#TheExpanse #TheDrive in Orbital 2100 – #TravellerRPG & #CepheusEngine Mods

In a previous post, I discussed the role-playing game Orbital 2100 – A Solar System Setting Using the Cepheus Engine Game and how it could possibly be used for playing in The Expanse setting. In The Expanse, the Epstein Drive is the engine that powers spacecraft across the Solar System. But just how does the Epstein Drive perform, and how could it be portrayed in the Orbital: 2100 setting using Cepheus Engine?

When playing Traveller or today’s Cepheus Engine games like Orbital 2100, I tend to be (using Marc Miller’s definitions from T4) a “Detailed Role Player.” I stray into the “System Engineer” role at times, like for this post. Part of my intention here is to show RPG players and referees/GMs that “this isn’t rocket science” – between the setting, game rules, and the internet (and with the help of a spreadsheet/calculator) it is actually fairly easy to do this analysis.

Fortunately, we have a “canon” story that we can draw inspiration from. The novella The Drive (published in 2012 and available for free online) takes place 150 years before the events of the first novel in the series, Leviathan Wakes. The Drive tells the story of Solomon Epstein, the inventor of the Epstein Drive. It is a very short novella coming in at a mere seven pages. Those seven pages, however, give us plenty of information that can be used to derive the performance of the Epstein Drive.

“By the way, we’re accelerating at four gravities. Five. Six. Seven.”

“He wonders how much above seven he’s going. Since the sensors are pegged, he’ll have to figure it out when the run is over.” – p. 1

In the first pages of the novella, we find a common language between the novels and the Orbital 2100 setting. Like Cepheus Engine and the Traveller RPG it derives from, spacecraft performance is expressed in g’s of acceleration. One g (1g) of acceleration is 9.8 meters/second/second. [Cepheus Engine and Traveller round this to 10 m/s/s…but we will use the actual value for the purposes of this discussion] Seven g’s of acceleration works out to 68.6 m/s. Since Sol’s “sensors are pegged,” this passage also establishes an instrumentation limit of the time.

“The yacht is built for long burns, and he started with the ejection tanks at ninety percent. The readout now shows the burn at ten minutes. The fuel supply ticks down to eighty-nine point six. That can’t be right.

Two minutes later, it drops to point five. Two and a half minutes later, point four. That puts the burn at over thirty-seven hours and the final velocity at something just under five percent of c.” – p. 1

These passages help determine a fuel consumption rate.

  • Using the 90% full tank as a beginning, and given that after 10 minutes 89.6% remains, we see that .4% was consumed in that short time for an hourly consumption rate of 2.4%.
  • “Two minutes later,” or after 12 minutes of total burn, the tanks are 89.5% full; meaning that .5% has been consumed at a rate of 2.5% per hour.
  • Finally, after 2.5 minutes more – or 14.5 minutes total – the tanks are at 89.4% full, or .6% consumed for a rough consumption rate of 2.4% per hour.

Using the 2.4% rate, 90% divided by 2.4% gives us 37.5 hours of “burn” endurance – right in line with Solomon’s “over thirty-seven hours” statement.

The later passage helps compute the acceleration performance of the Epstein Drive.

  • The speed of light – c – is 299,792,458 m/s. Five percent (5%) of c is 14,989,623 m/s.
  • The formula for acceleration is a=v/t where a equals acceleration in m/s, v is velocity in m/s, and t is time in seconds.
  • Plugging in our numbers for velocity (5% of c) and time (37.5 hours or 135,000 seconds) we get an acceleration of 111.03424 m/s.
  • Dividing this by 9.8 m/s, we get 11.33g acceleration.

Eleven g’s of acceleration is quite a lot, even for Cepheus Engine/Traveller where a top-grade maneuver drive is no more than 6g performance!

“Only the acceleration isn’t the problem either. Ships have had the power to burn at fifteen or even twenty g since the early chemical rockets. The power is always there. It’s the efficiency necessary to maintain a burn that was missing. Thrust to weight when most of your weight is propellant to give you thrust. And bodies can accelerate at over twenty g for a fraction of a second. It’s the sustain that’s killing him. It’s going for hours.” – p.3

NASA and the military conducted many experiments in the 1950’s and 1960’s that established a 20g human limit to acceleration. Sol is obviously in pain, but in terms of Cepheus Engine and Orbital: 2100, just how much damage is he taking?

There are no specific rules in Orbital 2100 for acceleration effects on characters. Looking at “Falling and Gravity “in Cepheus Engine (p. 164), we see that on a 1g world, the character will get 1d6 damage per 2m of fall. The rules further specify that for higher g worlds, multiple the 1d6 by the planet’s gravity number. The Epstein Drive accelerates at 11g, which we can compute as 11d6 damage. The question is the time period in which this damage takes place. Falling is assumed to be instantaneous, but declaring 11d6 damage per combat round (6 seconds) does not seem to fit the events of The Drive. This seems excessive because an average character in Orbital 2100 (7 Strength/7 Dexterity/ 7 Endurance) only has 21 damage points until death. The “average” damage from 11d6 is 44, meaning the character is dead twice over!

Perhaps we should assume the 11d6 damage takes place every space combat round  (1,000 seconds/16.6 minutes) instead. This better reflects the painful, but non-instantaneous death like Solomon Epstein experiences. It still seems like an excessive amount of damage, guaranteeing character death.

Looking around for a solution, and not finding one in the rules, I suggest a “house rule” that acceleration couches absorb some of the damaging g forces. In This New Ocean: A History of Project Mercury, acceleration couches in the Mercury spacecraft were designed to absorb 9g (assumed to be the maximum g at reentry). If we use couches to absorb 9 of 11g, the character will have only 2g of damage (2d6) per space combat round. This means an average human may  last as long as three space combat rounds, or about 48 minutes, before sub-coming to the strangling g forces.

“Even as he struggles to make the terminal respond, he’s also thinking what the drive means practically. With efficiency like this, ships can be under thrust all through a voyage. Acceleration thrust to the halfway point, then cut the engines, flip, and decelerate the rest of the trip. Even a relatively gentle one third g will mean not only getting wherever they are headed much faster, but there won’t be any of the problems of long-term weightlessness. He tries to figure how long the transit to Earth will take, but he can’t.” – p.5

Ah, here we can use the classic formula for interplanetary travel time where a ship constantly accelerates to a midpoint, flips over, and then decelerates at a constant rate to the destination. The formula is t=2*SQRT(d/a) where like before = time in seconds, = distance in meters, and = acceleration in m/s/s. (see Cepheus Engine, p. 104)

Unlike Solomon, we do not have 11g’s of force crushing down upon us, so we can solve for the time it would take an Epstein Drive spacecraft to travel from Mars to Earth.

To figure distance, one must first realize that both Mars and Earth orbit the sun differently and the distance between the two planets is not constant. At opposition, the two can be as close as 56 million kilometers (Mkm); however, at conjunction the two can be as far as 401 Mkm apart!. On average, Mars and Earth are 225 Mkm apart.

[Interestingly, in Cepheus Engine, Chapter 6: Off World Travel, Interplanetary Travel, Table: Common Travel Times by Acceleration, there is a listing for “Far Neighbor” with a distance of 255 million km. This is close enough to the Earth-Mars average distance that I think it was the source for the entry. Orbital 2100, Chapter 6: Operating Spacecraft, Travel Times, Travel Between Inner Planets, uses a different process to determine distance (p. 71). In Orbital 2100 you start with the Basic Distance of 80 Mkm (Inner Planets: Basic Distance Table) PLUS seven squares of travel on the Travel Between Inner Planets chart (using the recommended starting setup). This works out to a total travel distance of 290 Mkm – within reason but a bit above the average.]

For the purposes of this example, lets use the 225 Mkm average. Using that average distance (225 Mkm), and Sol’s stated  1/3g (3.27 m/s acceleration), the formula gives us a travel time of Mars to Earth of just over 3 weeks. This may be a normal pre-Epstein Drive trip, given the 3.27g falls within the previously noted 7g instrument limit.

“The United Nations ordered that all shipyards on Mars shut down until an inspection team could be sent out there. Seven months to get the team together, and almost six months in transit because of the relative distances of the two planets in their orbits around the sun.” – p. 6

From this passage we can assume that Sol is telling us that the average transit time between Earth and Mars is about six months. The is an important figure to remember for later.

“And the war! If distance is measured in time, Mars just got very, very close to Earth while Earth is still very distant from Mars. That kind of asymmetry changes everything.” – p. 7

Once again, lets assume the Earth to Mars distance d to be 225 Mkm. Using the Epstein Drive with an acceleration of 11g ( a=111.03424 m/s) and solving for time t gets 12.5 hours. This is a major difference from the six months Sol was thinking about earlier. It is orders of magnitude better performance!

Think for a moment about Jupiter like Sol does. Assuming the Earth-to-Jupiter average distance is 588 Mkm, using the Epstein Drive the trip would take 1 day and 16 hours!

In Orbital 2100, the best TL 9 Nuclear Thermal Rocket (NTR, p. 41) can only travel a maximum of 330 Mkm per month, meaning it takes 1 month and 23 days to make the Earth to Jupiter transit. Even the best alternative TL 10 Fusion Drive, or Nuclear Pulse Fusion (NPF, p. 61), has an acceleration performance of 12 m/s for a travel time of 5 days and 21 hours. Even the best performing Maneuver Drive in Cepheus Engine (6g or 60 m/s acceleration – p. 122), takes 2 days and 7 hours to make the same trip.

Unfortunately, as much as we can learn from The Drive about Epstein Drive performance, the novella lacks other details like the size of the drive or the volume of fuel required. This means we will have to look elsewhere for that information, like maybe Leviathan Wakes.

In summary, the Epstein Drive is very efficient compared to the NPR and NTR in the Orbital 2100 setting. Even compared to maneuver drives available in Cepheus Engine the Epstein Drive is superior. The major drawback, as Sol discovered, is the crushing gravity of acceleration. In the default Traveller setting, the Original Traveller Universe, this is overcome by using handwavium acceleration compensators. In The Expanse, 150 years after Sol’s invention, you have “the juice.”

The juice was a cocktail of drugs the pilot’s chair would inject into him to keep him conscious, alert, and hopefully stroke-free when his body weighed five hundred kilos. Holden had used the juice on multiple occasions in the navy, and coming down afterward was unpleasant. –Leviathan Wakes


Leviathan Wakes, Copyright (c) 2011 by Daniel Abraham and Ty Franck.

The Drive (A Novella for The Expanse), Copyright (c) 2012 by Daniel Abraham and Ty Franck.

Cepheus Engine: A Classic Era Science Fiction 2D6-Based Open Game System. Copyright (c) 2016 Samardan Press.

Orbital 2100 Second Edition, Copyright (c) 2016 Zozer Games.

“The Traveller game in all forms is owned by Far Future Enterprises. Copyright 1977-2016 Far Future Enterprises.”

#Orbital2100 RPG – Hard Sci-Fi RPG for The Expanse?

Orbital 2100: A Solar System Setting Using the Cepheus Game Engine by Paul Elliott at Zozer Games is the “second edition” of Orbital but now based on the Open Game Content Cepheus Engine System Reference Document vice the Mongoose Traveller first edition rules. Coming in at 239-pages, Orbital 2100 provides a more hard sci-fi setting within our Solar System around the year 2100. According to the publisher’s blurb:

Orbital is a science fiction setting for Traveller with a fairly realistic (TL 9) feel that is set within our own solar system. The Earth is locked in a Cold War with the people of Luna. Both face off, 400,000 km apart, threatening mutual annihilation whilst they compete to colonise the moons of Jupiter and Saturn. Older colonies such as Mars and Mercury are independent and caught up in this struggle for solar system supremacy. Spacecraft use nuclear thermal rockets and create gravity by spinning pods or centrifuges, this is spaceflight as envisaged today!

What I think Mr. Elliott really means to say, to avoid legal troubles, is that Orbital is a science fiction setting for the Cepheus Engine. Orbital 2100 attempts to update the original material that used the Mongoose Traveller first edition rules. No longer a legal option, to avoid intruding on Mongoose Traveller’s second edition Closed Gaming Content and Product Identity, Orbital 2100 is based on the Cepheus Engine.

With Cepheus Engine providing the game rules, Orbital 2100 focuses on the setting. The first three chapters; The Situation, The Cold War, and Organisations provide a great deal of background and sets the stage for player adventure.

Character Creation follows the Cepheus Engine which allows this small section to focus on the differences in chargen from Cepheus. For instance, a new definition of Social Standing is introduced as well as new Background Skills. At this point, players “select a campaign” of which there are five. Depending on the campaign selected, different careers are recommended. There are no new career paths presented in Orbital 2100; rather, equivalent careers are mapped to Cepheus Engine careers. Other changes include a slightly modified Skills Cascade list, unique Military & Spacer Ranks, and modified Mustering Out process. The later is an interesting wrinkle to long-time Traveller RPG players because Orbital 2100 does not use the “traditional” end chargen and start adventuring. Instead:

In a typical game, characters must muster out before the game begins. In Orbital, it is more likely that characters will still be in employment within their chosen career. Player’s may finish character generation at any desired point and have their characters join the game, although an aging crisis or some events may also indicate a character has left the character generation process and begun the game. Orbital 2100, p. 35

Being set in 2100, the governing tech is generally TL 9 (with TL 11 in computing and electronics). There is no anti-gravity or jump drive. Trips are limited to inside the solar system using Nuclear Thermal Rockets and spin habitats. The next chapter, Spacecraft Design, introduces three classes of spacecraft that follow these setting restrictions. Deep Space Vehicles (DSV) are analogous to “starships”(100 tons or larger)  in Cepheus whereas Orbital Vehicles are this settings “small craft” (under 100 tons). The added vehicle class is Launch Vehicles (100 tons or less using regular chemical rockets. (Orbital 2100, p. 37). Although Cepheus Engine provides rules for building up to 5000 tons, the Orbital 2100 limit is 2000 tons (p. 60). Orbital 2100 does introduce an alternative drive, the TL 10 Fusion Drive (or Nuclear Pulse Fusion Drive – NPF p. 61). This vastly more efficient drive can make ships more akin to those seen in the TV series The Expanse.

Operating Spacecraft generally follows the Cepheus Engine rules with the greatest exception being travel time within the Solar System. Without getting too scientific, Orbital 2100 uses an orbital racetrack for travel between the inner planets and easy tables to assist in computation of travel times (p. 71). Fuel is also treated much differently, being defined in terms of “Burns” (p. 73) Bottom Line – The Expanse “Flip and Burn” is rare in Orbital 2100. Maintenance is also treated differently, as well as trade revenue. Setting-specific Encounter tables and updated Space Combat rules also are found here (remember – Trajectory is King! – p. 77).

The next chapter, Hardware, properly focuses first on Space Suits. Rovers, Orbital 2100’s version of vehicles, area also here but no rules for their design/construction are presented (nor are they found in Cepheus Engine). Computers are also redefined, and a section of Orbital and Launch Vehicles given. These Launch Vehicles go beyond chemical rockets by adding items like a Mass Driver Catapult or other alternate launch systems. Background and stats for common DSVs are also presented, as well as modular space stations.

Orbital Society is more setting background looking at Law Enforcement, Art, Colonies and the like, background on life aboard a DSV, various Treaties and Regulations and the Earth Orbit Network. There are many adventure seeds buried within these pages!

Working in Space is the Orbital 2100 version of the Environments & Hazards section of Cepheus Engine. The most interesting part to me was “Ways to Die in Space.” There are also rules for Astroid Mining found here as well as a basic outline of how to set up an outpost.

Worlds breaks from the Cepheus Engine design system and instead presents the planets and moons of the Solar System in UWP format. The real gems are found in  the extensive flavor text. Again, lots of great adventure seeds are found here.

Running Orbital is in effect the Referee’s section. I found this section a bit weak. It starts out with four different campaign types, seemingly ignoring the fifth one found in the character generation “select a campaign” at the beginning of the book. This chapter also introduces Secret Agendas and Status, character concepts that I strongly believe should be included in the character generation chapter and not buried here (p. 218-221). The section ends with a look at Aliens (again, nice adventure seeds).

Resources is the Orbital 2100 version of Appendix N; the inspirations for the setting. Good movie or reading list material here, although I can’t believe Paul didn’t mention  Atomic Rockets or the Encyclopedia Astronautica!

Overall, this is a good setting. I have always liked playing in a grittier, harder sci-fi setting like Orbital. I really appreciate the changes Mr. Elliott makes from the Cepheus Engine basic rules. If I have a criticism, it is that I wish Zozer Games had taken the opportunity to relook at the layout of the book and move some items around (especially Secret Agendas and Status) to make these distinguishing character features more prominent and not bury them near the end of the product.

If one is looking for a 2d6-based science fiction setting that can be adapted for The Expanse, Orbital 2100 is a very close fit. To avoid legal entanglements Mr. Elliott is obviously very careful with references to The Expanse with only three mentions in the entire book (one of which is The Expanse entry in Resources). The Expanse has its own spacecraft technology and combat vision, best shown in the episode “CQB”, but a moderately resourceful referee can probably make the adjustments necessary to capture an Expanse-like narrative. At the very least the Orbital 2100 spacecraft design sequence can make DSV’s with NPF in a tail-sitter configuration, and Mag Boots are found on p. 86!

Orbital 2100: A Solar System Setting for the Cepheus Engine Game. Copyright (c) 2016 Samardan Press, Author Jason “Flynn” Kemp.

Cepheus Engine: A Classic Era Science Fiction 2D6-Based Open Gaming System. Copyright (c) 2016 Samardan Press.

“The Traveller game in all forms is owned by Far Future Enterprises. Copyright 1977-2015 Far Future Enterprises.”