This is a post about an idea by Doug Plata. His idea is to put multiple propellant tanks on top with just enough structure to keep them intact and drop them in pairs as they drain. Under the tanks is the payload. Under the payload is a plug nozzle/heatshield that multiple engines expand against for altitude compensation. The configuration is chosen to be fault tolerant of both engines and tanks. This allows the use of somewhat questionable engines and tanks. The concept is specifically for the purpose of launching very large payloads on vehicles with relatively low development costs.
From the top down, this concept starts with a composite aerodynamic fairing that is also a fuel tank for kerosene. Next is a cluster of 19 tanks each of which holds either fuel or oxidizer with no common bulkheads. Next comes the payload volume which is the full diameter of the 19 tanks and whatever length required for the particular mission, with cylindrical sections added or removed as required. Under the payload volume is a full diameter plug nozzle that is also a heatshield for reentry. Against the sides of the plug nozzle are multiple engines of relatively low expansion ratio with the plug nozzle making up the difference at all altitudes.
The cartoon is a rough representation of Dougs’ concept. The payload can be a habitat cylinder 15 meters in diameter and 30-45 meters long. By launching it dry, all of the permanent fittings can be installed and tested on the ground as well as some of the transient components that are used early on.
The habitat is conceived as useful for an all up station in one go without the fitting problems of an expandable structure that is volume constrained at launch. It is also possible to design it in such a way that lunar mass could be added for radiation shielding in Lunar orbit or one of the L points. The rigid structure is also suitable for the Lunar surface with the capability of handling a thick regolith covering
The aerodynamic fairing that is also a fuel tank is handles aerodynamic loads only with both the fuel and pressurant gas providing support through the atmospheric portion of the flight. It is expected that mass of the shroud/tank will be on the order of 2% of the mass of fuel it holds. The shroud tank is sized to empty as the vehicle reaches low dynamic pressure at altitude when it is jettisoned.
The 19 tanks under the shroud/tank are protected from head on pressure while in the atmosphere. As the shroud tank is jettisoned, the empty oxygen tanks in the 1 ring are sent off as well leaving only full tanks to carry. As each pair of either oxygen or kerosene tanks are drained on opposite side of the vehicle, the empties are kicked off from the 1 ring, then the 2 ring. The pressurant gasses in each tank are used as a cold gas thruster to ensure clean separation. There are as many as 6 tank staging events as the vehicle climbs out. Each tank being 3 meters diameter by 50 or more meters long, available propellant volumes are at roughly 350 cubic meters per tank. Tank mass can be on the order of 1% of propellant mass. Along with the shroud/tank, propellant mass can be on the order of 7,000 tons.
At the plug nozzle/heat shield, there are as many engines as required by a given mission. Since the concept is for launching massive one off missions, engine mounting must be modular similar to the tank concepts. Shrapnel shields and other safeguards must be designed in as the concept is for very low flight number vehicles with inherent infant mortality. Extra engines are a requirement for a couple of reasons. One is that it allows a more efficient flight profile than the normal thrust limited take offs of most launch vehicles. The other, more important reason is that available engines will be used with variable reliability and availability. Careful attention to this detail should make it possible to use the remaining AJ-26 inventory of orbital-ATK as well as used Merlins and anything else the contractor can get his hands on. Unreliable engines can be compensated by having fail safe ways of shutting them down and jettisoning them. This approach allows buying engines from motivated sellers.
An additional advantage of Dougs’ concept is that it allows the multiple engines to use a variety of propellants. A mixture of kerosene, methane and hydrogen engines is quite possible in this configuration.
As the vehicle sheds propellant and tank mass, acceleration will rise. As it reaches the maximum desired, pairs of engines will be dropped in a manner similar to the way the tanks are treated. Each engine can be fitted with a decelerator and parachute as long as the velocity is low enough that there is a reasonable expectation of recovery. At higher velocities, expended engines will be lost as they are dropped. The ones that make it to orbit can be packaged into the heat shield for return to the ground.
The plug nozzle/heat shield has a multiple use as a heat shield in addition to its’ nozzle duties. In case of abort, engines and tanks are jettisoned and the heat shield is used to protect the payload for a return to sea level. The idea is that the nozzle and vehicle shell may be lost, but the interior equipment could be saved for use on a replacement mission. For returning the payload from orbit, or atmospheric entry to another planet, the heat shield works in the normal manner. On a nominal mission where the payload is not returning to Earth, the heat shield is used to return the remaining engines and any other valuable gear that needs to return to the ground.
Figuring the payload to orbit is interesting. It turns out that with so many small staging events, the dropped tanks and engines can be treated as propellant for calculation purposes. With ~7,000 tons of Kero/LOX propellant, it is possible to place a 350 ton space station in orbit with well over 6,000 cubic meters of living and work space. This is moreover, work space that doesn’t have to be cramped into tiny cubicals and corridors.