Say we have a large cargo spaceship and a small pirate spaceship. The pirate ship is the only one with any sort of weaponry (lasers, missiles etc.), but for the pirates to safely retrieve the cargo, they decide to board the ship and take it manually since teleportation is impossible.
So, how could a pirate board the cargo ship, without letting the precious cargo fly off into space, if a gaping hole in the side of the cargo bay would certainly mean the cargo would get ejected into space?
One way would be to use suction pads.
The problem is that suction pads use air pressure in order to work, so they do not work in a vacuum. That means that you cannot attach a suction pad on the outside of a spaceship. However you can attach it inside, in order to create an airlock. Here is how it would work:
Before creating an airlock, you need to be firmly attached to the ship. That can be made thanks to a sort of harpoon. Once the harpoon has gone through the hull of the big ship, it opens up and liberates a plastic circle, which mill make your suction pad. What happens is that when the air inside the ship will try to escape, it will actually push the membrane, so that it will cover the hole. Then the difference of pressure between the air inside the ship and the void outside will keep the membrane in place.
With enough harpoons, you can get a good grip on the big ship. Now to create the air-lock. It is the same principle. You project a big tube towards the hull of the big ship. Once you are through, you deploy membranes that will prevent the air from escaping.
Edit: The basic shape of the harpoons would be something close to umbrellas, which would open once inside the ship. The "inside" of the umbrella would at pressure zero thanks to the hole you just made, and the "outside" would be at room pressure. The ribs can also help to reinforce the harpoon, much as the barbs of real-life harpoons.
Concerning the air lock, the things you deploy have something like a corona-shape.
Two more things I did not mention:
The canonical solution in scifi is as mentioned by others threaten to open fire unless the ship allows you to board. This is not only practical (unless you have a reputation for killing surrendered crews), but low risk. It is also financially efficient as it keeps the ship and cargo value intact and the crew alive for ransom. It is also generally best solution for the owner as it only causes losses that can be covered by insurance. And paying insurance is generally cheaper than arming the ship so it can defend itself and paying the crew for fighting. Exception is if the pirates have a reputation for killing crews or the piracy is actually part of some sort of an ongoing larger conflict.
But that is rare as pirates generally hate fighting, there is no profit. Everything you spend on fighting reduces profit and increases risk. So most pirates try to keep surrender the best option by not killing surrendered people unless necessary. Incompetence and stupidity does exist though. As do insane people and religious fanatics.
In addition to being common in scifi this is how most real pirates have operated. Fighting is only done until one side thinks fighting is more dangerous than giving up. Usually which side is stronger gets obvious without unnecessary bloodshed.
Your question already assumes that the cargo ship is weaponless, and we'll also assume for the sake of simplicity that the pirates have already taken care of any escorts that might be around, so they aren't in any danger of getting blown out of the figurative water. The problem now is - they need to get the precious cargo off the cargo ship. For that, I see three possible ways to enter the ship:
Boarding Through Airlocks
The most obvious way is to simply slip spacesuits on and pop open an airlock from the outside. Any space-faring vessel should have some type of de-pressurizing zone the pirates can use to their advantage to access and enter the ship.
Depending on your priates' style, this can be done either after they've done precision damage to render the ship immobile, or mid-flight and stealthily to avoid any detection whatsoever.
Make-Your-Own-Airlock
If for some reason the ship doesn't have an entryway, they could make one on their own. Simply give your pirates an attachment to the ship that manually digs into the hull, preferably 'locking-on' like a claw, that allows the pirates to connect one ship to another for easy boarding.
It doesn't matter if they then leave a huge hole in the ship's hull after they leave - they're pirates. They got the cargo. Let the poor saps with a hole in their ship worry about that.
Just Use The Vacuum
The pirates are trying to get the cargo out of the cargo carrier. For them, letting the vacuum of space do the work for them. From there, it's just a matter of snatching the cargo out of space - either by hand with space suits, or just by flying by and scooping it into the ship through a de-pressurized 'mouth' piece, ideal for taking cargo out of the motionless vacuum of space. (Or Tractor Beams, if you want to have those).
Match speed and course.
Aim.
Fire all starboard harpoons!
Okay, maybe harpoons are bad idea... but they sure are cool so mount them anyway.
Everything depends on cargo ship design.
Cargo spaceship will consists of three main parts:
For commercial scale transit cargo hold would be big enough to make it main hull, while engines and crew compartment would be just an attachment on top of it. In pair with it comes space elevator and/or surface-orbit cargo shuttles. Launching single monstrosity between planets would be cheaper but in same time making in unable to perform planetary landing and having low thrust-to-mass ratio. Even entering parking orbit would be waste of money (both fuel and time equals money), so instead they would just come close enough for rendezvous with local high thrust-to-mass (which implies short range) haulers. In space you do not even need to dock, just drop cargo container with radio beacon for locals to detect and pick up while cargo ship uses its own hauler/tractor beams to pick up new containers.
So now we have giant cargo ship plowing slowly but without stop through space as parking this behemoth would just cost too much. As it is capable of transferring cargo containers in-flight all you need is to convince its crew to push a button, automatic storage of fixed-size containers is quite easy (when compared to space ships) in 0g environment. Or you can just convince their computers.
And here where gamma-ray lasers come in handy. Talking is bothersome, lasers are accurate.
So what if cargo ship refuses to comply?
In case sailors are suicidal:
It would create certain culture of piracy:
Currently most spaceships are expected to be made from metal. Most likely one that has magnetic properties (at least enough for this purpose).
Large electromagnets on cables can attach the pirate ship, and then it can have a tube with another electromagnet that is big enough to cut an entry port with, like a lamprey eel. The smaller cables could have smaller versions that could poke weapons through the hull to protect the door that is cut through the hull for the boarders.
They can promise that if there is no funny business, they will take what they want and leave, closing the holes behind them else...
I'd like to suggest something different. Given the huge success of shipping containers here on Earth in the last 50 years (http://www.amazon.com/The-Box-Shipping-Container-Smaller/dp/0691136408) I would suggest that a similar cargo handing system would also be more efficient for space travel. Moreover, as at least one of the other answers suggested regarding the cargo hold, each container would be individually powered, warmed, pressurised as required by it's contents.
On a large cargo vessel in space, moving the containers in and out would be an unnecessary waste of time and effort. Instead they could be attached externally to the ship, only requiring the connection of an umbilical for power, air etc as mentioned above, and sufficient clamping/bracing to cope with the expected acceleration/deceleration of the trip. So, I would expect some sort of modular racking system external to the ship's hull.
Given this situation, pirates would only have to draw near, and send over ROVs to detach and eject the containers, which the pirate ship could then collect. The ROVs could perhaps be adapted versions of the robots used to load/unload the cargo at space ports. Or, if it suits your story better, a giant articulated arm on the pirate ship could detach containers and move them over to attach them to the pirate ship. Or the cargo vessel's computers are hacked to order the release of the containers. Another angle could be that the containers are stripped off the vessel, to be collected later by the pirates.
I would personally favour a boarding pod. Essentially, you fill a torpedo-like pod with your finest pirates and fire it at the cargo ship. When the torpedo smashes into the side of the cargo ship, the impact activates expanding foam (like the polyurethane foams available to us right now) around the exterior of the boarding pod which seals the breach, while armoured pirates burst out of the doors at the front of the pod. Either the crew surrenders, or is gunned down, leaving the pirates to bring it back to space-Tortuga or drop the desired cargo into their primary vessel by whatever means they see fit to employ.
If the pirates miss, they can waste the cargo ship and collect the boarding pod for the next attack.
This has the benefit of being (aside from the assumptions of casual space flight) completely within our current technological reach.
Assumptions
Learn from the pro!
Similar to Ville Niemi's answer, but more detail on actual combat.
Demand the vessel surrender. If they do, board through whatever air-locks the ship uses while in port. This should be fairly standard, as air-locks are required everywhere.
If they resist, open fire. Use a weapon designed to penetrate, not explode (ie, something high density, high velocity). Aim for the engineering spaces, based either on known ship layout, or thermal imaging - power plants give off heat.
This should result in a minimum amount of structural damage, and a maximum amount of on-board casualties. Contrary to what TV and movies would have you think, it's pretty difficult to make things explode. It is easy to make them break. Damaging the engineering spaces is likely to release super-heated steam, high pressure hydraulic-oil, and potentially radiation, killing many of the crew members in that area. It will also shut off the source of electrical power. Without power, there will be no atmosphere control or temperature control.
Over the next few hours, the remaining crew will die of CO2 poisoning, hypothermia, etc. Space suits will only provide a few more hours of heat and oxygen. Wait as long as you can afford - days if the ship is far from potential assistance - and then board via the standard airlock.
It's likely that after a few of these encounters unarmed ships will simply surrender when the demand is given. They may also begin arming themselves and traveling in convoys for protection.
As a pirate, I would want to have a mechanism whereby I could administer the 'lamprey treatment'.
Might require a delicate touch from the operative to be effective beyond entry.
Cargo would be strapped down for the long time in zero-G in a vessel where position of center of mass is critical for precise maneuvering. It's also possible that the hold of a large freighter would be unpressurized.
Also a sudden hole in the side of the craft doesn't mean that everything is jettisoned, the gas inside will rush out but the heavier objects take time to get moving. A quick vent will also have the air move around the slow object instead of pushing it out.
First of all, the cargo and the crew will have entered the ship somehow. That means that there is a door.
There have been quite some movies that sported some kind of fabric covered tunnel that can be secured on the side of the vessel to be boarded.
Since your pirates will plan to do such boardings more or less frequently (it being their profession, after all), they will have a device like that.
Now all they need to do is open the door, by the handle, if possible, or with some force, and they are good to go.
Of course there would also be the possibility to take over the cargo vessel completely. That would save quite some time dragging cargo around, plus they had a nice ship to sell.
Tractor beam as used repeatedly in Star Trek. Pull the ship in close were you make some impromptu passages, or pull the ship into a cargo bay and lay siege.
A tractor beam is simply a sci-fi version of grappling hooks. Grappling hooks were standard fare of pirates.
A version of the Roman Corvus on the pirate vessel would be needed.
The ship would have a set of grappling arms to attach itself to the cargo ship, and the Corvus would provide a cutting head to breach the hull and a pressurized tunnel into the interior of the "enemy ship". (think of an extendable pressure tunnel with a cutting head on one end).
Once the Corvus had drilled/burned/lasered its way through the cargo ship's hull, the pirate ship could increase the air pressure on board to blow the hull section away and waft the boarding party onto the cargo ship. Once they collected the booty, lowering the air pressure in the pirate vessel brings the boarding party back. Once the inner airlock of the Corvus is secure, the cutting head is retracted (breaking the seal between the Corvus itself and the cargo ship) and the grappling legs are released, "blowing" the pirate ship clear of the cargo ship, and giving the cargo ship crew something more important to think about than giving chase.
As an incidental, the Marines or police would probably use something similar to send their boarding parties aboard pirate vessels.
Why is the cargo hold pressurised in the first place?
It would prevent explosive decompression if it were not, and would be cheaper to build the ship. You may have a pressurised hold for carrying animals or whatever needed it, or they might just have pressurised cargo containers in an unpressurised hold (the same way we have refridgerated containers on cargo ships).
If i was in a space battle, i would depressurise the ship and use space suits. Nothing would catch fire and no-one would get sucked out.
I would simply develop a virus to inject inside the target vessel (maybe by communication, or maybe with a little drone witch goes to connect to some terminal outside the ship, or even cut its way to the wires) and then just board as you would with a normal ship; just do it before the attacked vessel as the time to restore the system.
Obviously this mean you KNOW the system and the blueprint of the target ship, so you would be very selective, or there should be some sort of "pirate hackers" that sell that virus/drones.
The pirates would use the deadliest weapon known with application in space. This is, of course, the can opener. For ease of application, large can openers are preferred. Spacecraft will be made of thin materials, to save on mass and to thereby reduce the energy expenditure at propelling any spacecraft up to cruising velocity.
This also means any gunfire will have bullets punching through internal walls and exiting via the outer hull. Very rapidly the ship will lose air. Space pirates only need to match velocities, pull alongside and knock on the airlock door, waving their can openers and hand guns. The threat would be immediately apparent: Open up or we will open you up and vent all your air.
Now there is one problem with the OP's scenario. The cargo ship is large, obviously carrying a lot of valuable cargo, ripe for the picking by space pirates, but the space pirates themselves are flying in a small spaceship. The whole object of piracy is economic, i.e., to make money by stealing cargo from otherwise helpless vessels. Unless a small amount of that rich cargo is exorbitantly valuable, then this act of space piracy isn't economically viable. Take your can openers and go home.
Except the space pirates will probably do real pirates do. namely, force the cargo vessel to change course and head to a location where the cargo can be transshipped and redistributed to destinations where the space pirates sell it and make a handsome profit. Otherwise you're out of the space piracy business.
This is glamorous version of space piracy. A more 'realistic' version would be that the cargo freighters loaded to the jets with rich bounty, will be robot vessels. There's no need for a human crew. Besides humans take up a lot of space and they need roughly a ton of life support per person. A waste of good propellent and energy to dispatching a manned vessel across space. Also, humans will have to be paid. Robot freighters are so obviously the way to go.
Do the space pirates turn up threatening the computer systems to heave to and prepare to be boarded? Of course, the space pirates will have the same problems. So the most likely scenario is a space pirate vessel, entirely computer controlled, matches velocities and dispatches a robot to board the freighter and reprogram its navigation systems to change course for the pirate base.
Piracy is a business and will be run like a business. The Galactic Jolly Roger Corporation will continue launching its pirate drones to intercept innocent cargo freighters and then redistributing the pirated plunder to the outer colonies until such time as they can no longer turn a profit. The company would then be wound up and taken off the stock exchange of Capella VII.
Actually the simplest, and also the most profitable way of doing space piracy would be to hack the navigation and control systems of a cargo freighter. When the right moment happened during its voyage this 'hack' would take control and steer the vessel to the pirate base.
Sorry, no glamour, no piratical hi-jinks, and no Yo Ho Ho. Just the cool headed mechanics of commerce, astrodynamics, and robot vessels with a dash of cyber-crime.
Patch-able Airlock The problem of using an external airlock to board the cargo ship, is that once you disembark you have 2 options:
Solution Having deployed your airlock (probably using a grappling system as earlier answers have postulated) and stolen all the precious loot of the cargo ship, patch the hole you've left behind! Even if its just a canvas patch (as used in The Martian ) to prevent the cargo ship depressurizing. This reduces the risk to cargo ships overall and would reduce the chance of them carrying weapons.
Hope this comes close to an answer, this is my first time posting to worldbuilding, so any advice would be appreciated :)
They could coerce the crew of the transport ship to let them board via threats and intimidation, or by good old-fashioned lies.
First think would be to stop the ship. Eather by threats, intimidation, promises of no harm to their crew or by force. If it comes to violance primary targets would be the engines and communication. Once the ship is crippled and silenced, boarding and plundering will be piece of cake.
Hey Samuel so there's a number of ways to go about robbing a merchant ship blind of all their goods. the first way is to use electromagnetic weapons like an emp gun/torpedo etc to shut down a ships systems whether fully or temporarily it depending on how good the technology of the time period and then having a boarding party board the ship through its airlock armed to the teeth of course and then either imprison the crew or dispatch them so the pirates can take all the cargo or the whole ship. the second way would be something like having a smaller boarding ship with a drill like piece in the front and ramming it into the merchant ship and then the drill would open depositing the pirates inside basically making a hole and plugging it with a ship. third way just shooting their engines and either going through the airlock or using a breaching cube which basically is a pressurized cube that attaches to the ship using magnets and your crew is in the cube you cut into the ship no depressurization you stack all the cargo in the cube and the part that connected to the ships hull you cut seals shut and the cube detaches for your pirate ship to pick up leaving the empty cargo hold depressurized and open to space. by the way what do you need this for? writing a sci fi book or something? :)
Ramming speed!
If it's a private merchant ship, then its hull will likely be thin and made of a cheap material like aluminum. Have a pirate ship with a nose like a brass paper fastener. e.g.
http://cdn.www.officedepot.com/pictures/us/od/sk/lg/909432_sk_lg.jpg
Pierce straight in, then separate, making a hole. Then send in guys with suits to tie cables to whatever you want to take, hoist them back to the ship, close the cutter, and leave. Yeah, the interior will depressurize, but so what? If you are worried about any cargo leaving the hole, have a funnel around the cutter to collect it. In and out before the Space-Police arrives.
Magnetic grappling hooks, disable the engines and dock, shuttles that adhere to the hull and then cut through, infiltrate the crew, wormhole (which technically isn't teleportation, hack a bio printer and print clones, hack a 3d printer and print robots, hack a robot that is already built, hack the ship and get control, psychic control of a crew member, blackmail a crew member or some other form of traditional coercion, pretend to be dead in space and trick someone into docking, grappling hooks that 'cold vacuum weld' to the hull (it's real google it), psychic hallucinations that trick the crew (like psirens in Red Dwarf), disable the engines and starve out the crew.
