Telescope Reviews: Rocket Engines 101

Ok, well, keeping with the original theme of the thread, there's a whole host of issues that come into play contemplating rocket engines. The one most folks get focussed on, is the efficiency, typically in terms of the Isp. Isp mattters a lot if your rocket has already been delivered to an orbital location (or higher), and you have expended great energy into designing the maximum benefit, from the minimum mass delivered to that location. But, in the early lift phase of a rocket flight, Isp is only a small factor.

An all up stack sitting on the pad will have a given mass, which translates into a weight based on local gravity, typically considered to be 9.81 m/s^2, or 32.2 ft/s^2 for those still living in the dark ages

In order to lift that mass vertically, the first important detail, thrust has to exceed weight. So, it doesn't matter how efficient the rocket is, if it cannot deliver the impluse fast enough, it isn't going anywhere, and the age old adage of 'gravity prevails' will win. How fast the rocket will accelerate off the pad is trivially simple to calculate, thrust/weight is a known quantity, and will translate into a value for acceleration, and that's exactly the acceleration you get, in the very early stages of the lift, the gravity well is the predominant factor, and outweighs all other factors by orders of magnitude. Ofc, when you look at how fast a rocket consumes it's fuels, and, how much of the actual launch mass is fuel, you realize, acceleration isn't constant, it increases dramatically thru the initial burns, because weight is decreasing at a substantial rate. Efficiency is a _bit_ of a concern for a first stage lifter, but, of far more concern, is the ability of the engine to deliver enough thrust. Getting fuel to the launch pad is cheap, delivering fuel to orbit is expensive, so brute force prevails over efficiency for the initial lift.

Now, apply that acceleration for a little while, and, now you start to get an appreciable velocity, and since it's travelling within the atmosphere, that means aerodynamic drag. Drag can be calculated quite simply as well, the surface area and geometry are fixed, do not change. But, the other really important factors are the velocity and altitude. Drag is a function of the square of velocity and the density of the air. Air density is a logarithmic function of altitude. Both of these explanations are rather simplified, because in reality, the equations get somewhat more complex for altitudes above troposphere, and velocities running into hypersonic, but, the gist of the issue is, the heavy lift phase occurs in the troposphere and stratosphere, at subsonic and supersonic velocity ranges.

So, when you light the fires, and let go of the rocket, it's going to accelerate up off the pad. In a short time, aerodynamic forces start to build, and, after another period, they actually become larger than gravitational forces. The key to designing a configuration that can actually get thru all of this becomes a huge balancing act in the 'number crunch'. The slower the vehicle accelerates, the longer it will have to do so, meaning more fuels required. The faster it acclerates, the quicker we get into huge aerodynamic force ranges, and the more fuel we need to overcome aerodynamic drag. Somewhere in the middle of all that, is a the sweet spot, a trajectory that requires the minimum total fuel to get above the atmosphere, and up to orbital velocity. Accelerating a little faster while still at low altitude, burns enough fuel early on, it wont have enough left to actually reach orbit. Accelerating a little slower down low, means there is still not enough fuel to actually reach orbit, it was burned early on. Somewhere between these points is the optimal trajectory, which actually puts the rocket above the atmosphere, with enough fuel still in the tanks, it can accelerate to an orbital velocity. Finding this trajectory is a balancing act playing acceleration / velocity against altitude / air density.

Now, there's a few other places that constraints will come into play. It's generally accepted that if the cargo is human, 9g is a maximum accleration the cargo can sustain for a short period, and 5g for a longer period. The vehicle itself will have maximum aerodynamic loading limits. On the shuttle, this comes from the fuel line joints between the external tank and the shuttle body, it is the limiting factor on the 'Max Q'. Q is the aerodynamic pressure that increases with the square of velocity, and drops logarithmically with altitude. These factors make the number crunch even more complex, because not only does one have to find the trajectory which burns mimimum fuel as altitude and velocity vary, that trajectory cannot introduce conditions that exceed these limitations. The shuttle has throttles, which make it a lot easier for the trajectory folks, they can dial down the thrust a bit after weight drops, but air density hasn't dropped enough yet. Without throttles, that's not possible, so you end up re-thinking the whole mess. Maximum g loading at the top end of a lift stage was a serious consideration for the Apollo launches, they had 3 fragile humans, and one rather fragile lunar lander module sitting atop the launch stack.

Solid rockets are really good at providing brute force thrust for short periods, but, they quickly become to inefficient for longer duration periods of thrusting. And that's why so many vehicles in use today, use a number of solid rocket stages for the initial lift, to get out of the heaviest part of the atmosphere, then use far more efficient rockets to finish the job of climbing thru stratosphere and accelerate to orbital velocity.

People always refer to the 'gravity well' of mother earth as the thing making access to space difficult, but, it's not the most important factor. The atmosphere is actually an equal or larger factor. On a typical launch trajectory to orbit, as much or more fuel is expended combatting aerodynamic drag as is expended on the accleration to overcome gravitational forces. On the bright side, the atmosphere provides a 'free ride' for the return trip, decelerating down from orbital velocity. For a round trip, the atmopshere becomes 'a wash', problem outbound, asset inbound, and the two issues offset each other. for the one way to orbit trip, it's just a problem, not an asset.

As for the term 'rocket scientist', pretty much everything in this field is a large compilation of engineering problems, normally worked and solved by the engineers. The scientists are to busy writing grant proposals to bother with mundane things like rocketry.

Edited by groz (06/12/09 12:12 PM)