Sunday, February 10, 2008

SIDEBAR: GODDARD

Sidebar: Robert Goddard

Father of Modern Rocket Propulsion

Robert Hutchings Goddard (October 5, 1882-August 10, 1945). Throughout his extremely productive life, Goddard realized the potential of missiles for space flight and greatly contributed to practical realization.
As a teenager, Robert Goddard, read a serialized version of H. G. Wells's War of the Worlds and began filling notebooks with ideas for interplanetary travel. In 1907, Goddard first gained public notice from a powder rocket which produced a huge cloud of smoke in the physics building of Worcester Polytechnic Institute.
In 1914, Goddard received his first two U.S. patents.
  • Liquid fueled rocket.
  • Multi-stage rocket using solid fuel.
Altogether, Robert Goddard developed 214 patents; many were awarded posthumously.
Goddard built a liquid-fueled 10-ft. rocket (called “Nell”) and used it for the first liquid-fueled rocket on March 16, 1926. The flight lasted just 2.5 seconds, reaching an altitude of 12.3 meters (41 ft.) and crashed 55.2 meters (184 ft.) away in his Aunt Effie's cabbage patch.
In 1919, Goddard wrote a scientific article, "A Method of Reaching Extreme Altitudes," describing a high-altitude rocket; this ground-breaking article was published in a 1920, Smithsonian report. Goddard was cautious not to mention flights to Mars or any other planet, as any celestial object beyond the Moon was considered by many contemporary scientists as too distant for human travel. However, Goddard briefly speculated about a small, unmanned rocket to Earth's Moon; upon reaching the surface, it would explode a payload of flash powder for observers with telescopes to mark the landing location.
Due to this brief speculation, the prestigious New York Times ridiculed Goddard in a Jan. 13, 1920, editorial, stating that space travel was impossible, and that Goddard "seems to lack the knowledge ladled out daily in high schools." A Times science writer incorrectly thought that rockets could not work in the vacuum of space. NY Times finally printed a retraction 3 days before men landed on the moon (p. 43, July 17, 1969); unfortunately, this was many years after Goddard’s death. From then on, Goddard remained skeptical of the press.

Luckily, aviator Charles Lindberg took an interest in Goddard and convinced philanthropist Daniel Guggenheim to help. Thus, Goddard was able to test his concepts in the wide open desert near Roswell, NM. This included many items in following list of firsts.
Goddard's Historic Firsts Robert H. Goddard contributed a long list of “firsts” to demonstrate many fundamental principles of rocket propulsion.

  • First explored mathematically the practicality of using rocket propulsion to reach high & altitudes and even the moon (1912).
  • First static test to prove a rocket will work in a vacuum; it needs no air to “push” against.
  • First developed and shot a liquid fuel rocket, March 16, 1926 .
  • First scientific payload (barometer and camera) in a rocket flight (1929, Auburn, Massachusetts).
  • First use of vanes in the rocket motor blast for guidance (1932, New Mexico)
  • First developed gyro control apparatus for rocket flight (1932, New Mexico).
  • First U.S. patent in idea of multi-stage rocket (1914).
  • First pumps for rocket fuels.
  • First launch of a rocket with a motor pivoted on gimbals connected to a gyroscopic mechanism (1937).

Despite all this work, Goddard and his rockets were generally unknown to the American public; however, many foreigners paid close attention to his ideas.. Notably, Wernher von Braun incorporated many of Goddard's published concepts in the V-2 rocket, which caused much Allied damage in the last two years of World War II.

Goddard provided essential service to the US military. In World War I, US Army adopted a major facet of Goddard's concepts in a well known antitank weapon. The "bazooka" was successfully tested two days before the Armistice in 1918 at the Aberdeen Proving Ground.

In WW II, the U.S. Navy employed Goddard to develop two rocket systems:

  • Practical jet assisted takeoff (JATO) for aircraft.
  • Variable thrust, liquid propellant rocket motors.

In Goddard's" autobiographical essay: "on the afternoon of October 19, 1899, I climbed a tall cherry tree and ….. looked towards the fields at the east, I imagined how wonderful it would be to make some device which had even the possibility of ascending to Mars ...."

He died on August 10, 1945, four days after the first atomic bomb was dropped on Japan.

Electric Propulsion

RHG's" early career as a young academic physicist was divided between his official research on electricity and his personal passion for propulsion . This would naturally lead him to think of Electric Propulsion (EP).

In particular, he posed the question: “At enormous potentials can electrons be liberated at the speed of light, and if the potential is still further increased will the reaction increase (to what extent) or will radio-activity be produced?"

Goddard quickly demonstrated that he was quite aware of the most recent developments in physics concerning the nature of cathode rays . He was well aware of Walter Kaufmann’s careful measurements, published in 1901, which indicated that the inferred mass of the electron increased as its speed neared that of light.

Although he was, apparently, not yet aware of Einstein’s special relativity theory, which was only published a few months before and had not yet gained much acceptance.

An excerpt of the the entry dated 6 September 1906 in Goddard’s hand written notebook showing some of the questions he attempted to answer quantitatively to assess the feasibility of electric propulsion using electrostatic potentials to accelerate electrons to the speed of light, self contending with the conjecture that the electron’s inertia at the speed of light might be infinite. He did remain hopeful, however,that experiments might determine “the voltage necessary to give aspeed equal to the velocity of light.”

Why was Goddard more concerned with the electrostatic acceleration of electrons rather than ions? (Recall that ions are much more massive and contribute far more to momentum exchange between the rocket and exhaust particles.)

An online paper suggests five reasons (apologies to author for errors introduced by myself):

  1. The nature of cathode rays was still debated at that time and the ionization physics underlying the production of electron-ion pairs was not clear.
  2. There was the implicit belief in these early writings that high accelerating voltages (and not high beam currents) were the main technical difficulty. This, consequently, favored electrons as the propellant needed to reach extremely high velocities.
  3. There was still a lack of appreciation of the immense difficulty, stipulated by the laws of special relativity, in accelerating a particle having a finite rest mass to a speed very near that of light.
  4. It is doubtful that Goddard, at this early time, had fully appreciated the practical (i.e., system-related) penalty incurred by an electric rocket with an exceedingly high exhaust velocity.
  5. There is another system-related penalty that was likely far from Goddard’s mind. Electrostatic acceleration of lighter atoms, let alone electrons, although less demanding on the voltage, results in beam currents which, because of space charge limitation, incur adverse demands on the required area (and therefore size and mass) of the accelerator.

Goddard’s notebooks show that EP was ever present in his mind. Between 1906 and 1912 the evolution of his thoughts on that subject led him to appreciate the advantages of relying on the reaction of ions in an electrostatic accelerator, and the need for neutralizing the charged exhaust with a stream of oppositely charged particles.

He explicitly stated the latter realization in the following quote from the March 9, 1907 notebook entry: If negative particles are shot off, the car will have an increasing positive charge until the potential is so great that negative particles cannot be shot off. Hence positive particles must be emitted in a quantity equal to that of the negative particles.

As in many instances in the career of this ingenious and practical scientist, his ideas culminated, by 1917, in two inventions whose importance to the history of EP has been largely unrecognized.

0 Comments:

Post a Comment

Links to this post:

Create a Link

<< Home