Quote of the Day
Our educational system is like an automobile which has strong rear lights, brightly illuminating the past. But looking forward, things are barely discernible.
— Hermann Oberth, German rocket theoretician, describing the German education system in the 1920s. He was bitter because his doctoral thesis on rocket propulsion was deemed utopian. His work became the basis of all spaceflight today. In my opinion, his criticism of the German educational system could be applied to the US education system today.
I just read a news article about Japan launching a 3 kg satellite into orbit using a 9.7-meter-long, two-stage rocket called the SS-520 (Figure 1). The 9.7 meter length was interesting to me because I recalled an Air & Space magazine article from 1999 that stated that the smallest rocket capable of achieving Earth orbit would be "about 30 feet long." Since 9.7 meters is 31.8 feet long, it appears that Japan's SS-520 is very near the lower size limit for rocket that can put an object into Earth orbit.
The size limit for an orbital rocket is driven by the amount of momentum lost because of atmospheric drag. As with artillery projectiles, larger rockets are more efficient in retaining momentum against drag. For a given shape, larger rockets are more aerodynamically efficient because frontal area increases by the square of the linear dimensions and volume (and mass) scales by the cube of the linear dimensions (see this detailed discussion). Drag is a function of the frontal area of the rocket, thus larger rockets have more mass (and momentum) relative to their drag. Another challenge with implementing a small launch vehicle is the difficulty of efficiently implementing a high specific impulse, liquid-fuel system because of the overhead of all the pumps, plumbing, and cooling.
Because I am still tied up with my cabin project, I have not gone through the minimum-sized orbital rocket calculations myself. Air & Space magazine states that:
A terrestrial rocket has to push through a plug of air equivalent to a 30-foot column of water, and physics dictates that the smallest vehicle capable of moving all that atmospheric mass without paying a penalty in momentum is about 30 feet long.
Historically, the orbital launch market has been dominate by customers who want to put large payloads into space. The advent of CubeSats has created a market for these small rockets. For example, a company called Rocket Lab uses their Electron rocket to launch small groups of CubeSats.
NASA has been researching the smallest rocket that can return a sample from Mars to Earth. According the Air & Space magazine article, the smallest orbital rocket is "about the size of a pencil" for essentially zero payload. NASA's Mars return mission is targeting a 1 pound payload and the mass is about 170 kg. Having lower gravity and a much thinner atmosphere make the job of getting into Mars orbit much easier than getting into Earth orbit.
People have been discussing these small rockets for many years. In fact, people have tried to motivate innovation in this area with the N-Prize, which is focused on putting a small payload (10 - 20 grams) into Earth orbit for less than 1000 £ . For an excellent discussion on micro-rocketry, see this forum thread. The following Google talk on microlaunchers is also useful.
|Figure 2: Microlaunchers – The Case for a New Generation of Very Small Spacecraft.|