Traveller Psionics: Break/Fix

I love Mongoose Traveller and I say that as a diehard Classic Traveller fan. I started playing Traveller in the early 80s and I still use my Little Black Books.

I’ll admit I was a bit sceptical when I first saw the big shiny Mongoose books and their fancy pants artwork but I bought the Core Rulebook anyway. As soon as I started reading I recognised its clear kinship to Classic Traveller and I soon realised that my house rules had been heading in the same direction anyway. 

But, Mongoose Traveller Psionics rules are broken. (Well, I guess Cepheus broke them first.)

Let me explain. I’m afraid we need a short history lesson. There will also be number crunching.

In Classic Traveller there was no dice rolling to use Psionics. You rolled dice to learn Psionics, and that was usually during gameplay, not during character generation. You made a single dice roll to determine your Psionic Strength characteristic and further dice rolls to see which talents you received: Telepathy, Clairvoyance, etc. There was an Experience system for Psionics which seemed at odds with the rest of Traveller, with dice rolling over several months to increase talent levels from 0 to 10 or higher, limited by the Psionic Strength characteristic. These were not like skills used elsewhere in the system which typically ranged from 0-4.

During gameplay you temporarily expended Psionic Strength points to use an ability, like magic points to power a spell. No dice rolls. Bit odd. It kinda made sense at the time…I think.

MegaTraveller introduced dice rolls for using Psionics. They kept the same 0-10 talent levels. There was a wide range of target numbers depending upon the specific talent ability being used: 7+ for something simple like Telempathy (sending basic emotions), 11+ for actual Telepathy (sending or receiving thoughts), and 15+ for an Assault (a brain-melting mental attack). MegaTraveller had silly jumps for dice rolling difficulty: 3+ (Simple), 7+ (Routine), 11+ (Difficult), 15+ (Formidable) with nothing in between. All rolled with two six-sided dice.

The easiest MegaTraveller Psionic task required a 7+ which might appear to give little more than a 50/50 chance of success (58% 21/36). An ‘average’ Psionic task required an 11+ which looks only marginally likely to succeed (8% 3/36). The most difficult Psionic tasks required a 15+ which obviously look completely impossible (0%). But these target numbers were just base rolls, which were modified, and that’s important.

First of all, you added your Int characteristic divided by 5. That was a +1 for most characters with +0 or +2 unlikely. Crunching these numbers we get 6+ (72%) or 10+ (17%) or 14+ (0%).

But here’s the important part: you also got to add your talent level. That was the rating you’d been increasing with Experience. It started at 0 for someone who had only just discovered their Psionic abilities but it would hit 3 or 4 within a few months and max out after a couple of years at your Psionic Strength characteristic. If you had an ‘average’ Psionic Strength of 7 that was a +7! If you were a gifted prodigy, like Akira or Eleven, you could get a solid +10!

In the rest of the MegaTraveller system you were lucky to get +3 or +4 on a dice roll, and there was a rule actually limiting this to a maximum DM+8. This rule was specifically waived for Psionic talents (Players’ Manual, p.97). 

Let’s crunch an ‘average’ MegaTraveller Psionic. You’ve got Psionic Strength 7 and your talents have increased over time with Experience to their maximum, giving you a DM+7. You have an average Int 7 for another DM+1. That’s a total DM+8. You will auto-succeed with any talent ability that’s Routine or Difficult and you only need a 6+ for the tricky stuff like Assault. No wonder the Imperials were terrified of the Zhodani! 

Let’s take a step back. An actual average character probably served 3 or 4 terms which would give them a DM -3 or -4 on their Psionic Strength roll due to their age. Let’s assume they get Psionic Strength 3 and after a few months they max’ed out their talent levels for a DM+3. Let’s assume they have the same Int 7 for a further DM+1. That’s a total DM+4. Let’s crunch: 3+ (97%), 7+ (58%), 11+ (8%). That’s more like it! They’re only going to fail the simple stuff if they roll snake-eyes. Average talents will work 50/50. Tricky stuff is unlikely…but it might happen. This feels about right to me. They already have a day-job piloting the ship or manning a turret. Psionics is a bonus. 

I loved MegaTraveller back in the day but I always struggled with the rules. I don’t mean the typos–which were notorious–I mean the constant rule breaks and retcons required to support a supposedly ‘accurate’ rules system which was brittle due to overcomplexity. I’ll blog specific examples in future posts. 

Saying that, I think MegaTraveller does a stellar job with Psionics, albeit with rules that feel like a hack. Player characters with entry-level talents get some side abilities that feel more like ‘cantrips’ unlikely to imbalance the game. Higher level Psions have the potential to be incredibly powerful, though still limited by rapid expenditure of Psionic Strength points. More on that below. 

Along came Cepheus and Mongoose. They kept the Psionics rules mostly as-is but they rationalised the whole ‘talent level’ thing in line with skills. I really like this. It makes sense. It no longer feels like a hack. But, oh dear, it breaks Psionics and that’s why I’m writing this blog post. 

The skill/difficulty system in Cepheus/Mongoose adopted a smoother granularity with difficulties ranging from 4+ to 14+ in increments of 2, averaging at 8+ (very Classic Traveller), rolling 2 six-sided dice as usual. I had been using this system as a house rule for years before I discovered either of these new systems. I suspect many did. 

Cepheus/Mongoose ported over Psionic abilities from MegaTraveller with much the same difficulties, such as Assault which needed a 14+. Some lower level abilities had their difficulties reduced, such as Life Detection, which now only needed a 4+.

To succeed with a Psionic ability in Cepheus/Mongoose you expend the same Psi points as Classic or MegaTraveller. You make a roll with similar target numbers to MegaTraveller. You add your Psi characteristic modifier to this roll, which averages at +0 compared with the almost default Int DM+1 from MegaTraveller, so you’re typically a point worse off. Mmm. Then you add your Psionic skill which, like any Traveller skill, averages at 0, 1 or 2, with 3 or 4 less likely. Gosh. So now I’m only adding DM+0 to +4.

Let’s crunch. Base rolls for simple Psionic abilities are a 4+ (92%) or 6+ (72%). For average abilities they’re 8+ (42%) or a 10+ (17%). Tricky abilities such as Assault, already mentioned, need a 14+. I’m going to use Assault as my ‘upper boundary’ example.

If you have Psi-10 (DM+1) in Cepheus/Mongoose you will fail Assault unless you have Telepathy-1 or 2, and even then you need a 12+ (3%) or 11+ (8%). Bear in mind that in Classic this was an automatic success and in my MegaTraveller example earlier on, a lesser Psionic with Psi-7 was only rolling 6+. That’s a major step-change.

Let’s up the stakes: a Cepheus/Mongoose Psion with Psi-10 and Telepathy-4 giving them a DM+5. This individual should be deadly! But when they attempt an Assault they still need a 9+ (27%). And if they actually succeed, in Cepheus, they only do 2D damage plus Effect. In Mongoose it’s Effect x 3 damage, that’s equivalent to 1D per Effect so they’re back to needing an 11+ or 12+ for any level of meaningful damage. That’s terrible.

If I was metagaming a street kid with an autopistol, Dex-7 and zero-level skill, I’d be laughing as I emptied my magazine at this supposedly deadly Psion with their Psi-10, Telepathy-4 powers. I’m rolling 8+ for 3D-3 damage each round at the same 10m range as them. I have 15 bullets in my mag. Who do you think’s gonna win?

It seems to me that when Cepheus ported over the Psionic difficulty levels from MegaTraveller they didn’t fully understand how dice rolls previously worked with those wacky 0-10 talent levels. Mongoose ported over the same rules from Cepheus. Same problem.

My House Rule

I ask for 8+ rolls for every single Psionic talent ability. I decrease or increase difficulty based on situation, not the ability being used. For example 6+ if meditating whilst using Telekinesis to move rocks on some slimy, mudhole of a planet. Or 10+ if using Telepathy whilst hanging from a pylon underneath a floating anti-grav city.

If this seems overly generous just consider gameplay. Not some justification over ‘accuracy’. This is a game, not real life. If you give your players an ability or a skill or a piece of equipment, it needs to be balanced with the equivalent from another player or NPC. You wouldn’t give a Monopoly player half the starting cash because they picked the hat, would you? Really?

Another House Rule

I also reduce Psi cost by 1 point per Effect. That’s potentially game imbalancing, increasing the deadliness of Psions in your universe. Discuss this with your players before use. I like it because it gives the Psions in my game, which are very rare, longer staying power. I also allow them to voluntarily burn Endurance points instead of Psi with some juicy physical description: bleeding nose, bloodshot eyes, the usual.

Last Word of Advice (especially if you’ve skim-read down to here, that’s cool, this post is long)

If you’re going to stick with the Cepheus or Mongoose Psionics rules as they are, that’s cool, just please do one thing…

If you have a player who’s going to roll a Psion, have a chat with them first. Explain their chances of success. Decide what to do together, whether that’s switching to another character or proceeding anyway.

There is nothing worse than a player getting pumped about a new character, investing all that time and effort, only to discover that the rules system has let them down.

Remember, it’s a game, we’re here to have fun. Enjoy!


Classic Traveller Book 3: World and Adventurers, p.38-47 (1981)

MegaTraveller Players’ Manual, p.96-102 (1987)

Cepheus Engine System Reference Document p.58-65 (2016)

Mongoose Core Rulebook p.228 onwards (the silly expensive 2022 leather one, I told you I loved it!)


  1. Thomas says:

    I like this fix quite a bit. I am new to Traveller, MgT2e being my entry point, so I really enjoyed the history behind the fix to give it context. I will definitely be using this houserule.

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