Dropships are a broad category of spacecraft whose primary purpose is transporting cargo or personnel from orbit to the ground as quickly and as efficiently as possible. Most other orbital vehicles discussed in the Orbital Travel Section are dedicated to getting cargo from the ground into orbit; a drop ship’s main purpose is to move payloads in the opposite direction, from space to the surface.
Orbital re-entry has been completed successfully on hundreds of space missions, both by manned and unmanned vehicles. Even so, it remains the most dangerous part of any trip into space, as the ship has to deal with many extremes as it careens back to the surface.
First and most famously, there’s the heat of re-entry, caused by the extreme air friction the craft encounters as it screams through the outer atmosphere. The heat on the outside of the vessel can reach in excess of 2500 degrees celsius.
The most common way of dealing with this heat so far has been with an ablative heat shield on the surface of the vessel expected to take the brunt of re-entry. These are usually made of heat-resisitant metals and/or ceramics, designed to vaporize away millimeter by millimeter from the heat.
Another tried and true method, first introduced on the Space Shuttle, is to use tiles made of durable ceramic insulators that can withstand extreme temperatures without deforming. These tiles conduct heat very, very slowly, so much so that only the outer layers of the tiles will become superheated during re-entry.
Secondly, there’s the structural stress the vessel undergoes as it enters the atmosphere at a typical re-entry velocity of about 7 km/sec, or roughly 20 times the speed of a bullet. Typical manned missions pull around 2 to 3 g’s of force, and unmanned payloads have been known to take up to 10 g’s and beyond. This combined with the heat and the buffeting the ship takes as it encounters thicker and thicker layers of air can threaten to rip a craft apart.
There’s also the angle of descent to consider. If a spacecraft comes in at too steep an angle, the air friction will become overwhelming and the ship will burn up no matter how well protected it is. If it comes in at too shallow an angle, its great velocity will "bounce" it off the outer layers of atmosphere much like a flat stone off the surface of a pond, sending it careening back out into space. Most spacecraft, therefore, have a fairly narrow "window" of re-entry to adhere to in order to avoid disaster.
Most of a spacecraft's re-entry velocity is usually bled off from blazing through the upper atmsphere, but once it slows down enough that air friction is no longer a major problem, it has to still worry about making it to the ground in one piece. Slowing the vehicle to a soft landing can take several different strategies. By far the most prevalent way of landing is with a parachute or a parasail, deployed only after it reaches the significantly dense parts of the atmosphere. Retro-rockets, so loved by golden-age science-fiction, are also used, but usually in conjunction with a parachute. A third method, pioneered by the Space Shuttle, is to give the spacecraft an aerodynamic shell and wings and allow it to glide back to Earth much like a conventional aircraft.
A few other soft-landing methods have been experimented with, but have yet to see widespread use. Aerobraking shrouds have been used on Mars missions, and are being developed by the ESA and private interests. These shrouds are usually meant to work in concert with retro-rockets or true parachutes, however. The Roton SSTO concept introduced the idea of using helicopter-like blades to soft-land a spaceship, and had three successful in-atmosphere tests.
Most of the space capsules from the Cold War Era were in effect small, one-use drop ships, meant primarily to get their crews back to Earth safely after they had been shot into space.
|CREW RETURN VEHICLE/CREW TRANSPORT VEHICLE|
These were to be small vessels designed to help personnel safely evacuate space faclities in low orbit during an emergency. A prototype, the X-38 Crew Return Vehicle (CRV), was being pioneered by NASA primarily for use as a lifeboat on the International Space Station until budget concerns put the project on indefinite hold in April 2002. The project at the time was 80% complete and had undergone extensive flight-testing, proving the viability of the concept.
The CRV uses a aerodynamically optimized lifting body borrowed from the X-24A project of the 1970s. It also has a rear engine module, with which it uses for a de-orbit burn. When this is exhausted, the engine module is jettsoned and the CRV glides back to Earth unpowered like the Space Shuttle and a steerable parafoil parachute for its final descent phase. Its life-support system was designed to support six passengers for up to nine hours, though de-orbit would take two hours at the most.
|The CRV with engine module attached.|
Though the project is dead at NASA, the European Space Agency (ESA) has expressed interest in using the design as a Crew Transport Vehicle (CTV), basically an updated space capsule if and when that organization ever begins to accomodate manned missions.
|INDIVIDUAL RE-ENTRY MODULE|
Tech Level: 12
|Videll-style IREMs in action. Art (c) Blair Reynolds.|
The Individual Re-Entry Module (IREM) is a bare-bones re-entry system designed primarily as a one-use emergency evacuation measure for facilities in Low Earth Orbit. They appeared in Allen Steele’s novel Orbital Decay and in several Traveller RPG articles, particularly "Rapid Repo" by Greg Videll in Megatraveller Journal #3.
The IREM consists of a hemispherical, foamed, ablative heat shield typically no more than two meters across, a sling to hold the single passenger, a gimballed, manually controlled thruster system for deorbiting, and a parasail for soft landing. Steele’s version was enclosed; Videll’s was open with a barebones framework. In both cases, no provisions were made for life support, as it was assumed that the passenger would be wearing a spacesuit.
Designed more with economy than safety in mind, using an IREM is risky at best. The passenger will typically pull 2.5 to 3.5 g’s for up to ten minutes during descent, and he or she must keep the IREM properly aligned the entire time or risk being incinerated by the heat of re-entry.
Videll’s version came in kit form, complete with an inflatable heat shield mold and a spray canister of ablative heat-resisitant foam, and could be assembled in about 20 minutes or so. He also mentioned that IREMs could be used for military purposes, for dropping personnel onto a planet’s surface more stealthily than a full dropship, and by extreme sports enthusiasts, who would use specialized ablative foams that would produce a variety of colored trails during re-entry.
A true dropship is designed to repeatedly carry large payloads from orbit to the ground. Unlike most orbital craft discussed in other articles in this section, a dropship is optimized for carrying cargo down, instead of up, and works most efficiently in that capacity.
Dropships may or may not be able to achieve orbit on their own once on the ground, depending on their exact design, especially if their means of ascent depends on fuel-intensive technologies such as standard chemical rockets. Getting to orbit takes a much greater expenditure of energy than going from space to the ground. They may need to use a seperate booster, drop tanks, or be towed by a dedicated lift vehicle in order to get back into space. If they can achieve orbit on their own, much of the weight carried as payload down to ground may be taken up by fuel on the way back up.
Dropships are usually depicted in one of three different configurations: capsule, spheroid, or spaceplane.
A drop capsule is very much like a modern-day space capsule, basically a small vessel of either conical or spheroidal configuration designed to carry only a few tons or a small group of personnel to the ground. Usually a drop capsule uses retro rockets or a parasail to initiate a soft landing. Many of the SSTO concepts discussed in that section could be modified to work as a drop capsule.
|A spheroidal dropship is designed to carry large, bulky payloads that may weigh hundreds or even thousands of tons to the ground. They are shaped generally as spheres or ovoids, with a blunt "nose" at one end and a cluster of powerful rocket engines at the other. They enter the atmosphere nose first, where the heaviest heat shielding is located, then flip and use their powerful retro rockets to initiate a soft landing once in the lower atmosphere.||A Fortress-class dropship from the Battletech universe. (c) FASA Corporation|
|A spaceplane dropship is what its name implies; a space plane, either powered by rockets or a combined cycle scramjet/rocket engine, optimized to carry cargo from orbit to a planetary surface. These are capable of gliding to the ground unpowered much like the Space Shuttle, but can also facilitate powered landings if needed because of either adverse conditions or excessive payload weight.||An Avenger-class dropship from the Battletech universe. (c) FASA Corporation.|
Tech Level: 15
|The dropship from the movie Aliens with weapon booms deployed. (c)Twentieth Century Fox|
Military dropships have been featured in many science fiction sources. Dropships can be seen in the novel and movie Starship Troopers, the movie Aliens, in the RPG/Video Game universe of Battletech, in the many various incarnations of the anime series Gundam, and in the recent Playstation 2 game titled, appropriately enough, Dropship.
Military dropships are differentiated from their civilian cousins in several significant ways. First, as it is assumed that they will come under fire, they are likely to be heavily armored and armed. Also because of the danger from enemy fire, military dropships may be expected to undergo heavy maneuvering during the decent phase, even if surrounded by a re-entry corona. Finally, some military dropships may carry heat sinks or refrigeration systems to allow the ship to withstand greater re-entry heat, and thus take advantage of steeper descent angles and quicker orbit-to-ground landing times.
Military dropships by necessity would almost certainly be able to achieve orbit on their own, as they may have to drop their cargo and ascend fast while under fire. Dropship armaments may be in the form of hardened turrets or barbettes, or they may be deployed on retractable arms as per the dropship in the movie Aliens.
Because of their inherent maneuverability, spaceplane configurations would probably predominate military dropship designs, though Spheroidal configurations may be used for unusually bulky payloads or for dropping payloads where there is no significant atmosphere, such as the Moon.
If a major military landing operation is expecting heavy resistance fire from the ground, they may employ multiple dummy dropships to draw fire away from the real ones. These vessels would have the same profile and EM signature as the real ones, but would only be cheap shells outfitted with guidance systems, ballast, and maneuvering engines.
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