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Why Launchers Use High Acceleration

Get to orbit as fast a you can

Rockets want to get into orbit as quickly as they can because the faster a launcher accelerates, the less fuel it uses. This causes some problems for rocket developers -- humans and most payloads don't tolerate high acceleration very well -- so we have to trade off acceleration vs. fuel.

A rocket uses its engine thrust to support its weight against the pull of gravity during the ascent to orbit. As it accelerates closer to orbital velocity, more and more of its weight is balanced by orbital mechanics.

You can visualize this by imagining a rocket that rises a bit off the launch pad and then reduces its acceleration to zero. At that point, the rocket is hovering above the the launch pad, burning fuel to support its weight but otherwise not going anywhere. The rocket would stay there until it ran out of fuel and then crash back onto the pad.

You want to reduce the time the launcher spends supporting its weight with rocket thrust. To do that, you accelerate as quickly as is practical until you get to orbital velocity.

Launcher ascent profile Aerodynamic forces present a trade-off. The force of air resistance is called "dynamic pressure," usually expressed with the symbol q in aerodynamic equations. Dynamic pressure is a function of the velicty of the rocket and the density of the atmosphere (rho) it is traveling through. (The equation for dynamic pressure is q = 1/2 * rho * V^2.) If you want to limit the dynamic pressure, you might have to shape the velocity profile so that your vehicle does not exceed a certain maximum velocity until it rises into the thinner atmosphere at altitude.

The Space Shuttle does this, throttling back its engines slightly for a few seconds during ascent to limit the dynamic pressure on the vehicle (hence the phrase "max q" followed by "throttle up" in the callouts). But even then, this condition lasts for only a few seconds and the vehicle resumes its maximum acceleration as soon as it can.

Climb fast. Accelerate down range.

The Shuttle trajectory is actually more vertical than you would expect, getting out of the atmosphere as quickly as possible and then bending over to accelerate more parallel to Earth's surface.

If you can rise above most of Earth's atmosphere before you accelerate to orbital velocity, you avoid structural loads on the spacecraft as well as aerodynamic friction. Once you are out of the atmosphere, you can bend your trajectory downrange -- that is, in the direction you want to go -- and then accelerate. Your rocket might even lose some altitude during this maneuver, using Earth's gravity to trade some of its potential energy for forward velocity. If you plan it carefully, you can reduce the amount of time the spacecraft spends standing on its rocket exhaust and get back most of the energy you put in during the climb to altitude.

So, in general, for ascent, faster acceleration is better.

Coming home without a heat shield

At the other end of the trip you might be able to get by with nothing more than a stainless steel hull for your spacecraft if you have enough fuel to burn.

For the return trip, the rocket will be traveling at least 25,000 ft/sec in orbit, and more than 35,000 ft/sec coming back from the moon. To reduce atmospheric heating, you have to decelerate very quickly. That would require a retrorocket burn that gives the vehicle very high acceleration. If you delerate slowly, the rocket's trajectory will take you deep in the atmosphere while you still have most of your orbital velocity.

If you could decelerate very fast, essentially to zero forward velocity, you could handle the reentry with nothing more exotic than stainless steel. David Burkhead demonstrated this in his studies for his suborbital sports rocket. You would need to burn almost as much fuel for this maneuver as you needed to get to orbit in the first place, which might make it impractical, but that's how to avoid exotic materials for a heat shield.

So, again, for descent, faster acceleration is better.

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