Ascent

The future is coming.


In my first post I mentioned that re-usability would immensely cheapen space travel, specifically the ascent to orbit phase of space travel.  This is because fuel costs are vanishingly tiny compared to the cost of the rockets themselves, around $200k vs. $60,000k.  Therefore, ability to recover and re-use the rocket is key.  Pictured above is a recent long exposure of a SpaceX Falcon 9 launch, and the successful recovery of its first stage.  This was the first time the company succeeded in doing so.  The system is imperfect, there are repair and refit costs (the rocket is generally damaged by temperature changes during descent), and they have no way to recover the second stage.  However, they estimate that they will be able to save around 30% with things as they are.  They hope to continually ruggedize the design and eventually reduce launch costs to somewhere within the region of one million dollars per launch.

The United Launch Alliance, old guard of the payload carrying rocket industry, have been working on the Vulcan, their own re-usable design.  The details of this system remain vague, for now.

The profile of a typical rocket launch consists of a couple of phases.  In the first phase, you are boosting upward to begin ascending to get out of the thick parts of the atmosphere.  In the second phase, you begin rotating and thrusting to the side in order to achieve orbit.  This is the part where you are thrusting sideways until the earth curves away under your trajectory and you lose the ability to reach the ground.  Generally the people designing the launch profile are trying to balance destroying the rocket with aerodynamic forces and getting into orbit as quickly as possible.  There tends to be a (more or less) straight upward portion of the launch where you focus on leaving the atmosphere quickly, so that you can build up your sideways velocity without destroying yourself.  They want all of this to happen quickly in order to save fuel.  I explained earlier that fuel is cheap, however adding more fuel will exponentially increase the total size of the rocket.  Rockets are expensive, so you want to use as little fuel as possible in order to have a smaller rocket.

Anyways, rapidly ascending saves you fuel because you are fighting gravity.  Imagine a series of rocket launches with lower and lower rates of upward acceleration.  Eventually you are left with a rocket that is hovering in place, which would require infinite fuel in order to make orbit.  As you increase your acceleration (decrease the ascent time), you asymptotically approach a fuel cost equal to the difference in energy between sitting on the launch pad and coasting along in orbit at high speed.  They haven't reached that ideal, but that is in part why rockets tend to break the sound barrier as a matter of course.  It would be wasteful not to do it that way.

Generally speaking, cargo rockets like the Falcon 9 ditch their first stage well after they have begun thrusting down range.  Its therefore somewhat fuel-intensive to turn completely around and return to the launch pad.  In fact, when lifting especially heavy payloads it is impossible.  SpaceX's solution to this was to try to land the rocket on a drone ship out on the ocean, downrange from the launch site.  After several failures, they recently succeeded in doing so, as part of a supply run to the ISS.

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To quickly recap, a critical cost saving measure for space travel is re-using the rocket.  The fuel is much cheaper than the rocket itself, so if you can re-use the rocket then things are great.  SpaceX and the United Launch Alliance have both been working on ways to achieve this.  You also want to be able to ascend quickly, in order to save fuel and keep rockets down to a manageable size.  Finally, rockets generally want to ascend directly upwards before curving their trajectory so that they don't destroy themselves on the atmosphere.  In my next post I will talk about some of the basic things you can do once you have reached orbit.