### Walter Hohmann

In 1912, Hohmann read a book on astronomy which sparked his lifelong interest in space flight. During the 1920s, it was a part of everyday life within the Hohmann family: poems, bookmarks decorated with rockets, and even birthday celebrations were infused with extraterrestrial enthusiasm.

Born March 18, 1880 in Germany, Hohmann became an engineer and worked for various companies in Vienna, Berlin, Hanover, and Breslau. In 1912, he became city engineer of Essen, Germany. in 1915, he filled a war-service position for eight months in WWI. Walter and Luise Juenemann were married in 1915 and had two sons, Rudolf in 1916 and Ernst in 1918. ********** |

After WWI, many Germans contemplated space travel. Using basic math: basic calculus, simplifying assumptions, and numerical experimentation; Hohmann published an important work in astronautics. As Hohmann later explained: “I'm an engineer, not a mathematician; thus, clumsy approximations occasionally appear in the calculations; the results should still be valid.” Hohman presented his ideas with clarity, without mathematical formalism.

Walter Hohmann was a huge influence on a famous rocket society, VfR. Other members included Hohmann, Oberth, Wernher von Braun (1912–1977) as well as many other Germans involved in early space and rocket work. In 1929, the society accomplished a series of designs and tests served to advance the discipline from infancy to a credible branch of engineering.

Hohmann invokes precedents for the concept of a rocket-in-space in science fiction (e.g., Verne and Lasswitz), engineering (e.g., Oberth, Tsiolkovsky, Hermann Ganswindt, and Valier), and science (e.g., Newton).

Free-Space “Maneuver Analysis.” He envisages a vehicle departing radially from Earth and designs maneuvers for a parabola suitable for reentry. (Here, he introduces the important concept of ΔV (“delta vee”): change in velocity of a spacecraft by means of propulsion.)

For spacecraft orientation, current technology uses thrusters (high intensity nozzles that expel high speed gases; i.e., little rocket engines); however, Hohmann devised a method for crew members to clamber about the walls of the vehicle to cause desired rotation.

**Circumnavigation of Other Heavenly Bodies**

The first interplanetary trajectory used the now well known "Hohmann Transfer" to travel from Earth to Venus. Similar calculations are done for a trip to Mars. After a flyby, orbit about the Sun will bring it back to the point of departure. Of course, Earth continues in its orbit while the the spacecraft is off on its trip. Thus, Earth will probably not be at departure when the spacecraft returns to Earth's orbit.

For the spacecraft to return to Mother Earth, Hohman considers two methods:

- Maneuver into a holding orbit about Venus and, waiting until Earth is suitably positioned, thrust out of Venusian orbit and rendezvous with Earth.
- Conduct a space maneuver and return to Earth without going into orbit about Venus.

A trip to Mars is similar in principle, but he notes that the greater eccentricity (compared to Earth and Venus) of the Martian orbit must be taken into account. A single trajectory, departing Earth and passing by Venus and Mars before returning home, is possible when the three planets are suitably configured: the length of the journey is 580 days. Hohmann adapts his previous estimate of spacecraft mass and arrives at a figure of 16,720 kg, not including fuel.

Considering several onerous requirements with regard to mass for landing and return to Earth (humans are aboard), Hohmann specifies, “The fuel necessary for a return [should] be manufactured by simple means of raw materials available [on Venus]” (Hohmann 1960, 91). This technique of “in-situ propellant production” is now under consideration for certain NASA missions.

Landing on Mars is analyzed without aerobraking: the engine is used to decelerate the vehicle and place it on the surface. The results, in terms of mass and energy requirements for the system are, of course, less favorable than for landing on Venus. Again, in-situ propellant production is prescribed for powering the return to Earth.

Hohmann addresses optimal transfer orbits between planets: “For simplicity, we have up to now only discussed those connecting elliptic segments between planets, which touch the two planets, which are to be connected… It is not obvious that these tangential ellipses constitute the most favorable connection. Rather it is conceivable that other ellipses, intersecting planetary orbits, would be more expeditious, since without doubt they would provide shorter connections.” By comparing tangential transfer orbits with ellipses that cross one or both of the planetary orbits, he establishes his famous result that the smallest ΔV is required for the tangential case.

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