The Martian revenge
First the spoiler: this blog is not about any type of the revenge. Instead it is a speculation about upcoming SpaceX Mars Colonial Transporter (MCT). But at the end, maybe there could be a little irony that would explain the post title.
This is a summary of what we "think" we know about MCT:
1. It is going to be an upper stage of BFR
2. It is going to be powered by metholox Raptor engine with vacuum Isp of 380
3. It can be refueled in space (typically LEO)
4. It will be capable of providing deltaV budget of roughly 7km/s
5. It will be (eventually) capable of delivering 100mT to Mars surface or 100 people
6. It will be capable of aerocapture and aerobraking in Mars and Earth athmosphere
7. It will be capable to refuel and launch from Mars surface.
So a rough estimate of its mass is 100mt payload, 150mt dry weight, and around 1200mt fuel. BFR diameter has been speculated between 10m and 15m, with tendency to assume the 15m diameter. It should be reusable 1st stage with mass between 3500-4000mt. Now the core speculation of this post is to challenge belief that MCT will be bi-conic capsule shaped similar to over-sized Dragon capsule. Due to square-cube law, such shape is increasingly inefficient for larger sizes than Dragon especially at aerobraking. It would also be notoriously inefficient for Mars EDL or aerocapture phases. An obvious alternative is variant of bi-conic shape in which the heat shield in on the side of the MCT. This can significantly increase cross-section and thus decrease the balistic coefficient. For example, if a hypothetical MCT would be 60m tall, such "one-sided" reentry profile would increase BC 4-5 times. The main issue is that it is unstable.
Another possibility is to add delta wings (like STS) or lifting body (like DreamChaser) design. There are two crucial differences between STS/DC and MCT which do not seem that suitable for MCT. The first different is that STS/DC are designed for horizontal landing on runways, while MCT shall land vertically. The second important difference is that both STS/DC do not include main propellant tanks inside its frame, which leads to smaller internal volume.Thus the wings are essentially a dead weight without much use through other flight phases except EDL. But there is a third type of shape, which was considered early during the space age but never actually flown. It it known under a fancy name of lenticular vehicle. In the popular culture, it is better known as flying saucer. The advantage is "flying saucer" shape is that it maximizes cross-sectional area used during the EDL phase, greatly reducing thermal and acceleration stress on the vehicle. One side of the "flying saucer" would contain heat shield, landing gear and rocket engine nozzles. The other side can contain solar panels, radiators, docking adapter, antennae and other equipment that would not be directly exposed to hot plasma during reentry. In this design, rocket engines and nozzles would be "inside" the vehicle. This design could also allow low artificial gravity and simple sloshing support by slowly spinning the vehicle.
So what would be advantages of such design?
I have created a rough drawing, in which I used an oval shape (so it looks more acceptable at the first sight).
Main disadvantages of this approach?
This is a summary of what we "think" we know about MCT:
1. It is going to be an upper stage of BFR
2. It is going to be powered by metholox Raptor engine with vacuum Isp of 380
3. It can be refueled in space (typically LEO)
4. It will be capable of providing deltaV budget of roughly 7km/s
5. It will be (eventually) capable of delivering 100mT to Mars surface or 100 people
6. It will be capable of aerocapture and aerobraking in Mars and Earth athmosphere
7. It will be capable to refuel and launch from Mars surface.
So a rough estimate of its mass is 100mt payload, 150mt dry weight, and around 1200mt fuel. BFR diameter has been speculated between 10m and 15m, with tendency to assume the 15m diameter. It should be reusable 1st stage with mass between 3500-4000mt. Now the core speculation of this post is to challenge belief that MCT will be bi-conic capsule shaped similar to over-sized Dragon capsule. Due to square-cube law, such shape is increasingly inefficient for larger sizes than Dragon especially at aerobraking. It would also be notoriously inefficient for Mars EDL or aerocapture phases. An obvious alternative is variant of bi-conic shape in which the heat shield in on the side of the MCT. This can significantly increase cross-section and thus decrease the balistic coefficient. For example, if a hypothetical MCT would be 60m tall, such "one-sided" reentry profile would increase BC 4-5 times. The main issue is that it is unstable.
Another possibility is to add delta wings (like STS) or lifting body (like DreamChaser) design. There are two crucial differences between STS/DC and MCT which do not seem that suitable for MCT. The first different is that STS/DC are designed for horizontal landing on runways, while MCT shall land vertically. The second important difference is that both STS/DC do not include main propellant tanks inside its frame, which leads to smaller internal volume.Thus the wings are essentially a dead weight without much use through other flight phases except EDL. But there is a third type of shape, which was considered early during the space age but never actually flown. It it known under a fancy name of lenticular vehicle. In the popular culture, it is better known as flying saucer. The advantage is "flying saucer" shape is that it maximizes cross-sectional area used during the EDL phase, greatly reducing thermal and acceleration stress on the vehicle. One side of the "flying saucer" would contain heat shield, landing gear and rocket engine nozzles. The other side can contain solar panels, radiators, docking adapter, antennae and other equipment that would not be directly exposed to hot plasma during reentry. In this design, rocket engines and nozzles would be "inside" the vehicle. This design could also allow low artificial gravity and simple sloshing support by slowly spinning the vehicle.
So what would be advantages of such design?
- order of magnitude higher ballistic coefficient compared to bi-conic capsule shape
- simple "artificial gravity support"
- engine nozzles protected from open space environment during cruise stage
- large surface available for radiators and solar panels available in space and on the Martian surface 5. no flip maneuvers during EDL needed.
- simpler design of the escape pod (maybe)
- it would be really, really cool. By far the coolest spaceship ever.
I have created a rough drawing, in which I used an oval shape (so it looks more acceptable at the first sight).
- additional drag losses
- flip maneuver needed after separation from the 1st stage BFR
- unusual shape would require additional modelling and testing in supersonic ascent regime
- it might require some kind of retractable nozzle to maximize vacuum performance
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