6:

   $$T_{0,\infty} \approx 226.5 \,\text{K} \Bigl[ 1 + 0.2 \times (6)^2\Bigr] \approx 226.5 \,\text{K} \times 8.2 \;\approx\; 1857 \,\text{K}$$

2. At Mach 8:

   $$T_{0,\text{target}} \approx 226.5 \,\text{K} \Bigl[1 + 0.2 \times (8)^2\Bigr] \approx 226.5 \,\text{K} \times 13.8 \;\approx\; 3129 \,\text{K}$$

So ideally, to go from Mach 6 to Mach 8 at the same altitude, the flow's total temperature must increase from roughly 1857 K to about 3129 K - an increase of 1272 K. Next, we estimate how much enthalpy (energy) that translates to.

Enthalpy Increase per kg of Air

Enthalpy change $\Delta h$ can be approximated by:

where $c_p$ (the specific heat at constant pressure) for air in this high-temperature range might average 1100 J/(kg.K). Thus:

So for each kilogram of air flowing through our "heater," we must supply about 1.4 MJ to accelerate from Mach 6 to Mach 8 ideally.

Well, that is a lot of energy. To put it in perspective, supplying 1.4 MJ every second (1.4 MW) is like lifting 140 one-ton trucks 1 m off the ground every second.

Approach 1: Using Li-ion Batteries

Imagine we have a magical array of Li-ion batteries powering tungsten rods (or a high-temperature heat exchanger). Each rod is heated electrically, and the passing air picks up heat from the rod surface. Simple enough in principle.

But let's look at some numbers for power requirements. If each kilogram of air needs about 1.4 MJ of energy to accelerate from Mach 6 to Mach 8, then the total power ($\dot{Q}$) required scales with the mass flow ($\dot{m}$). Specifically:

• If $\dot{m} = 1\,\text{kg/s}$:

  $$\dot{Q} = 1.4\,\text{MJ/s} = 1.4\,\text{MW}$$

• If $\dot{m} = 10\,\text{kg/s}$:

  $$\dot{Q} = 14\,\text{MW}$$

• If $\dot{m} = 100\,\text{kg/s}$:

  $$\dot{Q} = 140\,\text{MW}$$

Here, $\dot{Q}$ is the rate of heat (energy) transfer - that is, power - you must provide to the airflow each second to achieve that required enthalpy increase. Since $1\,\text{MJ/s} = 1\,\text{MW}$, multiplying the energy per kilogram by the mass flow rate directly gives the power in megawatts.

Those are enormous power levels. By comparison, Li-ion batteries today store around 0.7-1.0 MJ per kg of battery mass (about 200-300 Wh/kg). If our engine runs even briefly at tens of megawatts, we'd rapidly drain or overload any realistic battery pack. You could carry more batteries, of course, but then your vehicle mass skyrockets.

Meet the Wall

That's no good - we hit the wall. The good news is that's what engineering is about: trying ideas, hitting dead ends, and learning from them. We now know pure electric heating from an onboard battery is not the right path. So let's try another approach.

Approach 2: Harnessing an External Energy Source

If batteries aren't suitable, maybe there's another way. We need a powerful, lightweight, and effectively "infinite" source of energy (yes, we can dream) at these speeds.

Wait, remember the last time we watched videos of large objects falling from the sky - rockets and shuttles - and they had that cool-looking glowing plasma that seemed pretty hot? Could we, you know, use that heat instead of just surviving it?

Ha!

After all, rocket engines have long used regenerative cooling, where the fuel is circulated through tiny channels around the nozzle to absorb heat and prevent meltdown. What if we flip that concept around: instead of simply cooling our vehicle's outer surfaces, we harvest that heat and feed it back into the airflow?

The Plasma Sheath and Aerodynamic Heating

When a vehicle flies at extreme speeds (think Mach 6+ or reentry conditions), the leading edges and surfaces experience intense friction, compression, and shock-wave heating. You've probably seen the dramatic footage of SpaceX's Starship reentering Earth's atmosphere with a glowing plasma layer around it.

In that situation:

1. Molecules break apart, losing electrons and forming ions.
2. The boundary-layer temperature near the vehicle can soar into the thousands of kelvins.
3. A radiant "plasma sheath" forms around the craft.

Usually, we treat this heat as a destructive threat to be withstood or dissipated. Let's see if we can capture it instead.

A Heat Exchanger Under the Skin

1. Picture the hot surfaces on your hypersonic vehicle - leading edges, nose tips, underside, etc.
2. Beneath that skin, we embed a closed-loop working fluid (like helium) that:
   • Picks up heat from the vehicle's extremely hot outer surface (or from the plasma interactions near the skin).
   • Routes that hot helium into a heat exchanger inside the scramjet flowpath, transferring heat to the incoming air.

Effectively, the faster you go (and the hotter the skin), the more energy is available to feed back into your airflow - helping sustain or further boost speed. It's a self-reinforcing cycle, provided you manage:

• Structural integrity (materials must survive the heat).
• Efficient heat transfer (so you don't lose it all to radiation).
• Overall drag vs. thrust balance.

Of course, this energy does not come from nowhere: we are tapping into the massive kinetic energy of the onrushing air molecules, converting part of it into thermal energy, then re-injecting that heat into the flow. As long as net thrust exceeds drag, you can ride this loop upward in speed.

Lets Crunch Some Numbers

Disclaimer: the following is a preliminary approximation for a feasibility check, not a high-fidelity study. We just want an order-of-magnitude sense of whether it seems workable.

We need to know two key variables:

1. The leading-edge area exposed to a high heat flux.
2. The magnitude of the heat flux on that area.

The first is relatively straightforward: the exact area depends on the airframe design, but let's pick a small example: 0.5 m^2 of a leading edge.

Estimating the Heat Flux

This part is trickier. Here are two approaches:

1. Using a known order-of-magnitude formula for stagnation-point heating on a blunt body (from reentry research): $$\dot{q}_{\text{conv}} \approx C \,\sqrt{\frac{\rho_{\infty}}{R_n}} \; V_{\infty}^{3},$$

At Mach 6-8, even moderate vehicle sizes and small leading-edge radii (sharp geometry) can see tens to hundreds of MW/m^2 of convective heat flux. For instance, 20 MW/m^2 on the low end to 100+ MW/m^2 at the higher end, depending on altitude, angle of attack, etc.

2. Historical and experimental data:
   • The Space Shuttle during reentry sees localized heat fluxes well over 100 MW/m^2 on its nose cap and leading edges.
   • Capsule reentry (Apollo, Orion) can see peak fluxes over 1000 W/cm^2 (that is, 10 MW/m^2) to hundreds of MW/m^2 for faster entries (e.g., from the Moon or Mars).
   • Hypersonic test vehicles at Mach 7-8+ often reach tens of MW/m^2 on sharp, slender leading edges.

So let's pick 50 MW/m^2 as a plausible middle range for a Mach 6-8 leading edge.

Putting It All Together

1. Leading edge area: 0.5 m^2
2. Heat flux (assumed): 50 MW/m^2

So the total heat load on that region is:

If you capture around 60% of that, that's 15 MW piped into your scramjet flow.

Now, assume the scramjet mass flow is about 10 kg/s of air. If each kilogram of air needs 1.4 MJ to go from Mach 6 to Mach 8:

You're in business! That 15 MW captured from aerodynamic heating is enough (in theory) to boost the flow from Mach 6 to Mach 8.

Of course, real vehicles might see bigger or smaller fluxes, and you'd have to handle plasma chemistry, helium pump power, etc. Still, the concept stands: if you can harness the huge aerodynamic heating already happening on the surface, you might self-supply enough enthalpy for sustained hypersonic flight.

Grain of Salt

Our back-of-the-napkin calculations suggest a promising concept, but turning that concept into a real, flying scramjet is a different story. Every assumption must be tested in both ground facilities and actual flight conditions. Here are a few open questions:

1. Helium Pump

   • How large must the pump be to circulate helium effectively under hypersonic conditions?
   • What power source will drive the pump, and how do we ensure it is reliable at high altitude?
   • How do we handle the potential mass penalty of a high-power pump and its supporting systems?

2. High-Mach Complexities

   • How do we transition smoothly from turbojet or ramjet modes to scramjet operation (if such a transition is required)?
   • Can we maintain stable airflow shaping at Mach 6+ in spite of shocks and high temperatures?
   • What thermal loads and structural stresses arise at these extreme speeds, and how do we manage them?

3. Materials & Maintainability

   • Which high-temperature materials (e.g., tungsten alloys, ceramics) can withstand shock waves, vibration, oxidation, and repeated thermal cycling?
   • How do we ensure vehicle reliability so not toy limited to a small number of flights?
   • What about the broader cooling strategy for avionics, payload, and crew (if any)?

.. and more.

Feasibility check is just the beginning. Turning raw theory into working hardware demands iterative design, systematic testing, and often a healthy dose of reality check - this is where the fun is.

What's Next?

The next step is to build the thing, test it to the limit, break it, iterate, and keep going until we have a working scramjet.

For testing purposes, once we get closer to an integrated scramjet-powered prototype, I’ll need to channel my inner Kelly Johnson and reach out to a DoD general, asking to donate a few missiles to get the prototype up to speed and test it in flight.

But that’s a story for another time.

Final Thoughts

The 'c' in scramjet is for combustion, but if there's no combustion, what do we call it then?