Russell, Graetz and Wiitanen developed the basic
Spacewar! concept in the summer of 1961, in anticipation of the PDP-1 being installed. Russell had recently finished reading the
Lensman series by
E. E. "Doc" Smith and thought the stories would make a good basis for the program. "His heroes had a strong tendency to get pursued by the villain across the galaxy and have to invent their way out of their problem while they were being pursued. That sort of action was the thing that suggested
Spacewar!. He had some very glowing descriptions of spaceship encounters and space fleet maneuvers." Other influences cited by fellow programmer Martin Graetz include E. E. Smith's
Skylark novels and Japanese pulp fiction
tokusatsu films. s in a holder, a punched tape reader, and the computer's control panel|alt=Mainframe computer with punched tapes For the first few months after its installation, the PDP-1 programming community at MIT focused on simpler programs to work out how to create software for the computer. During this period, Russell visited his old friends in the community frequently and described the
Spacewar! concept to them. Russell hoped someone would implement the game, but had no plans to do so himself. Other members of the community felt he was the logical choice to create the game, however, and began pressuring him to program it. In response, Russell began providing various excuses as to why he could not do so. One of these was the lack of a
trigonometric function routine needed to calculate the trajectories of the spacecraft. This prompted Alan Kotok of the TMRC to call DEC, who informed him that they had such a routine already written. Kotok drove to DEC to pick up a tape containing the code, slammed it down in front of Russell, and asked what other excuses he had. Russell, later explaining that "I looked around and I didn't find an excuse, so I had to settle down and do some figuring", started writing the code around the time that the PDP-1's display was installed at the end of December 1961. The game was developed to meet three precepts Russell, Graetz, and Wiitanen had developed for creating a program that functioned equally well as an entertainment experience for the players and as a demonstration for spectators: to use as much of the computer's resources as possible, to be consistently interesting and therefore have every run be different, and to be entertaining and therefore a game. It took Russell, with assistance from the other programmers—including Bob Saunders and Steve Piner (but not Wiitanen, who had been called up by the
United States Army Reserve)—about 200 total hours to write the first version of
Spacewar!, or around six weeks to develop the basic game. It was written in the PDP-1's
assembly language. Russell had a program with a movable dot before the end of January 1962, and an early operational game with rotatable spaceships by February. The two spaceships were designed to evoke the curvy spaceship from
Buck Rogers stories and the
PGM-11 Redstone rocket. That early version also contained a randomly generated background star field, initially added by Russell because a blank background made it difficult to tell the relative motion of the two spaceships at slow speeds. The programming community in the area, including the Hingham Institute and the TMRC, had developed what was later termed the "
hacker ethic", whereby all programs were freely shared and modified by other programmers in a collaborative environment without concern for ownership or copyright, which led to a group effort to elaborate on Russell's initial
Spacewar! game. Consequently, since the inaccuracy and lack of realism in the starfield annoyed TMRC member Peter Samson, he wrote a program based on real star charts that scrolled slowly through the night sky, including every star in a band between 22.5° N and 22.5° S down to the fifth
magnitude, displayed at their relative brightness. The program was called "Expensive Planetarium"—referring to the high price of the PDP-1 computer compared to an analog planetarium, as part of the series of "expensive" programs like Piner's
Expensive Typewriter—and was quickly incorporated into the game in March by Russell, who served as the collator of the primary version of the game. The initial version of the game also did not include the central star gravity well or the hyperspace feature; they were written by MIT graduate student and TMRC member Dan Edwards and Graetz respectively to add elements of a strategy to what initially was a shooter game of pure reflexes. Russell had previously wanted to add gravity, but was unable to get the program to perform the calculations fast enough; Edwards optimized the drawing functions to free up processing time to calculate the effects of gravity. The initial version of the hyperspace function was limited to three jumps, but carried no risk save possibly re-entering the game in a dangerous position; later versions removed the limit but added the increasing risk of destroying the ship instead of moving it. Additionally, in March 1962, Saunders created gamepads for the game, to counter "Space War Elbow" from sitting hunched over the mainframe toggles. The game was a multiplayer-only game because the computer had no resources left over to handle controlling the other ship. Similarly, other proposed additions to the game such as a more refined explosion display upon the destruction of a spaceship and having the torpedoes also be affected by gravity had to be abandoned as there were not enough computer resources to handle them while smoothly running the game. One feature, having the speed and direction of torpedoes differ slightly with each shot, was added and then removed by Russell after player complaints. With the added features and changes in place, Russell and the other programmers shifted focus from developing the game to preparing to show it off to others such as at the MIT Science Open House at the end of April 1962. The group added a time limit, the hyperspace function, and a larger, second screen for viewers at the demonstration, and in May Graetz presented a paper about the game, "SPACEWAR! Real-Time Capability of the PDP-1", at the first meeting of the
Digital Equipment Computer Users' Society. The demonstration was a success, and the game proved very popular at MIT; the laboratory that hosted the PDP-1 soon banned play except during lunch and after working hours. Visitors such as
Frederik Pohl, the editor of
Galaxy Science Fiction, enjoyed playing the "lovely game" and wrote that MIT was "borrowing from the science-fiction magazines", with players able to pretend to be
Skylark characters. Beginning in mid-1962 and continuing over the next few years, members of the PDP-1 programming community at MIT, including Russell and the other Hingham Institute members, began to spread out to other schools and employers such as
Stanford University and DEC, and as they did they spread the game to other universities and institutions with a PDP-1 computer. As a result,
Spacewar! was perhaps the first video game to be available outside a single research institute. Over the next decade, programmers at these other institutions began coding their own variants, including features such as allowing more ships and players at once, replacing the hyperspace feature with a
cloaking device, space mines, and even a
first-person perspective version played on two screens that simulates each pilot's view out of the cockpit. Some of these
Spacewar! installations also replicated Saunders' gamepad. DEC learned about the game soon after its creation, and gave demonstrations of it running on their PDP-1, as well as publishing a brochure about the game and the computer in 1963. According to a second-hand account heard by Russell while working at DEC,
Spacewar! was reportedly used as a
smoke test by DEC technicians on new PDP-1 systems before shipping because it was the only available program that exercised every aspect of the hardware. Although the game was widespread for the era, it was still very limited in its direct reach: while less expensive than most mainframe computers, the PDP-1 was priced at and only 53 were ever sold, most without a monitor and many of the remainder to secure military locations or research labs with no free computer time, which prevented the original
Spacewar! from reaching beyond a narrow, academic audience. Though some later DEC models, such as the
PDP-6, came with
Spacewar! pre-loaded, the audience for the game remained very limited; the PDP-6, for example, sold only 23 units. ==Distribution and legacy==