Thinking to Much about SciFi-Hail Mary
Today is the first of most likely many fun trips into, what if engineering scenarios where I take technologies/rules of physics/McGuffins what have you from books, short stories, or movies that I really enjoyed, and explore what could be done with those tools within the universe they were presented in.
Today I will be going down a rabbit hole of Andy Weir’s “Project Hail Mary”, as it is the most recent example of a story that I truly enjoyed and devoured pretty quickly.
Note: SPOILERS, you have been warned, if you haven’t read the book yet and don’t want to have the details revealed check it out and come back later or read on. I’m not here to control you.
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You sure about the spoilers?
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Ok, seriously spoilers in
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At this point no one has an excuse for griping to me about spoilers.
For those who haven’t read Project Hail Mary, but would rather read the book first, here’s the bullet points.
Our narrator wakes up in a spaceship not sure why he is there.
Through a series of flashbacks the narrator begins to recall why he is on the mission.
An unexpected change in the properties of the sun’s output leads to the discovery of unique microbes named astrophages,
These astrophages have an incredible property where they can absorb any and all forms of electromagnetic energy and can use that energy to propel themselves through space.
Unfortunately for the residents of Earth the life cycle of astrophages leads them reproduce to such a degree that they can impact the amount of sunlight that reaches planet Earth.
Scientists estimate that the effective output of the sun will be reduced by 10%, enough to cause a global climate catastrophe.
Eventually humanity realizes that countless stars have become hosts to the invasive astrophage, with the exception of one star, Tau Ceti.
Project Hail Mary is conceived as a mission of hope for humanity, sending a small team of researchers on a one way trip to Tau Ceti in the hopes of finding out how to stop the astrophage.
Humanity begins to understand astrophage and use some of its properties to make Hail Mary work.
Eventually they build a spacecraft that uses light emitted by the astrophage to accelerate to near light speed to get to Tau Ceti and go about saving humanity.
The astrophage has the following properties (that at first I tried to go off memory and then realized the internet existed and used the fan wiki as my fact check
An 8 day reproductive cycle.
The ability for a single astrophage microbe to absorb 1.5 Megajoules of energy
The ability to release that 1.5 megajoules of energy as infrared light, which provides the thrust for the astrophage to perform its lifecycle
Astrophage stores energy non-chemically
So long as astrophage is alive it maintains a core body temperature of 96.4 degrees Celsius, almost regardless of how much heat is being put in or removed from the astrophage.
Now it’s time for epic thought experiments that the book, didn’t have time to explore, the author didn’t want to burden readers with, it interfered with other narrative goals, etc…
First and foremost the Hail Mary’s light engine is possible, the reason we use chemical rockets is we don’t have a way to store energy the same way astrophage do, if we did we could use it.
Light engines work because light has momentum, not a lot, but if you are clever or just have a lot of energy, light based propulsion is amazing because it allows you to go incredibly fast.
Traditional chemical rockets are limited to a maximum velocity about 2x the speed of the exhaust(assuming no tricky maneuvers using gravity assist). That means that no matter how large you build a chemical rocket you can’t go anywhere close to the speed of light if you only use chemical reactions to make thrust.
In the book the Hail Mary exclusively uses light based engines under the argument that it keeps the design simple. Now I agree simplicity is your friend, but that design simplicity also led to some events that a NASA engineering team would have said that simplicity “gains” were unacceptable. A photonic based thruster (ideally) produces a thrust equal to the effective momentum transferred by the photons leaving the system. The momentum of each photon is determined by the frequency of the photon, Plank’s constant, and the speed of light.
Momentum of photon equation
equations for solving the frequency of light and the momentum of light based on frequency
That seems fine until we look out how much energy that same photon should contain in total.
Equation for the energy of a photon
The difference is huge.
The energy of our photon is almost 300 million times greater than its momentum. That means that if you put your light energy into super heating something and throwing that super-heated exhaust out the back, you’re going to a lot more thrust per unit of light put into your system. Even if you assume some massive losses where only a fraction of a percent of each photon’s total energy can be converted into thrust, you are likely to want to use the energy of the light indirectly in cases where the exhaust gas velocity is high enough to be useful.
What would that mean for humanity in the book? Too much to cover in this post.
The two big things I want to talk about are what astrophage would mean for spacetravel in the context of the book, one for Earth and one for the spacecraft.
For residents of Earth launching rockets is going to become a lot easier/cheaper.
Modern rockets are a complicated balancing act to force as energetic of a reaction as can be made, without blowing up our equipment.
In the world of Hail Mary the energy that Astrophage is able to dump out per unit mass means that we don’t really need to worry about chemistry. Why use a chemical reaction to get hot, when light energy gets the job done.
Actual rocket designs would depend on many subtle questions about the properties of astrophage that were not answered to sufficient detail to be helpful to our needs.
How rapidly do astrophage release their light energy?
Is the amount of energy released variable, or is always a single blast?
How quickly can astrophage absorb the energy that they are exposed to?
What is the maximum ambient temperature in several atmospheres of pressure that astrophage are able to survive?
Without knowing those details I can’t outline anything super specific, but could give the broad strokes
Our spacecraft would likely use filtered water heated up by astrophage to a few to several thousand Kelvin. That super-heated steam would be thrown out one or more exhaust nozzles.
The size and number of exhaust nozzles would come down to what we could produce in a cost-effective enough manner. For most US produced launch platforms in the compromise between many small engines and a few larger engines. Larger engines won out as a result of a preference for building a smaller number of well-made elements. In the era of astrophage based technologies it seems less likely that we would go down the route of larger engines. Larger engines are helpful because they don’t need as many mechanically complex turbopumps. In the book we are told that astrophage electrical generators are both very effective and relatively small. This means our spacecraft could go down the route of Rocket Labs and use electric motors to pump water to the reaction chambers.
Smaller engines have another advantage in that, assuming you have enough of them, you don’t need to gimbal the engine to change direction, instead you simply throttle the engines to adjust your path. Rockets in the world of Hail Mary could have hundreds of small engines working dynamically to provide tremendous amounts of thrust and control. (Check out Stroke space as a cool example of the technology)
With respect to the Hail Mary spacecraft, this is going to get weedy and complicated, but you’ve survived over a thousand words of text, so sunk costs and all that.
As I noted previously, the momentum of light is a tiny fraction of the energy of a photon. For a spacecraft accelerating close to the speed of light, we are certainly going to need to use our emitted light to boost up, but there are several stages of flight where we are going to want to consider using something other than directed light.
Those stages are as follows.
Accelerating up to 2x maximum exhaust velocity
Higher precision maneuvers.
Deceleration.
Producing the thrust for our spacecraft would take a decent amount of engineering. The simplest approach (I can think of) would be to have an ignition chamber where small volumes of material are placed at the center and blasted by a huge amount of light energy from various astrophages is blasted toward the material. That light would superheat our material which would then rapidly heat up and produce a high temperature/pressure gas that would expand out the exhaust of the chamber.
Alternatively we could make something akin to a Bussard Ramjet, where interstellar gasses are collected at the front of the craft, guided into a central point where light is used to compress the gases into a superheated plasma, and if you’re lucky maybe even some amount of fusion.
Regardless of the method, these hot-gas based engines would be slightly regenerative, where the superheated gas would emit radiation that could be captured by the astrophages, helping to improve “fuel efficiency” of the spacecraft.
(Edit: author Andy Weir very kindly responded to my sharing this blog post and responded, and wondered what would be the reactant for light heated engine, which made me realize I didn’t include that detail, so I am inserting it with this parenthetical.
Once an asterphage has expended its optical energy it is no longer useful and can be discarded, no-need to hold onto dead mass. Used up astrophages and human waste products from the crew of the Hail Mary could produce anywhere between miligrams and grams of “fuel” every second. My back of the envelope excel math ballparked, that depending on how hot we can get the exhaust the ISP of our “fuel” would be as follows
ISP_100,000K = 1,550 seconds
ISP_1,000,000K = 4.900 seconds
ISP_10,000,000K = 15.500 seconds
ISP_100,000,000K = 49,0000 seconds
Realistically ranges of temps between 10 and 100 million Kelvin are “reasonable” for laser compression, which is close to what I imagine. Assuming it was “only possible to heat things up to 100,000 K, there is a reasonable argument for developing a technology like that within the Hail Mary universe, just not for the Hail Mary mission itself, as Andy Weir’s response indicated that reliability was core to the mission’s design requirements.
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OK so this got longer than I planned so I am going to stop now and leave you with a model of what these rockets could look like.
Also before I go here are some other high level thoughts
Power generation in the Hail Mary world would quickly become one of the cheapest things on the planet. Already wind and solar are pushing energy costs globally down tremendously, in Hail Mary, power costs would hit floors that are hard to imagine.
Ex a traditional solar panel has a conversion efficiency between 10-25% for normal consumers, and needs other gear to make itself work. As astrphages absorb 100% of light and are at a temp of 96.4 C, we can assume that an astrophage solar concentrator power source could convert sunlight at over 93% efficiency (1-(96.4+273.15)/5772). Oh and that energy can be stored near indefinitely. So assuming a similar price per square meter as classic panels which astrophage would likely be made cheaper than that, solar panels would produce 3x as much energy per square meter. That’s insane.
Wind would also benefit, either having surplus energy simply used to heat up astrophage, or alternatively we could have the electric generator be replaced by clever brake pads. As the turbine turned it would have metals rub against each other to produce heat that would charge the astrophage. While it wouldn’t be as sexy and probably quite loud, the cost per unit of generating capacity could be far lower due to reduced electrical/mechanical complexity.
More insane when we consider that researchers at MIT estimated that if we could produce grid storage batteries at a cost of $150 per kilowatt hour of installed capacity we could move the world to using 95% using wind and solar. Well in the world of Hail Mary the cost per kilowatt hour of storage capacity would be in the range of pennies, at most. At those prices fossil fuels would only continue in usage so long as humans needed the additional greenhouse gasses to offset lost energy from the sun.
Math on thermally enhanced rockets
https://www.nagwa.com/en/explainers/725104643703/
Other Technologies
Air breathing spacecraft
Things that could help in the book,
space based solar concentrators
manufacturing astrophage traps to capture additional energy sources