Heplar stands for High efficiency Plasma Re-combustion. A power plant, most usually a fission or fusion plant is used to heat liquid hydrogen fuel to plasma state, containing it in a magnetic or at later tech levels a gravitational bottle compressing it and containing it until damper mediated nuclear fusion begins. The plasma is then released as an incredibly hot and powerful drive, producing tremendous thrust.

Discovery Tech Level for the Drive is Tl-9 though once the principles are understood they can be reliably back engineered as low as tech 7 (see below), though maximum performance can be degraded, further advances in gravitational plasma containment at Tech Level 10 allow performance to be safely increased to 6G.

Heplar despite its low-tech nature is a robust deep space drive often found in older spacecraft within and without the growing Imperium, commonly used by designers/governments that due to the isolation and degradation caused by the long night have lost the secret of 'Reaction-less Thrusters', now once again made popular and aggressively marked by many Imperial companies including emperor Cleon's own Zhunastu Industries.

Most low tech spacecraft that use these drives either mount a secondary grav based drive if available for use within the atmosphere for easy take off and landing, or employ air-framed hull designs to allow aerodynamic flight in an atmosphere allowing the vessel to take off and land like an aircraft utilising a runway, or with sufficient modifications vertical take off and landing…

Cost is in Mcr Size is in Displacement Tons

Fuel required is in Displacement Tons per hour of drive operation (liquid Hydrogen).

E.g. when designing a 200-ton merchant ship, requiring 1G of Heplar thrust first reference the drive and armour table on page 259 of 'The Traveller's Handbook', checking the table it is apparent that such a vessel would require 4 drive units, as only 1G performance is required we have a cost of Mcr 1, the drive would consume 2 displacement ton of volume, require 1 Ep and consume 2 tons of fuel per hour of use.

A 200 ton military ship installing a 4G drive would also need 4 units, and would work out as costing Mcr 4, taking up 8 tons of volume, require 4 EP and consume 8 Tons of fuel per hour of use.

Tech Level 7 societies can build this drive as a back engineered prototype, in which case double all values including fuel required to reflect the experimental nature of this drive.

Even after the advent of Gravitic deep space drives (so called Reaction-less drives), many designers still prefer Heplar due to its low cost and ease of build, though it is severely disadvantaged in terms of both range and the volume it requires to support its fuel needs.

Heplar Performance

A handy way of recording performance is to include 'Endurance' in the Vessel description.

Endurance is the length of time the drive could operate for and usually in hours, not days or weeks, e.g. 10 hours, meaning that the craft could sustain maximum burn (highest acceleration) for 10 hours. Endurance does include jump fuel even though it is routinely used to supplement reaction mass on longer journeys.

Final Velocity (FV)

In order to compute travel times, the final velocity of the craft needs to be known, this is normally recorded in G-Hours. For example a craft that accelerates at 1G for 11 hours would have a final velocity of 11 G-Hours, whilst a craft that accelerates at 3G for 11 hours would have a final velocity of 33 G-Hours (3 x 11).

Interplanetary distance is normally measured in Light Seconds and A.U.

1 AU = 500 light seconds or 150,000,000 km.
1 light second = 300,000 km
1/10 light second = 30,000 km

1G Hour of thrust would enable a vessel to cross 1 Light Second in approximately 3 hours.

Travel Times in hours = Light Seconds x 3/FV

Hence a vessel with a final velocity of 20 light hours, travelling to a neighbouring world 2 Au (1000 light seconds) away would take-

1000 x 3/20 = 150 hours (6 ¼ days)

Remember though that a vessel with a final velocity of 20 G-Hours would then have to expend a further 20 G-Hours of thrust in order to slow down and enter orbit around the target world/object, so strict use of fuel is required. Typically a 1G rated ship would start to brake 20 hours out, whilst a 4G rated ship could either do the same or begin breaking at maximum burn some 5 hours out.

For longer trips, most starship operators would simply execute a micro-jump (a jump less than one parsec and normally with an exit point within the same system) in order to save time and arrive with plenty of reaction mass left to allow manoeuvres.

Contra Grav/Anti Grav Drives/Null Grav

Contra Grav

Contragrav drives are gradually being phased out as such on new build imperial spacecraft, and are based on the same technology that makes air/rafts lift off the ground, this is unsurprising as this technology was first used by races like the Villani and the Solomani etc to allow large craft to negate the gravitational attraction of their respective home worlds, and hence achieve orbit quickly and easily without the stresses and enormous fuel consumptions common to non gravitic spacecraft such as rockets.

Discovery tech level for Contra gravity is Tech Level 8, and comes in the following forms.

Null Grav

Null Grav (N.G.)discovered at Tech Level 8 though freely available at Tl-7+

Null Grav is simply that, a phased array of super conducting field emitters that negate gravitational attraction so that vehicles/vessels simply float within the gravity well and remain unaffected by it. Depending on build the level of negation is anything from 50 to 100 % of the vehicles weight but most commonly between 95 and 100 %. Thus at low altitudes most craft remain buoyant in an atmosphere and simply float. A secondary drive such as Heplar or a ramjet/rocket etc is commonly used to produce thrust to enable the vessel/craft to escape the atmosphere and achieve orbit.

Null grav can be reliably back-engineered by competent Tl-7 societies, it is simple and relatively easy to repair, hence is the drive most often used in conjunction with ship mounted Heplar Thrusters, though examples of ship designs utilising the Uni-directional and Omni-directional thrusters can be found, as per the designer's preference.

Uni-Directional Grav

Uni-Directional Grav Thrusters (U.D.G.T) freely available at mid Tl-8+

Further gravitic developments in Tl-8 lead to the uni-directional thruster, which is the first grav drive capable of reaction-less thrust by interacting with the local gravity field and using normally attractive gravitational force for thrust in one direction, normally from the aft of the vehicle. These drives incorporate all of the abilities of its Null Grav predecessor, and typically negate 99% of all gravitational attraction in addition to producing reliable thrust so that speeders, air/rafts etc can fly forward and behave more or less like an aircraft whilst a vehicle with a thruster dedicated to vertical thrust would behave more or less like a primitive helicopter (pilots of these vehicles would gain forward momentum by tipping the nose of the craft forward whilst increasing lift, causing the vehicle to slowly and gracefully accelerate, in order to brake such a craft the pilot would simply pull back (raising the nose) and allow the thruster to do its work, bringing the vehicle to a stationary position, or alternatively reducing thrust as he or she did so to reduce altitude .

This limitation can be negated by fitting the craft with two grav drives, one to provide horizontal thrust, the other to provide vertical thrust, so that the vessel could hover in place and move forward or climb/dive etc as desired.

Omni-Directional Grav

Omni-Directional Thrusters,(O.D.G.T) freely available at late Tl 8+

It is only natural that these two thrusters come combined into a standard grav array, later in tech level 8 as the Omni Directional thruster, ever more sophisticated computer control allow these vehicles to not only efficiently negate gravitational attraction (99%) but use this normally attractive gravitational force for thrust in any direction, making these vehicles the easiest and safest to fly, needless to say this drive is most commonly used by spacecraft in conjunction with an alternative deep space thruster.

Regardless of which actual type of gravitic suspension is actually used in a vessel, Null Grav, U.D.G.T or O.D.G.T, they are commonly referred to as Contragrav Lifters, or less frequently Anti Grav Lifters/Drives, gravitic thrust array/suspension etc.

The Deep Space Grav Thruster and its variants

Gravity based drives by their very nature are only effective within the presence of a gravity well (much like an aircraft propeller can only be useful in an atmosphere) and over 10 diameters out from a main-world thrust falls off dramatically to 1% of its thrust rating, severely limiting performance, (though this isn't much of an issue for a Null Grav drive as it only negates gravitational attraction anyway) so a solution was sought to this annoying problem (some societies never developed anything better than Heplar or other low tech equivalents, though thankfully for the growing Imperium/former Sylean Federation they developed (some would say stole or recovered) the secret of the Deep Space Thruster Plate Drive (or simply Thruster Drive or Thrust Plate), as originally developed by the Villani. Societies that discover the secrets of these drives in isolation generally do so at around tech level 9, with proper training and expert guidance even tech level 7 societies can manufacture low performance versions of these drives (see common design examples).

It should be noted that this 'new' technology would not be possible without the pre-existing developments that led to gravitic vehicles and air/rafts common on many high tech or developed worlds.

The original and some would say common deep space drive (C.D.S.D.) at tech level 9 is an outgrowth of the O.D.G.T. drive that actually locks onto the gravitational field created by not just the local world but the combined gravitational field created by the entire solar system small changes in the angular momentum of the system result in gigantic changes of velocity for the craft. Because these drives are based on the same technology as their O.D.G.T & C.D.S.D. ancestors they too negate gravitational attraction making it possible for even a 1-G rated ship to leave the gravity well of a large world. It is worth noting though that a 1-G drive will still only produce 1-G worth of thrust as the vehicles mass (as opposed to weight), will always be the same irrespective of gravity. These drives tend to produce reliable thrust out as far as 2000 A.U. from a normal main sequence (sol type) star or in some cases even further for a binary or trinary system beyond which quantum gravitic effects drop off drastically making this type of thruster is only good for station keeping at best.

A solution to this at Tech 10 is the S.I.T. (Standard Imperial Thruster), an outgrowth of gravitic technologies and a true gravity independent drive, technically referred to as an S.S.C.E.E. (Simultaneous Space Compression & Expansion Engine), again just referred to as the thrust driver or thruster for general reference. Even primitive societies as early as tech level 5 can have a working knowledge of the cosmos and an understanding that gravity is nothing more than a bend in space-time caused by the presence of a large mass. Unlike previous drives this engine does not interact with or negate the local gravity field but instead uses powerful, energy hungry artificial gravity fields to create a bottle of collapsed space time at the heart of the thruster array, the bottle is maintained by extensive computer control and when cycled causes space at the front of the bottle to collapse whilst simultaneously expanding behind it, with the result that the engine assembly and the rest of the craft bolted to it move slightly in the direction of thrust. Cycling this drive many thousands of times per second allows sizeable forward momentum to be generated at reliable rates, (1-6G). Needless to say these drives like their C.D.S.D forerunners produce a great deal of waste heat and light and a characteristic blue engine glow created by exotic particles trapped close to the 'Thrust plate' though unlike their C.D.S.D. predecessors these engines do not need a gravity field to interact with and may be reliably and safely used even in areas of dead space (an empty hex on the map- sometimes called deep space.)

Unlike other drives, local gravity will affect it as strong gravity wells will always pull on the mass of the ship, hence the sensible thing to do is to mount them with contragrav lifters (now considered to be an integral part of the drive), in order to negate gravitational attraction and gain all of the benefits available to the C.D.S.D. and earlier drives. Depending on the designers preference this could be Null Grav modules, a unidirectional lifter (to allow easy hovering and VTOL etc) or most commonly the Omni-directional thrust array to allow thrust in any direction so that modern starships/spacecraft can hover, take off and land safely and even reverse if required just like their grav vehicle cousins.

The sizes, volumes and power requirements of any needed contragrav components are subsumed into the costs, volume requirements and power needs of all of the drive options in the descriptions and table given on pages 265 & 266 of The Traveller's Handbook.

When flying a starship or space vessel in an atmosphere many trainee pilots are simply told, 'just think extra-large air/raft' though simplistic it's not too far from the truth, as can be gleaned from synergistic effects of research and development coupled with necessity of design.

Finally the S.I.T has one further advantage over previous drives, put simply, extra thrust may be gained by overdriving the plates, (channelling more energy through them and using the computer to cycle them faster), in order to increase performance).

A typical S.I.T. thrust drive could be overdriven by up to 40% for a few days before risking catastrophic burn out and drive failure or 50 to 100 % for up to a few hours or even up to 400% for a few minutes. This has important ramifications for civilians and military alike, consider the scenario of a merchant vessel damaged by a raider forced to crash land on a planet with a 1.5 G gravity well, (no problem you think, we've got contragrav built into our drive) consider that the contragrav might be damaged and non functional. With any other drive (Heplar included) you might never leave that rock again; but with your engineer keeping a skilled eye on the drive your 1G rated merchant could easily leave that 1.5G gravity planet with little difficulty, simply by overdriving the plates by 100% for the duration of the ascent. This would effectively give you 0.5G thrust (2G thrust - 1.5G local grav = 0.5 Acceleration).

This same drive in drastic circumstances could go from a performance of 1G to 4G in an emergency but it would require a skilled engineer to prevent drive failure in anything more than minutes.

The old saying is 'treat her bad and she'll treat you just as bad, but look after her and she'll look after you…'

Needless to say a 2G rated drive would rarely have this difficulty when leaving large worlds, hence the 2G rating of many 'standard' scout vessels built for durability and utility that routinely operate beyond the frontier where there's no help.

So it comes as little surprise that this is the standard Imperial Deep Space drive for civilians and military alike though discovered at tech 10 can be manufactured, maintained and repaired by technically able societies as far back as tech level 8 though performance may suffer as a result of this.

Common Design Examples (Reaction-less)

As in today's world the tech level of discovery or achievement is often higher than the tech level actually needed to maintain and manufacture the drives, when back engineered it is usual to expect some reduction in performance, the tables above and below are only intended to give the designer a feel for the likely explanations for locomotion within the Traveller universe and help tie up cannon and outright gaps in the technology and equipment chapters such as there being no explanation of how the various manoeuvre drives at available form Tl-7 to Tl-9, the only hint of an explanation is on THB page 198 which states that gravitic manoeuvre drives become available at tech 10. Being a long-term traveller fan I drew on the combined wealth of knowledge and materials from previous editions of Traveller (MegaTraveller, Traveller: The New Era, T4 (Marc Miller's Traveller)) plus a little inspiration of my own… Again if you've got your own explanations don't feel that you have to use these, I thought that people new to Traveller who purchased the T20 rules, might benefit from this as a good technological overview is the one thing that sorely missing from the T20 manual, probably deliberately so, in order to allow the imagination to flourish and not to be 'dated' by future scientific discoveries. Yet I personally find that the imagination can only flourish and breathe life into the vessels of our wonderful universe if it has a sound and common basis to work on, otherwise traveller technology just becomes a hand wave to true science fiction that's based on physical laws.

Checking the table above you can see that a tl-8 long-range shuttle craft could be built with either a CDSD or an SIT with a 5g maximum rating for the C.D.S.D. or a 4g maximum rating for the S.I.T., if built with a 1g or 2g drive any thruster type could be used as the tables above only represent maximum performances, as previously implied use the tables of page 266 of the Traveller's Handbook to actually build the drives as they can all be argued to share common characteristics, sizes and power requirements etc due to the synergistic effects of design and development.

Common Design Examples

A.G. = Anti Grav N.G.= Null Grav
U.D.G.T.= Uni-Directional Gravitic Thruster
O.D.G.T.= Omni-Directional Gravitic Thruster

The Tl-7 low performance basic system would consume twice as much fuel, volume, and power and have double the cost of the improved and standard Heplar engine types.

When designing or purchasing a Heplar drive assume that the associated costs, volume and power requirements of a supporting grav drive are subsumed into the cost, volume and power requirements of the main Heplar drive; so there's no need to reference another table you only need to decide which gravitic suspension you'd like the craft to have and note it in the craft's description/write up.

In all cases Heplar designers can elect not to have any gravitic components aboard their vessels, if this is the case use the normal tables but select the air-framed hull form when designing such vehicles so that they can be safely and aerodynamically controlled in an atmosphere, reduce the cost of the Heplar drive by 75% and reduce the drive volume by 50% to reflect this as starship sized gravitic emitter arrays/thruster assemblies are expensive,(though scale to scale, cheaper than small air/rafts and other grav vehicles which is the way it should be!)

Hence in our earlier example of the 200 ton merchant ship, suddenly electing not to have a gravitic counterpart the cost of the drive would fall to Cr 250,000, whilst the corresponding cost to the military vessel also mentioned would fall to Mcr 1, the volume taken up by the drive would fall to 1 & 4 Tons giving a substantial volume saving and cost reduction.

Fusion Rocket

Perhaps the most terrifying drive of all, and hence the least common, is the fusion rocket, a fusion rocket is nothing more than a fusion reactor with a blast combustion chamber mounted to the end as a thruster, a steady stream of hydrogen going in one end is fused in the reactor core releasing tremendous quantities of energy in the process not to mention a long plasma vapour trail/wake behind the accelerating vessel. The exhaust is extremely radioactive, and thankfully most vessels pass through these wakes quickly enough that they suffer no ill effects unless extremely close (any craft coming within 200 km of such a wake will take damage from the associated radiation, making the crew ill and probably in need of blood and bone marrow treatments for the next few months in order to survive).

Simply activating such a drive can be viewed as an act of war in busy systems with lots of traffic, and needless to say entering atmosphere in such a vessel would at the minimum sterilise entire continents or more than likely, that hemisphere.

Fusion rockets are most often used by long-range generation ships that cross the interstellar voids without the use of jump drive, as the thrust they generate is incredible for relatively small amount of fuel consumed.

When fusion rockets are used they tend to be on orbit-to-orbit vessels as the hull itself often becomes radioactive in the vicinity of the drive and often needs extensive decontamination procedures before safely setting down in a Starport or Spaceport, assuming that it can enter atmosphere using another cleaner type of drive.

Also many busy systems might have traffic control stipulate that the drive is run rich (10 times fuel consumption) which lessens thrust but also reduces the radioactivity associated with the exhaust.

Another useful feature of this drive is that it can also be used to generate electricity for other ship systems.

Again use the table on P-259 of The Traveller's Handbook to determine the number of drive units needed.

Volume = Displacement Tons
EPO = Energy Point Output (power produced by the drive)
Mcr = Millions Credits
Fuel (liquid Hydrogen/ Displacement Tons consumed per hour of drive operation per drive unit)

Again a 200 ton ship needing 1G acceleration would require 4 units giving us a cost of Mcr 40, a volume of 16 tons, a power output of 2 Ep and fuel consumption of 1 ton per hour of drive use.

It should be noted that the drive can only produce power when it's active; another power plant is going to be needed to power the ship when the drive isn't being used.

The high cost of the drive can be attributed to the fact that these things are not likely to be mass-produced anywhere, again, if the referee feels that he or she can justify a lower cost then go ahead…

Despite the radioactivity, the drive is twice as efficient as its Heplar counterpart, meaning that it can go twice as far on the same amount of fuel.

T20 & Milieu Zero Fusion+

When integrating T20 rules in the MO setting, it becomes apparent that cold fusion power production technology (marketed as Fusion+ in the Imperium) is an important part of the overall setting and feel of the era. Fusion + is referred to the technology that makes the Imperium possible, giving the Emperor a tactical advantage against his foes.

Fusion+ is 'cold fusion' only in comparison to the intense heat of a traditional shipboard fusion reactor and produces tremendous amounts of electricity for a relatively small plant, using heavy (deuterium rich) water as its fuel.

Tl = Tech level of power plant availability
Ep = Energy Point output
Cost = Mcr
Size = Displacement Tons
Fuel = Displacement Tons per 4 weeks of power plant operation (heavy water)