But you DO have to research Elerium, which is some really powerful stuff. Fixed CTD to reflect that. Comments are welcome.
there is no requirement to have BB Launchers to research the Grav Shield.
Here it is...
Gravity Shield Facility- V1.4
Major changes: Changed multiple engines to a single large engine, after looking at the facility design. Changed 'fear of blaster bombs' to 'fear of Xenium'. Minor grammatical stuff.
During the research and testing of the large gravity engines of the Avenger, our scientists made a startling discovery. They found that the large engines were able to control gravity waves for an incredible distance, greater than ever believed possible. The Avenger's engines were capable of generating and controlling a gravity field 10s of kilometers away from the craft, with no known limit. In theory, a large enough land-based Xenium engine could control gravity around an X-Corps facility, and protect it from attack.
The "Grav shield", as it's come to be called, was originally proposed to protect X-Corps facilities from possible alien bombardment. Research into the incredible materials at the alien's disposal has cast a dark shadow over the safety of our underground facilities. If the aliens were to attempt to make use of a Xenium bomb, they could theoretically create explosions with power greater than that of a Hydrogen bomb. Our underground facilities would not be able to hold against such an attack. The original grav shield design would deflect these "fusion bombs", and use gravity waves to keep the base integrity intact. However, research into this design was put on hold, as testing has shown that an extremely powerful field would be required to repel a device of any size, and creating such a complicated and massive device was completely beyond our reach.
This has led to a redesign of the grav shield. A gigantic specialized gravity engine is constructed within a 10MX10M facility. It is capable of generating a very powerful and very long distance gravity disturbance, with a minimal net gravity, which is directed at the incoming target. This field inteferes with the alien propulsion systems, causing them to produce a fraction of their power. As well, considerable pilot control and engine resources are required to protect against the powerful tidal forces. This in return reduces the incoming target's speed logarithmically, slowing it to a fraction of its speed. This will allow our slower base defenses to fire at the target with a vastly increased chance to hit, and will allow ample time to rearm the defenses to fire again, if the target is found early enough. The intelligent targeting computers lock out undesirable targets for these gravity waves, such as rural areas, or the center of an X-Corps facility.
Unfortunately, controlling such a large engine and keeping it stable stretches the limits of even the best of our Xenium engine pilots. The effect of having extra grav shield facilities active would be catastrophic, as the gravity wave interference that two engines would cause to each other will make them impossible to control. The result would send uncontrollable and extremely destructive gravity waves everywhere, capable of crushing an X-Corps base. We have built in internal failsafes to shut down the field as soon as the second wave of defenses are fired, but be advised, that only one grav shield facility can be safely active within an X-Corps base.
As a side note, the tidal forces of the field also cause considerable water condensation in the air. This has led independent contractors of the funding nations to pursue possible weather control technologies using this device.