You are assuming a standard jet propulsion or rocket system. Rather, this is a different propulsion system. In fact Nietzsche, we are already building engines uncanningly similar to the one described in the Sansrit text - and one of the fuels we have used is indeed mercury(as well xenon and ceasium):
A form of electric space propulsion in which ions are accelerated by an electrostatic field to produce a high-speed (typically about 30 km/s) exhaust. An ion engine has a high specific impulse (making it very fuel-efficient) but a very low thrust. Therefore, it is useless in the atmosphere or as a launch vehicle, but extremely useful in space where a small amount of thrust over a long period can result in a big difference in velocity. This makes an ion engine particularly useful for two applications: (1) as a final thruster to nudge a satellite into a higher orbit and or for orbital maneuvering or station-keeping, and (2) as a means of propelling deep-space probes by thrusting over a period of months to provide a high final velocity. The source of electrical energy for an ion engine can be either solar (see solar-electric propulsion) or nuclear (see nuclear-electric propulsion).
On Aug. 1, 1961, NASA awarded a contract to the Astro-Electronics Division of RCA to design and build a payload capsule for flight-testing electric propulsion engines. The program called for seven capsules, three for ground tests and four for actual flight tests. Each capsule was expected to carry two electric engines. The first was expected to carry one cesium-fueled ion-engine representing Stuhlinger’s design with the Hughes engine. The second was expected to carry one mercury-fueled ion engine representing Kaufman’s design with the Lewis engine. Plans called for the engines to operate from 1 to 2 kW of power. Hughes demonstrated an ion engine on Sep. 27, 1961, at its research laboratories in Malibu. Stuhlinger was among those on hand to greet the scientific and technical writers who attended the event.
Ion propulsion - also known as solar-electric propulsion because of its dependence on electricity from solar panels - has been under development since the 1950s. Dr. Harold Kaufman, a NASA engineer, built the first ion engine in 1959. In the 1960s, NASA Glenn undertook a spaceflight test program called Space Electric Rocket Test (SERT).
In 1964, a pair of NASA Glenn ion engines were launched on a Scout rocket under the name SERT 1; one of the two thrusters onboard did not work, but the other operated for 31 minutes. NASA Glenn also lead the way for a follow-up mission, SERT 2, which carried two ion thrusters, one operating for more than five months and the other for nearly three months.
Many early ion engines used mercury or caesium instead of xenon. SERT 1 carried one mercury and one caesium engine, while SERT 2 had two mercury engines. Apart from the fuel, these ion drives were similar to Deep Space 1’s; the mercury or caesium would be turned into a gas, bombarded with electrons to ionise it, then electrostatically accelerated out the rear of the engine.
But mercury and caesium proved to be difficult to work with. At room temperature, mercury is a liquid and caesium is a solid; both must be heated to turn them into gases. After exiting the ion engine, many mercury or caesium atoms would cool and condense on the exterior of the spacecraft. Eventually researchers turned to xenon as a cleaner and simpler fuel for ion engines.
Now if we compare the description given of the mercury-engine in the Sanskrit text we will find they are uncanninly similar:
The ion engine using the mercuy source first heats up the mercury source contained in the craft either using solar-electric means or nuclear means, which then become ionized and the high speed ions are then through a controlled manner(in this case using electric fields) is used to provide thrust.
The Sanskrit text says that the mercuy engine is the interior of the aircraft, containing 4 metal containers containing mercury, it is then headed in a controller manner through a special iron apparatus, which causes the latent power within mercury to be released providing a thrust causing an instant velocity. (Note, although such an engine would not work in the atmosphere, it would work in space and literally then it would become like a pearl in the sky. )
I think what is clear that this is a secondary description. This sanskrit text is suppose to be a redaction by a 10th century Indian king Bhoja of a much older literature on engineering. It is more a compilation than an actual how-to-build guide of ancient knowledge on engineering. The fact that it is describing in such clear detail a mercury engine for an aircraft cannot be ignored. The fact of the matter is this text is also describing machines that we know for a fact exist(such as mechnical computers to calculate astronomical positions)
And by the way, the Vimanas you speak of SD, are not aerodynamically savvy. I believe an Indian university examined the aircraft designs in these shastras and found that they violated the principles of aerodynamics AND Newton’s laws.
You are talking of the Viamanika shastra here and the pictures the designs were drawn up by an illustrater. This text has dubious origins because it has been channeled and there is no evidence that this text existed. The Sanskrit text I mentioned though is authentic.