Hypersonic Reentry

Harvard grad student Corin Wagen recently wrote a post on the “more pharma manufacturing” part of the Varda flywheel below.

I hope to achieve the same level of quality for talking about the other part of the Varda flywheel: More DoD rideshares.

I’ll be covering why we should care about rideshares in the first place, going in depth about how Varda’s reentry system could work and some thoughts of mine on future directions Varda could go with their reentry program.

Enjoy!

Why should we care about rideshares?

As noted in Not Boring’s analysis, hypersonic testing is critically needed because China has gained nuclear-capable hypersonic missile capabilities and our defense system isn’t yet prepared to fully handle that. However, preparing for this by conducting our own hypersonic testing is difficult.

Why?

First, to get realistic hypersonic conditions other than by launching a missile (which is a big no no because it could provoke enemies), you would have to drop an object from orbit without it immolating its insides and thus not giving any useful data.

So, the only real way to do realistic hypersonic testing is to do a rideshare on either SpaceX or Rocket Lab, but due to how advanced the Dragon capsule is, SpaceX. As Not Boring has mentioned, this would cost you around 200 million dollars per test (50 million for the capsule, 150 million for the flight) due to Dragon having to support humans.

Since Varda costs significantly less than 200 million dollars for each test, there’s a viable opportunity for the defense industry to take advantage of.

Second, which was not mentioned in the Not Boring article but is a motivator, is that defense agencies publicly failing can be seen as weakness. 

If SpaceX has a “rapid unscheduled disassembly” CNN complains about it for a couple days then moves on.

If the Air Force or NASA has that happen, then players around the globe doubt America’s defense capabilities.

There are different consequences of failure for startups vs government agencies, which helps to explain why Varda is able to thrive. Focusing on the role of startups in the defense industry could be a subject for another post. For now though, let’s move on to

How this reentry capsule could work?

Let’s start by talking about one of every engineer’s favorite subject: thermodynamics!

There are three main ways you can transfer heat: through conduction (you touch something hot), convection (something hot touches a medium (like air or water) which then touches you), or radiation (electromagnetic waves touch you).

In space, while these three methods of transferring heat can technically work inside of a spacecraft, transferring heat between the vacuum of space and spacecrafts is typically done through radiation and some conduction. (Note: In the lower parts of LEO you can still have natural convection since there is enough air to be able to transfer heat that way, but not enough to be very meaningful).

If you’re an astute student of thermodynamics, you might mention that radiation is faster at transferring heat than convection which is faster at transferring heat than conduction, so why do we care? Well, you’re right, but the amount of heat we need to transfer away from our capsule per unit of time is so high during hypersonic reentry that we want all the help we can get to avoid the insides of our capsule being damaged.

One way of being able to increase the amount of heat transfer we’re able to have is to use a thermal protection system (TPS). TPS are heat shields typically made out of numerous tiles that cover a spacecraft in order to protect the spacecraft from thermal damage.

In order to protect the spacecraft from the harsh environments of space, the tiles that make up a TPS are usually made out of exotic materials. The TPS that Varda was interested in using is made out of a very exotic material called C-PICA, or Conformal Phenolic-Impregenated Carbon Ablator.

Those are a lot of big words, so let’s first do a broad explanation of what this material does and then dive into the specifics.

At a high level, C-PICA is a material that contains an outer layer of plastic resin which is then heated into a gas that can then carry away additional heat via convection. This process of “burning up” a material to turn it into a gas to convect heat away from a spacecraft is known as ablative cooling. This is why this material is an ablator; it’s just like how a material that conducts electricity is called a conductor.

Now, let’s dive into what C-PICA actually is.

C: Conformal. The heat shield can be shaped or molded closely to “conform” to the shape of the spacecraft, which makes it more efficient and effective in dissipating heat. If the shield can’t properly conform to the shape of the spacecraft, the spacecraft could still survive, but it is much more difficult due to how high-speed the reentry is.

P: Phenolic. Phenolics are a type of resin, often used in high-pressure or high-temperature environments. They are known for their strength and resistance to heat and chemical damage.

I: Impregnated. This means that the phenolic resin is infused or filled into the material to enhance its characteristics, often improving its structural integrity, heat resistance, or other qualities.

C: Carbon. This is the primary material that makes up the ablator. Carbon is a good choice for TPS due to its high melting point, strength, and resistance to various forms of degradation.

A: Ablator. An ablator is a material used on the heat shield of a spacecraft. It's designed to "ablate", or erode away, in a controlled manner when subjected to extreme heat (like that encountered during reentry). As the material ablates, it carries away heat, thus preventing the transfer of extreme temperatures to the spacecraft.

Ok, so now that we know why we need a TPS, what a TPS is and what C-PICA is, let’s finish off with everything related to the sensors part of the reentry.

In order for Varda’s reentries to make money, they need to have sensors to collect data for the US military to buy.

What sensors would be useful and why? While I do not know exactly what Varda is using (as of writing this I am not affiliated with them professionally) but there are some basics we can determine.

Accelerometers, Barometers, and Pyrometers would be immediate candidates to put on Varda’s capsule. Being able to measure changes in acceleration, pressure changes, and high temperature changes would be vital for understanding hypersonic environments.

Outside of these three core sensors, I could see gyroscopes, strain gauges, and infared sensors as candidates I would consider. Gyroscopes are important for understanding changes in the reentry angle of the capsule, strain gauges are important to measure the forces put on the capsule from a steep reentry angle and infared sensors can provide further details on the heat experienced and (ideally) convected away via ablative cooling.

Where would these sensors go?

All of the temperature and pressure sensors (barometer, pyrometer, infared sensor) would be placed on the outside of the capsule, as if it was very hot inside of the capsule then that would be very bad.

The accelerometer and the gyroscope would be inside of the capsule, likely in a sensor bay similar to how Dragon does it.

Finally, the strain gauges would be placed at points where significant strain would be expected, which would include inside and outside portions alike.

How could Varda bury the lead on any hypersonic reentry competitors?

There is currently a scarcity of the hypersonic reentry services that Varda provides.

Since Varda is able to provide that, they have the potential to obtain rare data, but currently only in small quantities.

If they are able to scale up their programs to what they predict, then they will not only have rare data but large quantities of it.

When someone has large quantities of rare data, in this age of GPT-4, the answer is to use that to develop AI models.

Originally, I was planning to go into an additional 2,000 word rant on fourier neural operators but the gist of it is that if you use a very special AI architecture then, with the data Varda will collect, you can make quick, valuable predictions about how to improve your engineering.

As an ethical engineer, I want to note that the point of adding AI to a business is not to replace human engineers.

Instead, the point is to add a new knob to improve Varda’s designs even further.

Ad Aspera Per Astra Varda.

As a final note, you also want to help me improve even further I would be very happy if you subscribed to “The Final Frontier.”

I cover technical topics relating to the space industry and my next post is going to be a deep dive into either some fascinating MIT grad courses on satellite engineering, control systems for thrust vector control rockets, or a continuation of my previous post on hybrid rocket engines. It will likely be the first topic, but I expect to work through all of these and much more as the summer progresses.