A request for input, regarding a likely upcoming event in my campaign: exactly how great would the scale of the devastation be if an Imperial voidship (say, a Dauntless-class Light Cruiser) deliberately plows into a planet's surface at full throttle?

At ~20 megatonnes in mass, and assuming it's going full-bore no-safeties acceleration, it'd depend on how much distance it has to build up speed. Assuming Lexicanum has the stats right, it'd be pushing maybe 5 gravities if you disabled the safeties (using earth standard gravity of 9.8 m/s/s, that'd be 49 m/s/s).

If the ship were in a Geosynchronous orbit over Earth it'd be at 42,164 km. Pointed straight down and with the engines pushed past the red line, it should be pushing 6 gravities by the time it gets deep into the atmosphere (1 for the planet's pull, the other 5 from the ship itself). However, for the sake of making things easy, we can stick with just the ship's raw acceleration of 49 m/s/s.

My physics days are far behind me, but a cursory interwebs search gets me this equation:

v^2 = u^2 + 2as, with v = velocity, u = initial velocity, a = acceleration, and s = distance.

v^2 = 0 + 2*49m/s/s*42,164,000m = 4,132,072,000 m*m/s/s

v = 64,281 m/s

Using the mass of the ship estimated at 20,000,000 tonnes, you should be able to calculate energy from the impact.

Using the interwebs, I estimate maybe 29.8 equivalent megatons of TNT. Hiroshima was destroyed by 15 kilotons. That puts the impact at around 1,987 times more power than Little Boy unleased over Hiroshima.

This all assumes that my math was anywhere near correct, and ignores any previous velocity the ship had before reaching a point of geosynchronous orbit over the planet that it wanted to wipe clean of life, as well as any potential issues caused by the massive plasma generators, already over-stressed by red-line acceleration with disabled safeties, having their magnetic bottles catastrophically fail due to the impact. I assume the force of the blast would be sufficient to make any secondary explosions from on-board munitions a moot point, barring something like a Cyclonic Torpedo or other Exterminatus-grade warhead.

Nukemap estimates that a surface detonation of 29.8 megatons would have a fireball radius of 4.88km, an air blast radius of 6.75 km at 20 psi (even reinforced concrete fails, almost total fatalities), an air blast radius of 15.7 km at 5 psi (harmful to internal organs, mostly fatal), and a thermal radius of 43.6 km (massive, painful burn injuries).

Again, assuming my math isn't off (and it very easily could be), then a simple de-orbit at maximum acceleration directly into the ground would not be an extinction-level event, but it would be massively devastating, made significantly moreso if the Void Ship were to accelerate at the maximum safe rate for as long as possible prior to the safeties-off final run from orbital altitude to ground.

АН602 was 58 mtonns. https://en.wikipedia.org/wiki/Tsar_Bomba

Problem is a Dauntless-class Light Cruiser have at least one plasma reactor and armaments, so I believe if you want to finish something with a ram you'll do it. Planet will survive though.

Worst case scenario is the warp drive detonates. Going off the Rogue Trader ship Critical table everything within 2d10VU (1VU=10,000km) that fails to evade is struck by a chaos storm dealing damage like a lance strike and potentially being sucked into the warp. So a planet at the epicenter, being unable to evade, would be completely screwed. Assuming an average 10VU distance roll that's 100,000km (earth has approx. circumference of 40,000km) even a large planet would most likely be completely engulfed and pretty much instantly reduced to a smoldering barren rock/daemon world. Basically no chance of anyone surviving. Even in just a 2VU explosion that would mean half the planet has possibly been sheared off and sent to the warp or just completely covered in daemons.

Slightly less terrible but still world ending would be if the plasma drive detonated instead. In this case the explosion is 1d10VU and the explosive force and debris hit like 1d5 macrocannon battery hits. So still enough to likely engulf the whole planet, kill most life, level most structures and possibly burn off the atmosphere but at least no daemons and the planet would still physically exist in real space.

Best case would be roughly what Levi posted. Equivalent to a sizable nuke. On some worlds this might not even be that big a deal depending on where it lands. For example a large portion of Desoleum is already barren lifeless waste so as long as it didn't impact near any of the major hives the consequences would be minimal.

Or you create a stable large scale warp rift that then plunges the whole system/sector into a warp storm.

Worst case scenario is the warp drive detonates. Going off the Rogue Trader ship Critical table everything within 2d10VU (1VU=10,000km) that fails to evade is struck by a chaos storm dealing damage like a lance strike and potentially being sucked into the warp. So a planet at the epicenter, being unable to evade, would be completely screwed. Assuming an average 10VU distance roll that's 100,000km (earth has approx. circumference of 40,000km) even a large planet would most likely be completely engulfed and pretty much instantly reduced to a smoldering barren rock/daemon world. Basically no chance of anyone surviving. Even in just a 2VU explosion that would mean half the planet has possibly been sheared off and sent to the warp or just completely covered in daemons.

Slightly less terrible but still world ending would be if the plasma drive detonated instead. In this case the explosion is 1d10VU and the explosive force and debris hit like 1d5 macrocannon battery hits. So still enough to likely engulf the whole planet, kill most life, level most structures and possibly burn off the atmosphere but at least no daemons and the planet would still physically exist in real space.

Best case would be roughly what Levi posted. Equivalent to a sizable nuke. On some worlds this might not even be that big a deal depending on where it lands. For example a large portion of Desoleum is already barren lifeless waste so as long as it didn't impact near any of the major hives the consequences would be minimal.

Let's not forget that in space there is no any atmospheric protection in space.

Would the PCs be on the ship or the planet, if it strikes?

Because if yes I think you might be planning a TPK with no chance of survival - and they may not appreciate that.

Or do they have a plan to survive? I'm curious

I don't believe it SO bad. Fluff have many falled voidship hulks after all. And Orks count it as a safe landing!

Would the PCs be on the ship or the planet, if it strikes?

Because if yes I think you might be planning a TPK with no chance of survival - and they may not appreciate that.

Or do they have a plan to survive? I'm curious

The PC's will definitely be on the ship (down in the bowels, far away from the controls) when it starts its manoeuver. They can try to break through to the bridge and nudge the ship off-course (going by Levi's estimates, that will indeed save the population center it was aimed for from immediate destruction), reach the thrusters and physically sabotage them, or do one of several other things intended to limit the destruction caused. They have ample time to evacuate in the beginning, but the longer they stay on-board to try and save the city below, the more they hurt their chances of reaching the life-boats in time.

It's going to take some careful planning to balance the combat difficulty, travel time through the ship, time taken at attempts to take control of engines etc. so that the players can make meaningful choice, but I hope it'll be worth it.

I don't believe it SO bad. Fluff have many falled voidship hulks after all. And Orks count it as a safe landing!

Let's not pretend physics actually plays a role in anything orks do. :-D

There's talk in the RT books about a planetbound voidship, but that's with a controlled-ish landing.

I think you should pick whatever fits your narrative best. It's going to be a catastrophic event and probably destroy all life on the planet. I imagine something like a nuclear winter. Even large volcano eruptions can do this.

For a little physics, see here: https://what-if.xkcd.com/20/

Note: Maybe total energy is better. Pure KE from the number posted about 20,000 kg and 64,000 m/s produces 4.096×10^13 joules, which is ~2.5×10^23 GeV. That's a lot of energy. Hard to compare to the What If? since I'm not clear on how many carbon atoms are in a diamond 100 feet in diameter...

And now I'm looking that up. Please hold..... aggghh that's really hard to figure out.

The volume, in cuft: 500000 Pi)/3 ft^3. But the density...

Oh I could use the density of diamond/the mass of a carbon atom.

Eh. I'm going to lunch.

I believe there is so much variables (atmosphere density? geological specifics? water coverage? mantle state?) for that crash so it's better be narrative. It's just simplier.

If your craft crush into populated area, trying to deal maximum damage, with setting reactors wrong and so all... well, I believe something about BosWash will be destroyed utterly. With new gulf appeared.

I'd like to spend a Fate Point for an extra DoS on my Dodge, please.

I don't believe it SO bad. Fluff have many falled voidship hulks after all. And Orks count it as a safe landing!

Let's not pretend physics actually plays a role in anything orks do. :-D

There's talk in the RT books about a planetbound voidship, but that's with a controlled-ish landing.

I think you should pick whatever fits your narrative best. It's going to be a catastrophic event and probably destroy all life on the planet. I imagine something like a nuclear winter. Even large volcano eruptions can do this.

For a little physics, see here: https://what-if.xkcd.com/20/

Note: Maybe total energy is better. Pure KE from the number posted about 20,000 kg and 64,000 m/s produces 4.096×10^13 joules, which is ~2.5×10^23 GeV. That's a lot of energy. Hard to compare to the What If? since I'm not clear on how many carbon atoms are in a diamond 100 feet in diameter...

And now I'm looking that up. Please hold..... aggghh that's really hard to figure out.

The volume, in cuft: 500000 Pi)/3 ft^3. But the density...

Oh I could use the density of diamond/the mass of a carbon atom.

Eh. I'm going to lunch.

Hm. Volume of the meteor is (4/3)*pi*(100 ft/2)^3 = 14826.7 m^3. Wikipedia has diamond's number density as 176.2 * 10^27 carbon atoms per cubic meter, so it's 2.612 * 10^33 atoms total. But we're probably better off going for the mass density directly, which Wolfram Alpha lists as 3500 kg * m^-3, so a total mass of 51893450 kg = ~5 * 10^7 kg.

Would the PCs be on the ship or the planet, if it strikes?

Because if yes I think you might be planning a TPK with no chance of survival - and they may not appreciate that.

Or do they have a plan to survive? I'm curious

The PC's will definitely be on the ship (down in the bowels, far away from the controls) when it starts its manoeuver. They can try to break through to the bridge and nudge the ship off-course (going by Levi's estimates, that will indeed save the population center it was aimed for from immediate destruction), reach the thrusters and physically sabotage them, or do one of several other things intended to limit the destruction caused. They have ample time to evacuate in the beginning, but the longer they stay on-board to try and save the city below, the more they hurt their chances of reaching the life-boats in time.

It's going to take some careful planning to balance the combat difficulty, travel time through the ship, time taken at attempts to take control of engines etc. so that the players can make meaningful choice, but I hope it'll be worth it.

Don't forget that the players might decide to turn the ship around and give it a full burn. Which might save the ship while the engine exhaust burns down the city.

Go with whatever narrative makes sense for your game after you've seen what the players are trying to do.

Worst case scenario is the warp drive detonates. Going off the Rogue Trader ship Critical table everything within 2d10VU (1VU=10,000km) that fails to evade is struck by a chaos storm dealing damage like a lance strike and potentially being sucked into the warp. So a planet at the epicenter, being unable to evade, would be completely screwed. Assuming an average 10VU distance roll that's 100,000km (earth has approx. circumference of 40,000km) even a large planet would most likely be completely engulfed and pretty much instantly reduced to a smoldering barren rock/daemon world. Basically no chance of anyone surviving. Even in just a 2VU explosion that would mean half the planet has possibly been sheared off and sent to the warp or just completely covered in daemons.

Slightly less terrible but still world ending would be if the plasma drive detonated instead. In this case the explosion is 1d10VU and the explosive force and debris hit like 1d5 macrocannon battery hits. So still enough to likely engulf the whole planet, kill most life, level most structures and possibly burn off the atmosphere but at least no daemons and the planet would still physically exist in real space.

Best case would be roughly what Levi posted. Equivalent to a sizable nuke. On some worlds this might not even be that big a deal depending on where it lands. For example a large portion of Desoleum is already barren lifeless waste so as long as it didn't impact near any of the major hives the consequences would be minimal.

Let's not forget that in space there is no any atmospheric protection in space.

At least against the warp explosion the atmosphere wouldn't help at all. The damage specifically ignores even void shields and being a lance it also ignores starship armor. So based on that there would be very little left caught in the blast.

With the plasma generator overload, atmosphere or not, its still on average a planet sized plasma explosion. Really doesn't seem like a good thing.

I don't believe it SO bad. Fluff have many falled voidship hulks after all. And Orks count it as a safe landing!

Rogue Trader covers this. Plasma generator and warp engine overloads are pretty rare. Usually in a fight ship's systems just get crippled until its inoperable at which point the winner of the fight leaves and the crew dies from failed life support or uncontrolled fires etc. or the winner boards to finish off the crew, both lead to space hulk type wrecks.

However purposely slamming a ship as hard as possible into a planets surface seems like a situation that would more likely lead to one of the worst case scenarios. Especially if the ship was sabotaged to do as much damage as possible.

Orks are still technically trying to land. It may be a crash landing but they still may try to slow down last minute.

I will also mention that the ship could survive relatively intact and with mild planetary consequences if the party can mange to slow it down enough and steer it to bring it in at a better angle and keep the void shields up making it more of a crash landing. Could be a way for them to survive if they can't get off in time. For this situation picture Star Wars Episode 3 when they crash land the broken off front part of the separatist ship.

I don't believe it SO bad. Fluff have many falled voidship hulks after all. And Orks count it as a safe landing!

Let's not pretend physics actually plays a role in anything orks do. :-D

There's talk in the RT books about a planetbound voidship, but that's with a controlled-ish landing.

I think you should pick whatever fits your narrative best. It's going to be a catastrophic event and probably destroy all life on the planet. I imagine something like a nuclear winter. Even large volcano eruptions can do this.

For a little physics, see here: https://what-if.xkcd.com/20/

Note: Maybe total energy is better. Pure KE from the number posted about 20,000 kg and 64,000 m/s produces 4.096×10^13 joules, which is ~2.5×10^23 GeV. That's a lot of energy. Hard to compare to the What If? since I'm not clear on how many carbon atoms are in a diamond 100 feet in diameter...

And now I'm looking that up. Please hold..... aggghh that's really hard to figure out.

The volume, in cuft: 500000 Pi)/3 ft^3. But the density...

Oh I could use the density of diamond/the mass of a carbon atom.

Eh. I'm going to lunch.

Hm. Volume of the meteor is (4/3)*pi*(100 ft/2)^3 = 14826.7 m^3. Wikipedia has diamond's number density as 176.2 * 10^27 carbon atoms per cubic meter, so it's 2.612 * 10^33 atoms total. But we're probably better off going for the mass density directly, which Wolfram Alpha lists as 3500 kg * m^-3, so a total mass of 51893450 kg = ~5 * 10^7 kg.

You swapped cubic feet into cu meters, which is a factor... 27? error. Unless you converted the first number already.

But yeah, that's not going to be huge. If that's 70 GeV per atom then that's more energy than our ship (which makes sense at 0.99c). But, I really don't have a basis for comparison here.

Injecting reality into 40k is always a huge mistake. Just claim it blows up a hive.

Injecting reality into 40k is always a huge mistake. Just claim it blows up a hive.

Nah, say it skims the top off of a hive, eradicating a huge portion of the ruling nobility and infrastructure that runs is, before plowing a huge swathe on the planets surface before coming to rest and exploding, leaving half a continent barren and lifeless (not to mention a new valley and crater). The Underhive population spills forth in search of any goodies (they don't have anyone telling them what to do) and set up junk-towns and raiding bases. The entire area turns into Mad Max/Fallout world where everyone wears bits of tyre, scrap metal and leather, drives around in inexplicably fueled vehicles at high speeds whilst heavy rock and metal music blasts out, their mullets, mohawks and other seriously bad hair-styles streaming behind them.

I like to take the longer view...

I don't believe it SO bad. Fluff have many falled voidship hulks after all. And Orks count it as a safe landing!

Let's not pretend physics actually plays a role in anything orks do. :-D

There's talk in the RT books about a planetbound voidship, but that's with a controlled-ish landing.

I think you should pick whatever fits your narrative best. It's going to be a catastrophic event and probably destroy all life on the planet. I imagine something like a nuclear winter. Even large volcano eruptions can do this.

For a little physics, see here: https://what-if.xkcd.com/20/

Note: Maybe total energy is better. Pure KE from the number posted about 20,000 kg and 64,000 m/s produces 4.096×10^13 joules, which is ~2.5×10^23 GeV. That's a lot of energy. Hard to compare to the What If? since I'm not clear on how many carbon atoms are in a diamond 100 feet in diameter...

And now I'm looking that up. Please hold..... aggghh that's really hard to figure out.

The volume, in cuft: 500000 Pi)/3 ft^3. But the density...

Oh I could use the density of diamond/the mass of a carbon atom.

Eh. I'm going to lunch.

Hm. Volume of the meteor is (4/3)*pi*(100 ft/2)^3 = 14826.7 m^3. Wikipedia has diamond's number density as 176.2 * 10^27 carbon atoms per cubic meter, so it's 2.612 * 10^33 atoms total. But we're probably better off going for the mass density directly, which Wolfram Alpha lists as 3500 kg * m^-3, so a total mass of 51893450 kg = ~5 * 10^7 kg.

You swapped cubic feet into cu meters, which is a factor... 27? error. Unless you converted the first number already.

But yeah, that's not going to be huge. If that's 70 GeV per atom then that's more energy than our ship (which makes sense at 0.99c). But, I really don't have a basis for comparison here.

Injecting reality into 40k is always a huge mistake. Just claim it blows up a hive.

Nah, say it skims the top off of a hive, eradicating a huge portion of the ruling nobility and infrastructure that runs is, before plowing a huge swathe on the planets surface before coming to rest and exploding, leaving half a continent barren and lifeless (not to mention a new valley and crater). The Underhive population spills forth in search of any goodies (they don't have anyone telling them what to do) and set up junk-towns and raiding bases. The entire area turns into Mad Max/Fallout world where everyone wears bits of tyre, scrap metal and leather, drives around in inexplicably fueled vehicles at high speeds whilst heavy rock and metal music blasts out, their mullets, mohawks and other seriously bad hair-styles streaming behind them.

I like to take the longer view...

And I like to play in that kind of game

All you need is a bit of Chaos (and some background music) and you have a winner!