The Mono-Polarized Displacer is the world's first form of reactionless pseudovelocity drive. While in the global consciouness, MPDs tend to be depicted as 'I push a button and it goes', in truth even just the way this drive behaves is pretty complex. Here are the things that one needs to understand about MPDs before becoming a pilot or navigator:
In a nutshell, pseudovelocity is rate of displacement that occurs without obtaining enough kinetic energy that would normally accompany it. A high-pseudovelocity collision will result in an object coming to an abrupt stop with no impact damage. Most but not all of the displacement rate of an MPD consists of pseudovelocity, so collisions are mostly but not completely safe.
Pseudovelocity will be lost completely if the MPD is fully shut down or disabled. It can be maintained at the cost of negligible amounts of power (relative to usual craft power consumption levels). Pseudovelocity is transferred nearly-instantly to objects that are in rigid contact with the drive, and so an MPD thrusting constantly in a given direction does not result in a constant felt gravity (but do note the real-velocity fraction, below). Soft contact with the sealed contents of an object in rigid contact with the MPD produces only slight delays and inefficiencies in PV transference, not noticeable under normal circumstances). Soft external contact, such as that of the fuselage with the external atmosphere, gives some PV to the surrounding air molecules, which reduces but not eliminates friction; this reduction of friction more-or-less compensate the relative scarcity of kinetic energy that the drive can spare to push the atmosphere out of the way during high-speed flight; the end result is airspeeds largely comparable to those of conventional atmospheric engines.
- ⚠ Safety considerations: Since an object with pseudovelocity requires an active MPD to retain it, be aware that fully losing contact with a craft will result in losing all PV. This is a major hazard to anyone performing EVA during a craft's flight without a proper safety tether!
Real Velocity and Real Velocity Fraction
An MPD does produce some amounts of Real Velocity (RV) (also sometimes called Bleedover), and this happens through a phenomenan of real (normal) acceleration, that can in fact be measured and felt in the same way as acceleration produced by conventional propulsion. RV is always produced relative to the Local Dominant Gravity Well (LDGW/DGW). The Relative Velocity Fraction RVF varies depending on total velocity (real+pseudo), and for linear movement with uniform acceleration from a full stop, can be calculated based on the ratio of the amount of work actually done by the drive to the amount of energy that would be required to achieve the same total speed with real-only velocity (RVF 1). At speeds where actual work done exceeds the hypothetical real-only energy value, the drive is operating inefficiently, but also produces RVF 1 (obviously RVF can't exceed 1). For typical continental drive thrusts and efficiency levels, RVF=1 at speeds of 20m/s or less, while for Khænish designs it's at 10m/s or less.
- ⚠ Safety considerations: Since the first 20m/s of velocity are fully real, pilots, cargo and passengers will experience the full effects of active acceleration until the craft exceeds this speed, and the felt acceleration will diminish afterwards. Future pilots: to maintain safer flights, attain the first 20-40m/s of joint velocity at a reduced throttle. Passengers: remember that even though most of the velocity of a craft isn't real, a collision with a stationary object is likely to still be equivalent to at least a 45-90mph to zero stop in a fraction of a second.
The simplified 20m/s rule of thumb only holds true for linear acceleration from a full stop. Calculations become more complex when the acceleration vector and the current RV vector don't align. Suffice it to say that the possibility of producing felt accelerations exceeding those for which the engine is rated with a given craft mass exists in scenarios where the craft is 'braking' by thrusting in the opposite direction.
Also, for a flights along certain trajectories under certain conditions (usually linear ones, as well as elliptic and circular orbits), it's possible to gradually convert pseudovelocity into real velocity without changing total velocity, by pumping more energy into the drive in a different mode. This is done primarily to establish true orbits that will no longer depend on a functional MPD to keep running in circles, and is colloquially known as Deliberate Bleedover. With the more typical drive power and energy allowances, establishing a geostationary orbit around Etéra or a similar planet in such a way takes a couple weeks.
- ⚠ Safety considerations: if your pilot is about to engage in evasive manoeuvres, abandon everything and ensure that you are protected against possible extreme whiplash, even if to the best of your knowledge the ship's agility is modest.
Remember that RV is always measured against the Local Dominant Gravity Well (LDGW/DGW)? Naturally, some flights will take a craft from an area dominated by one gravity well to one dominated by another. As the the gravitational dominance shifts, the craft will experience gradual changes in real velocity. This involves 'stealing' some kinetic energy from the new gravity well. Such an effect of gravity wells on the real velocity of craft tends to be colloquially called Gravity Drag, but people are leery of using it in more formal scientific context applied to MPDs.
It's possible to slightly adjust how much gravity drag affects a craft, and it's possible to use it to deliberately gain a certain real velocity vector relatively quickly. The most common application of that is stealing some kinetic energy from one of the moons in order to establish a real orbit without the need to spend weeks on PV->RV conversion.
On Overcoming Gravity, and of Potential Energy
Another thing to consider about MPDs is what happens when their thrust vector is directed away from a gravity well. Yes, moving an object away from a gravity field takes energy expenditure. However, with MPDs, this energy need not be spent during the thrust itself. Instead, it's spent in advance, during the production of a given drive, and is 'locked up' in some of its components (Potential Energy Capacitors, PECs). Don't worry, it can't be released explosively; in fact it's extremely hard to get it out of the salvaged materials in case of a drive's destruction, and the extraction process is even too slow to use as a battery (without invoking Rube Goldberg machines involving gravity wells, that is). Colloquially, PECs are sometimes called Rubber Bands, with various adjective such as 'gravitic' thrown in occasionally.
Just how much energy needs to be invested 'manually' in the production of a given MPD's PECs will depend on how far from gravity wells a drive is produced, and how high it is rated in terms of its ability to move away from wells. Nowadays, most deep space capable drives are rated for being able to push a craft whose total mass is 20 times greater than the drive itself up to 50au from Lumenoferos. The further from Lumenoferos the alchemists transmute the metals into PEC components, the less energy the process takes, since the raw materials already have a larger fraction of the needed potential energy.
Advanced students may also consider the effects of orbital energy on the energy requirements of moving objects up from a gravity well, but that is beyond the scope of this introduction to MPDs.