The Tyranny Of The Rocket Equation

So I just finished watching Don Pettit on the Tyranny Of The Rocket equation and I think he sums up beautifully the current problems with our multi stage to orbit capabilities and has captured my thinking exactly.

In a nut shell a turbo compressor driven high exhaust velocity liquid fueled rocket engine does a terrible job while it is going slow and must carry an enormous penalty in the form of highly pressurized mass to get to the point where it can get fast enough to be efficient and hit that magic 11180.6 m/s. This requires multiple staging complex sub systems complicated ground handling and expertise in dangerous oxidizers like super concentrated hydrogen peroxide and toxic hypergolics.

If you look at the companies involved in getting us in to orbit at the moment ( note here I am just supposing that Nasa is a dead loss by default ) they all have different approaches to the tyranny and I find them all lacking.

(a) SpaceX

If you look at what SpaceX are doing it is all about cycle time. Sure they want to build cheaper more efficient rockets but no matter how well they design these systems unless they do something really radical they are still constrained by the same problems as any other rocket system. They aim to address some of these problems by building cheaper recoverable rocket systems and having a turn around time measured in days not months. They aim to build rockets that are simply BIGGER to lift more mass. But observe that none of this solves the basic problem of having to loft an enormous tank of oxidizer/fuel mass all the way to the edge of the atmosphere.

(b) Copenhagen Suborbitals

To my mind Copenhagen Suborbitals are not really INVENTING anything. Rather they are following a very specific well trodden path for a sub orbital shot and trying to do so on a small budget using open source hardware. I am all for this and it is excellent to see them throwing together items like turbo pumps from used turbines and so on. This is all commendable but once again none of this solves the basic problem.

(c) Armadillo Aerospace

Armadillo for the most part seem to be messing around in a highly focused but pointless kind of way. Powered descent capability. Various kinds of hybrid engine. This is all well and good but once again none of this is addressing the tyranny.

(d) Virgin Galactic & Scaled Composites

Both of these companies are repeating what I would call the ‘X’ flights. These are essentially small aerodynamic jet type vehicles lifted by a secondary flying vehicle from which they drop and make a shot. Vehicles like White Knight will get us to sub orbit but they wont be carrying much in the way of useful mass. Once again none of the tyranny is being solved here.

(e) Reaction Engines Limited

Now this is where it at last gets interesting. These guys are directly addressing the tyranny in a way nobody else is. The Sabre system is accepting the harsh reality that for us to get single transition to orbit systems with decent turn around and conventional landing and take off ability we are going to have to fundamentally rethink our rocket systems and think of something new.

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So where does that leave us. Well I think the Sabre guys have pointed the way to a whole different conception of the traditional rocket engine. We can now begin to do the math to ask serious questions about hybrid systems. As soon as you begin to think about replacing the tank mass and going partially ambient the rocket tyranny starts to look less and less like a problem. If all your assumptions about transfer to orbit are not based on an enormous highly pressurized expensive one shot tube of metal you get a lot more options.


At the moment I am busy working out compressor sizes for a given flow rate to get some given mass in to orbit. It gets interesting. For example I can see that even a 20% reduction in total fuel mass to orbit would be pushing the engineering in to the realm of achievable for an organisation like Copenhagen Suborbital. Basically solve the high pressure tank mass problem and figure out a way to harness some of that air mass your already flying through and everything looks better. This is also changes how you think about the design of the vehicle. So for example so what if you need a compressor with a 3 meter inlet diameter. Nobody said a system like this had to all be in a long tall thin metal tube.