Jon
04/07/10 05:18 AM
24.21.3.118
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Ok, has anyone ever run a game where they battle underwater? My question is, can you use your jump jets or not. I looked through both Maximum Tech and BMR and couldn't find that tidbit anywhere. I know that energy weapons work under water at reduced ranages, and missiles have to be swapped out for torpedos, but no info on jumping...
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Cray
04/07/10 08:49 AM
147.160.136.10
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Ok, has anyone ever run a game where they battle underwater?
A lot of my games have had 'Mechs stray into water on regular maps. A handful of involved extended underwater movement or combat, but the response of most players is to quickly get out of the water if they can.
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My question is, can you use your jump jets or not. I looked through both Maximum Tech and BMR and couldn't find that tidbit anywhere.
"Jump jets underwater" is addressed in the first section of the BMR that addresses jump jets: Movement Modes. In the BattleTech Master Rules, Revised, page 20, under "Jumping":
"Water: Jump jets cannot be fired while submerged in water,
and so a ’Mech standing in Depth 2 or deeper water cannot jump.
If a ’Mech is standing in Depth 1 water, it may not fire jump jets
located in its legs, but it may use any jets located in the torso,
each one providing 1 Jumping MP. For example, a ’Mech with a
Jumping MP of 5 that has one jump jet in each leg and each torso
location may only use 3 MP when jumping out of Depth 1 water"
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I know that energy weapons work under water at reduced ranages, and missiles have to be swapped out for torpedos
Missile-for-torpedo swaps are difficult. Missile launchers cannot fire torpedoes; you need to replace the missile launchers with torpedo launchers, so there's a lot of re-engineering of the 'Mech.
If you want expanded underwater rules, pick up the Steiner Handbook or Tactical Operations. Both considerably expand the rules for underwater operations by adding issues like crush depths and modified breach rules.
I believe Tech Manual (if not, then Tactical Operations) supplies rules for the underwater equivalent of jump jets, underwater maneuvering units.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Zandel_Corrin
04/08/10 04:04 AM
123.2.140.247
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Interesting to note that while under the rules you CAN'T use jets under water you CAN use them in ZERO Gravity and/or ZERO Atmosphere....
I've always found this strange.... how can jets work with no oxygen?
As stated in some of the fluff battlemech jump jets work like mech flamers.... Venting excess plasma from the reactor to achieve lift...
IF this is indeed the case then i see no reason (other than the rule) as to why jets can't be used under water....
and in some fluff they do work... in one tech manual there was a mech that was described as fireing off it's jets breaking clear of the surface of a river and landing in an ammo dump....
Galaxy Commander
Zandel Corrin
Night Dragon Clan
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Cray
04/08/10 08:18 AM
147.160.136.10
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Quote:
Interesting to note that while under the rules you CAN'T use jets under water you CAN use them in ZERO Gravity and/or ZERO Atmosphere....
I've always found this strange.... how can jets work with no oxygen?
As stated in some of the fluff battlemech jump jets work like mech flamers.... Venting excess plasma from the reactor to achieve lift...
You answered your own question about the lack of oxygen. Jump jets are not chemical rockets or jet engines, they're fusion-powered like flamers. See Tech Manual pg38-39 for a detailed explanation.
Now, jump jets have a lot of similarities to the fusion rockets
of aerospace fighters. They use a magnetically shielded reaction
chamber to superheat some material, the reaction mass, and
squirt it out of a rocket nozzle. Like fighter engines, an
electron beam, an electrical arc, is used to superheat the
reaction mass.
However, jump jets do not add plasma vented from the fusion
engine, unlike fi ghters. Aerospace fi ghters are interested
in maximizing their engine effi ciency because they have to
carry all their reaction mass onboard, and thus want the exhaust
as hot as possible. On the other hand, jump jets usually
have an endless supply of reaction mass and don’t need all
the complexity of venting their fusion engine’s plasma. These
jets thus usually capture air as their reaction mass through a
system of turbo-compressors.
This use of atmosphere also contributes to jump jets’ short
burns. Remember what I said about outside air getting inside
an operating fusion engine and causing an explosion?** Jump
jets can be eaten alive by hot oxygen if they keep running
too long on air. This—as well as the possibility that the ’Mech
might be called to action in vacuum—is why most jump jetequipped
BattleMechs also carry a small reserve of reaction
mass, like hydrogen, water or mercury, for jumps. No, it’s still
not enough to get them into orbit.
**See pg34-35 Tech Manual, the "Great Myth" sidebar.
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IF this is indeed the case then i see no reason (other than the rule) as to why jets can't be used under water....
Consider back pressure and the incompressibility of liquids. As noted on pg39 of Tech Manual:
It’s also worth noting that jump jets can’t handle operating
underwater. Lighting up a jet when it’s filled with incompressible
water generates high pressures that rupture even
the toughened jump jet’s casing. Even jump jets trying to
operate on stored reaction mass don’t work right with water
plugging their nozzles.
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and in some fluff they do work... in one tech manual there was a mech that was described as fireing off it's jets breaking clear of the surface of a river and landing in an ammo dump....
The guidelines for fact checking new drafts in BT is that rules always trump fluff. If there's a dispute between the two, then the fluff writer was incorrect (and the fact checkers missed it.)
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Frabby
04/08/10 09:30 AM
79.224.150.19
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Missile-for-torpedo swaps are difficult. Missile launchers cannot fire torpedoes; you need to replace the missile launchers with torpedo launchers, so there's a lot of re-engineering of the 'Mech.
Now that does raise an eyebrow. Given that virtually anything is merrily swapped around 'Mechs otherwise, "omnifying" regular 'Mechs (ever note how any item A from, say, the right torso can easily be swapped for a totally different item B that gets glued onto, say, the left arm, slots and tonnage allowing and ignoring any engineerng concerns?), I would say it is the work of an hour or less to exchange a missile rack for a corresponding torpedo rack.
Ammo feed mechanisms seem to be able to move anything, its opposite or a rock in a pinch in whatever condition from virtually any spot to virtually any other spot on the 'Mech.
In the fluff for the Neptune submarine, the torpedo launchers are explicitly described as navalized versions of proven missile launcher systems.
Edited by Frabby (04/08/10 09:33 AM)
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Cray
04/08/10 11:08 AM
147.160.136.10
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Quote:
Quote:
Missile-for-torpedo swaps are difficult.
Now that does raise an eyebrow.
I meant "difficult" as in "more work than simply loading torpedoes into a missile launcher's ammo bin."
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Given that virtually anything is merrily swapped around 'Mechs otherwise, "omnifying" regular 'Mechs (ever note how any item A from, say, the right torso can easily be swapped for a totally different item B that gets glued onto, say, the left arm, slots and tonnage allowing and ignoring any engineerng concerns?)
What does Strategic Operations' 'Mech modification rules say about the difficulty of swapping missile launchers for torpedo launchers?
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Zandel_Corrin
04/08/10 07:45 PM
123.2.140.247
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Whoa! now this is a surprise... i never expected such an in depth book for this game... hoped for but never expected... will have to get online and order these new books asap.
I can buy that explanation about how and why they work / don't work... I still think it's strange tho that they can get jets working in no atmosphere but can't in water....
might have to house rule that one for fun and see how it goes... maybe cutting down the distance possible with a 'from water' jump might be an idea to account for the water weight resistance...
thanks cray for your time and explanation.... you have sold me on the new books as well as fully described where the rules stand... as a matter of curiosity what is your personal opinion on this issue? Do you think house ruling the use of jets underwater would be over powered or over balanced in the game?
Galaxy Commander
Zandel Corrin
Night Dragon Clan
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Cray
04/08/10 08:37 PM
173.168.115.68
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Quote:
I still think it's strange tho that they can get jets working in no atmosphere but can't in water....
The difference between vacuum and air is minor. Air is a low density, compressible fluid that gets out of the way when you fire a rocket into it. Vacuum is more of the same: a minimal obstruction to the expanding rocket exhaust. Water is quite different: it doesn't compress or move quickly when hit by a sudden explosion.
You can see the differences between water and air in a couple of areas.
First, there are a multitude of rocket engines that work readily from sea level to vacuum. The shuttle's main engines are a standout example, as is the core of the Delta IV heavy. However, very few rocket engines work through a water-to-air transition. Most sub-launched nuclear missiles, for example, are ejected from the submarine and out of the water by a steam blast. (Trident missiles are ejected and kept in a bubble sheath to keep water from ever touching the missile.)
Second, there's a huge difference between air and water explosions, evidenced by the radically different armoring strategies used on real warships to deal with each threat. A proper battleship will have heavy, thick armor plate above and at the water line to stop cannon shells, armor piercing bombs, and explosions. But starting below the water line (and below the depth to which cannon shells might plunge), warships use multiple layers of thin, ductile steel (separated by air gaps and partially filled oil- or water tanks) to act as "pillows" to absorb underwater explosions. Air gives and moves away from an explosion, while water is unwilling to budge quickly, greatly concentrating the force of an explosion. See:
http://www.kbismarck.org/forum/viewtopic.php?f=13&t=1730
When you light up a jump jet underwater, especially if it has any chance to be flooded, then the first ignition of the electric arc is going to blow it apart because the water just won't clear our of the reaction chamber fast enough. Air in the chamber, on the other hand, can be compressed and blown out of the way.
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thanks cray for your time and explanation.... you have sold me on the new books as well as fully described where the rules stand... as a matter of curiosity what is your personal opinion on this issue? Do you think house ruling the use of jets underwater would be over powered or over balanced in the game?
No, I don't think it'd be a problem. I was a little surprised by the rule, personally, since I can think of some perfectly reasonable ways of clearing jump jet reaction chambers to fire them underwater.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Zandel_Corrin
04/11/10 11:21 AM
203.208.78.129
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That's good to hear, i find it strange some times that so far into the future minor things like that stop stuff from working.
Will work on house rule for this sometime in the future.
Galaxy Commander
Zandel Corrin
Night Dragon Clan
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Cray
04/11/10 02:29 PM
173.168.115.68
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Quote:
Will work on house rule for this sometime in the future.
Honestly, the simplest house rule for underwater jump jets is to ignore the water and treat the underwater area just like an air-filled canyon. Unless you're using a particularly exotic map with water over depth 3, water should only be a temporary hindrance to boosting above it and then jumping normally.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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