OPT-MKII
Using some revised mathamatics, I present the Orbital Propellant Tanker Mark II, with revised dimensioning and some more carefully calculated mass figures:
OPT-MKII stands almost 77 meters tall and is roughly 10.5 meters in diameter.
Changes include:
- Widening of tank diameter to 10.5 meters up from 8.4 meters (the result of an attempt to have pointless commonality with the Shuttle ET). A small increase in width can result in a large decrease in height- improving the ease of vehicle integration, transport to the launch site, and launch pad operations.
- Mass figures are more carefully calculated, including more attention paid to insulation foam.
- Extra engine added (up to six from five) to increase liftoff thrust, decrease gravity losses during the early portion of the ascent, and add non-RCS roll-control later in the ascent.
However, the main difference here is the inclusion of a 1.5 staging system as seen on the Atlas rocket. This makes OPT conceptually similar to the National Launch System, a shuttle-derived concept studied in the '80s by NASA and the USAF. The NLS-2 was a vehicle derived from an STS ET, powered by 6 hydrolox 'STME' engines, derived from the SSME.
Also of note is the Saturn V-B, a 1.5 stage launcher made up of an S-IC derived stage (the S-ID), which could lift over 22 tons to LEO. The S-ID was also intended for use in other, multi-stage Saturn studies to increase payload, and is simulated in the Velcro Saturns addon.
Why drop four engines? Well, the answer is simple: after a certain amount of propellant has been burned, they are simply not needed. A five-engine OPT reaching burnout would experience very high accelerations, perhaps in the range of 10G or more, even with all engines throttled down to the lowest possible setting.
The solution then, to reduce stresses on the vehicle's lightweight structure, would be to shut down some engines. However, these engines would then become dead weight and do nothing but serve to reduce capability unecessarily.
A single RS-68 engine masses 6.6 tons, and there would be multiple uneeded engines. In addition, some of the piping and structure needed to support those engines would also be dead weight.
Ergo, the 1.5 stager is born. Extra mass is added in the seperation system for the discarded 'engine pack', but the mass of this system that remains on the vehicle after seperation is far less than the mass of four engines and their support structure. Disconnecting propellant feed lines are nothing to fear- they have been practiced for 30 years with the Shuttle.
An extra engine can be added. Although this engine will increase the amount of mass the vehicle needs to take to orbit, it will pay for itself by improving performance early in the flight and reducing gravity losses. In addition, it will allow a degree of roll-control after seperation, negating the need for a seperate thruster system to do the same job.
After seperation, the two remaining engines can be throttled down as required to achieve optimum acceleration.
A 1.5 stage vehicle offers improvments over a vehicle using parallel-burning boosters, for example, by offering increased production runs of common components (namely engines), therefore reducing cost and increasing reliability.
Depending on seperation velocity and engine properties, it might be possible to recover and reuse the engine pack. The booster pack of a 1.5 stage vehicle being mainly engines and their support structure, could be easier to recover than a flimsy mass-optimised propellant stage.
The performance gain by seperating unecessary engines can be used to:
1. Increase payload.
2. Ensure a viable mass ratio or make the vehicle structure sturdier.
3. Reduce vehicle size.
4. Two or more of the above.
The common propellant tank, however, is justified by easier handling, construction, and integration, less "wasted mass" in the late portions of ascent, and potential on-orbit applications.
A whole family of vehicles is potentially possible by stretching or "squashing" the propellant tanks. There is no reason not to include conventional payload-launching vehicles in this potential family, as well as derivatives of the Orbital Propellant Tanker concept itself.
The propellant tank is still white. This is in an attempt to slow propellant boiloff on orbit. A payload-launching design would leave the tank unpainted, and a signature orange colour (or grey... or light green... it depends on the insulation used).
Another new addition is the nosecap, which is not actually intrinsic to the MKII design. It is larger, but aerodynamically far superior to the silly nosecap of the first version (see the beginning of this thread).
Comments, suggestions and criticism are welcome.
Using some revised mathamatics, I present the Orbital Propellant Tanker Mark II, with revised dimensioning and some more carefully calculated mass figures:
OPT-MKII stands almost 77 meters tall and is roughly 10.5 meters in diameter.
Changes include:
- Widening of tank diameter to 10.5 meters up from 8.4 meters (the result of an attempt to have pointless commonality with the Shuttle ET). A small increase in width can result in a large decrease in height- improving the ease of vehicle integration, transport to the launch site, and launch pad operations.
- Mass figures are more carefully calculated, including more attention paid to insulation foam.
- Extra engine added (up to six from five) to increase liftoff thrust, decrease gravity losses during the early portion of the ascent, and add non-RCS roll-control later in the ascent.
However, the main difference here is the inclusion of a 1.5 staging system as seen on the Atlas rocket. This makes OPT conceptually similar to the National Launch System, a shuttle-derived concept studied in the '80s by NASA and the USAF. The NLS-2 was a vehicle derived from an STS ET, powered by 6 hydrolox 'STME' engines, derived from the SSME.
Also of note is the Saturn V-B, a 1.5 stage launcher made up of an S-IC derived stage (the S-ID), which could lift over 22 tons to LEO. The S-ID was also intended for use in other, multi-stage Saturn studies to increase payload, and is simulated in the Velcro Saturns addon.
Why drop four engines? Well, the answer is simple: after a certain amount of propellant has been burned, they are simply not needed. A five-engine OPT reaching burnout would experience very high accelerations, perhaps in the range of 10G or more, even with all engines throttled down to the lowest possible setting.
The solution then, to reduce stresses on the vehicle's lightweight structure, would be to shut down some engines. However, these engines would then become dead weight and do nothing but serve to reduce capability unecessarily.
A single RS-68 engine masses 6.6 tons, and there would be multiple uneeded engines. In addition, some of the piping and structure needed to support those engines would also be dead weight.
Ergo, the 1.5 stager is born. Extra mass is added in the seperation system for the discarded 'engine pack', but the mass of this system that remains on the vehicle after seperation is far less than the mass of four engines and their support structure. Disconnecting propellant feed lines are nothing to fear- they have been practiced for 30 years with the Shuttle.
An extra engine can be added. Although this engine will increase the amount of mass the vehicle needs to take to orbit, it will pay for itself by improving performance early in the flight and reducing gravity losses. In addition, it will allow a degree of roll-control after seperation, negating the need for a seperate thruster system to do the same job.
After seperation, the two remaining engines can be throttled down as required to achieve optimum acceleration.
A 1.5 stage vehicle offers improvments over a vehicle using parallel-burning boosters, for example, by offering increased production runs of common components (namely engines), therefore reducing cost and increasing reliability.
Depending on seperation velocity and engine properties, it might be possible to recover and reuse the engine pack. The booster pack of a 1.5 stage vehicle being mainly engines and their support structure, could be easier to recover than a flimsy mass-optimised propellant stage.
The performance gain by seperating unecessary engines can be used to:
1. Increase payload.
2. Ensure a viable mass ratio or make the vehicle structure sturdier.
3. Reduce vehicle size.
4. Two or more of the above.
The common propellant tank, however, is justified by easier handling, construction, and integration, less "wasted mass" in the late portions of ascent, and potential on-orbit applications.
A whole family of vehicles is potentially possible by stretching or "squashing" the propellant tanks. There is no reason not to include conventional payload-launching vehicles in this potential family, as well as derivatives of the Orbital Propellant Tanker concept itself.
The propellant tank is still white. This is in an attempt to slow propellant boiloff on orbit. A payload-launching design would leave the tank unpainted, and a signature orange colour (or grey... or light green... it depends on the insulation used).
Another new addition is the nosecap, which is not actually intrinsic to the MKII design. It is larger, but aerodynamically far superior to the silly nosecap of the first version (see the beginning of this thread).
Comments, suggestions and criticism are welcome.
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