300 ANlSnjAL REPORT SMITHSONIAN INSTITUTION, 196 2 



Table 1 . — Mission velocity requirements 

 Mission Velocity , ft.jtee. * 



Low earth orbit 30,000 



Earth escape! ^2 qoo 



Lunar hit J ' 



High earth orbit 1 



Lunar orbit [ 45,000 



Mars, Venus probesj 



Lunar landing 50,000 



Lunar round trip ] ^^ ^q^ 



Escape from solar system] ' 



Mars, Venus round trip 60,000 to 90,000 



♦Including losses. 



atmospheric losses (such as aerodynamic drag). These losses are 

 frequently evaluated m terms of velocity and included in the mission 

 requirement. Table 1 gives the approximate requirements for some 

 missions of interest. A lunar round trip has only twice the velocity 

 requirement of orbital missions, and interplanetary trips need only 

 three times orbital velocity. However, this implies that the rockets 

 must be respectively four and nine times as large as the orbital vehicles 

 for the same payload and propulsion system. 



ROCKET PRINCIPLES 



Almost all types of rockets are based on the principle of action and 

 reaction, and are similar in action to the recoil of a gini or to the mo- 

 tion of a balloon which is rapidly losing its gas. The motion depends 

 upon expelling some material (propellant) , be it gas, solid, or charged 

 particles, from the vehicle. Thus rockets carrying their own propel- 

 lant are able to operate in a vacuum outside the atmosphere, just as 

 a gun's recoil is independent of the air about it. The velocity of the 

 expellant with respect to the vehicle (called the exhaust velocity) is 

 a measure of how effectively the propellant is used, and is comparable 

 to the miles per gallon of an auto engine. The higher the exhaust 

 velocity, the more effectively the propellant is being used, and al- 

 though this requires more energy per unit mass of propellant, it is 

 advantageous to have high exhaust velocity. Note that the original 

 source of energy does not have to be in the ejected material, although 

 this is true for chemically propelled rockets. In our earlier illustra- 

 tions, the energy was stored in the gunpowder, not the lead projectile, 

 and in the stretched rubber of the balloon as well as in the compressed 

 gas. The initial gross weight of a rocket for a given payload depends 

 exponentially upon the ratio of the mission velocity requirement and 

 the exhaust velocity, making the results quite sensitive to these quan- 

 tities. Since the mission velocities are relatively fixed, major reduc- 

 tions of vehicle sizes for given payloads and missions can come only 

 through increasing the exhaust velocity. 



