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The Limits of Chemistry

In the last post, I talked about how it was basically impossible for humanity to get to another star using modern technology. For this post, I would like to talk about why that is, and why we don’t have space hotels or moon bases yet.

The whole reason comes down to chemistry. The vast majority of rockets that exist and all rockets that take anything into outer space use chemistry to make the rockets go.  A few posts ago, I talked about thrust. Thrust is a pretty simple concept – basically, a rocket moves forward by expelling things quite quickly out the back.  There are two terms in the thrust equation, the mass flow rate (how much stuff the rocket spits out), and the exhaust velocity (how fast it spits it out).  Simple.

The mass flow rate is pretty easy to understand also.  It basically is just how much fuel the rocket uses per second.  In some ways, it is like hitting the gas pedal on your car: the harder you push on the gas pedal, the more gas flows into the engine and the faster you go.  That is a pretty simplified version, but it is about right.  A larger rocket really just has a larger mass flow rate.  The space shuttle had pipes that fed into the main engines that were about a foot in diameter.  That is a LOT of fuel!  The Saturn V used roughly 1000 gallons of fuel per second.  They actually had a very hard time mixing the fuel with oxidizer on the Saturn V, since the flow rate was so high (they didn’t have great fuel injectors in the 60s!), and they would get explosions in the engines.  Instead of giving up, they simply made the combustion chambers more sturdy to handle the explosions.

Anyways, the mass flow rate is how much fuel the rocket uses per second.  This is set by how big the engine is, and there is no real limit, except how big you can build the engines (or how many engines you can stick on a rocket – yes, I am talking to you Space-X with your 27-engine Falcon Heavy rocket).

The other term in the equation is the really tricky one – this is the exhaust velocity, which is how fast you can expel the mass out the back. Simplistically, you would think that this would be easy to turn up, but it is not. There has not really been any big revolutions in the exhaust velocity in a long time (like the 60s). The most common way to make a large exhaust velocity is to make an extremely hot gas, and direct it into a nozzle.  You mix fuel with an oxidizer, and you get an explosion. Then you turn the explosion into directed energy using a nozzle.

We can design pretty good nozzles.  They can be something like 90%+ effective at turning the thermal energy into kinetic energy.  That is great.  There is no factor of 10 improvement or anything that can be gained from nozzles.

The big problem behind this is chemistry. Let’s take the space shuttle’s main engine. This engine used two of the most abundant elements we have on Earth: Hydrogen and Oxygen.  You cool them both down until they are liquids, store them until the rocket is ready to fly, then combine them in the engine.  What is the result?  Water!  The space shuttle’s main engine exhaust is water!  Crazy, eh?  The amount of energy that is released when 2 molecules of Hydrogen are introduced to one molecule of Oxygen is exactly the same every time – about 6 eV, which is a tiny bit of energy.  The fundamental issue here is that we get only a very specific amount of energy out of the reaction.  If we take the 6 eV of energy and we turn that into an exhaust velocity, it ends up being about 3,000 m/s.  This is very fast at first glance, really it is not.

631808main_1981-04-12_full
Space Shuttle Columbia taking off for the first time.  There are really 5 engines that you can see if you look really closely.  The big white stick things are solid rocket boosters – they don’t burn hydrogen and oxygen). On the back of the shuttle proper (orbital vehicle, to be more precise), you can see three engines.  The huge white thing that the shuttle is attached to os a gigantic fuel tank.  That is where the hydrogen and oxygen are located.

This small amount of energy totally limits us so that rockets have to be huge.  If the chemistry were such that these elements released 10 times more energy, then we could (in theory) make rockets that were much smaller (more than 10 time – by a lot). In fact, we play around with different chemicals to try to make a larger exhaust velocity, but the problem is that the chemicals that produce the most wickedly large exhaust velocities are horrific to work with – like super caustic and really, really bad for humans. So, there has to be a balance between safety (which costs a LOT of money or lives) and exhaust velocity too. This huge Russian rocket explosion that killed over 100 people, was while they were trying out new fuels that would have larger exhaust velocities.

We have not invented a better way to get off the ground than using a chemical rocket engine.  There are a TON of other ideas out there, but it is this fundamental limitation of the exhaust velocity that limits our ability to actually go very many places far away from Earth.

Next time, I will go through a simple formula that was invented in the early 1900s that predicted this whole problem. It was a good 40 years before modern rockets were even invented! And then, I will start posting about all of the absolutely crazy ideas that could possibly get us to the stars. Well, ok, maybe not.  But, they are awesome anyways!

 

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