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About sevenperforce

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    Senior Rocket Scientist

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  1. Oh, I know; I was just wondering if DECQ was proposing an identical stack or if he has any suggested improvements. That's GLOW for each version, not payload for each version. F9FT certainly is not putting 550 tonnes into LEO.
  2. There's a splendid blending of graceful awe and stunning complexity with a liquid-flyback-booster stack. Though I think the payload-to-dry-mass ratio is...not great. How would you choose engines/body/fuel types/etc.?
  3. At least until McDonald's can figure out a way to put it into a sandwich.
  4. Not even close. If you divide the total number of deaths attributable to nuclear power to the total amount of energy it has produced, it is orders of magnitude safer than any other major power source. Keep in mind that coal-powered plants operating at minimum pollution levels still release more radioactive material into the environment than nuclear reactors.
  5. With all There are many areas of inquiry in which our feelings about what "matches the evidence" happen (coincidentally) to line up with reality, but that is not how science (or spaceflight) are conducted. If you "feel" that thrusting radially "ought" to increase your apogee, or that thrusting prograde "ought" to get you closer to another ship in front of you, orbital mechanics will correct your feelings rather hastily. The scientific consensus concerning anthropogenic climate change is based on the product of rigorous, peer-reviewed scientific research. Climate change denial might not be as inane as Flat Earth nonsense or young-Earth creationism, but it's every bit as foolhardy as insisting that smoking doesn't cause cancer.
  6. Elon said that the ITS Spaceship and Tanker will both use split body flaps to control roll and pitch during re-entry. It will have plenty of yaw authority from its auxiliary thrusters. One nice thing about using a biconic re-entry is that you do get substantial body lift, which is AOA-dependent. So there's an element of pitch self-correction in the overall design: if your COM is farther back than it should be and it kicks your nose up, your lift increases, which raises your altitude and decreases your drag, allowing you to pitch back forward. Split flaps are best at giving roll authority, perhaps aided by auxiliary thrusters, so hypersonic attitude control shouldn't be too difficult to manage. With my design, it might even be possible to actuate the four underside panels differentially to provide the same sort of attitude control. If not, split flaps on the tail would do the job well enough. The overall aerodynamic design would most likely be tuned to passive re-entry with a crewed-version mass distribution, since that's the one version you are most concerned about re-entering safely. The cargo version would rely more heavily on the split flaps or actuated panels. My mass-fraction numbers may be slightly optimistic, but I don't think so. At least, they're no more optimistic than Elon's structural mass numbers for the ITS system. I filled up about three Excel spreadsheets making sure all the math came out right. For structural mass, I took middle-of-the-road estimates for dry mass on the Raptor engines, adjusted based on TWR for the Vacuum Raptors, deducted total engine mass from the quoted ITS system dry masses, and used that as the structural/tankage mass. I took appropriate square-cube reductions and I added in my engines and my auxiliary thrusters on top of the calculated dry mass for the smaller vehicle. Just shy of 12 km/s. No, really. With the structure alone as the payload and a full tank in LEO, it would have a whopping 11.678 km/s of dV. Of course, actually having positive payload is important. With LEO refueling it could deliver 19.2 tonnes to the lunar surface one-way or a more modest 7.6 tonnes round-trip. That 381 seconds of ISP, along with the really good tankage ratio of composites, can do a lot. I mentioned this before, but the placement of the auxiliary thrusters within the wings means that the ship can land on unprepared surfaces with relative safety, since the thruster wash doesn't impinge on the ground nearly as harshly as with more conventional landing systems. I try not to succumb to Rule of Cool too often, but I couldn't resist this. Mostly because it does genuinely offer some real advantages. And, good grief, who doesn't want to see a sleek spaceship rise straight off the lunar pad on thrusters, rotate gently to orient properly, and then fire up its big engines in the back to blast into orbit? It's exactly how the Millennium Falcon takes off. I set the total auxiliary engine thrust high enough to allow the same vertical takeoff on Mars, which (coincidentally) is precisely what you need for a nice tight landing on Earth. One thing I'm unsure of is the pressurization issues for the auxiliary thrusters. They are autogenously pressurized off the main Raptors, so I'm not sure how much dV they can push before they start to lose tank pressure.
  7. About what I expected. If we could make a tank out of woven carbon nanotubes or straight-up graphene, then yes, high-pressure GH2 would be the way to go. But we can't...not yet, anyway. So until then, we're stuck with liquid hydrogen.
  8. Recovery mode for the second stage is the tricky part.
  9. Liquid mercury, maybe.
  10. Landing on the tail (e.g., ITS Spaceship/Tanker) isn't possible because the smaller vehicle only has the high-thrust vacuum-expanded engines in the tail. You could put thrusters in the tail, but there isn't a lot of space back there, and you run the risk of damage to the engine bells, either from plume impingement or from debris being kicked up. The tail-first landing also requires very large fold-out landing legs to clear the engine bells. Landing on the nose might work a little better, but it isn't suitable for manned launches for several reasons. First, emergency abort during landing isn't possible if you're coming down cabin/capsule-first. Next, there is an increased tip-over risk. Finally, there's no seating arrangement which can support rear g-forces during launch and forward g-forces during landing. Coming down like a modern-scifi spaceship is by far the safest and most stable landing mode.
  11. To the re-entry question, again.... IIRC, the Shuttle's external tank was so lightweight and yet so large that when it hit the atmosphere, it didn't immediately burn up. It would tumble, efficiently dissipating heat due to its high drag coefficient and low mass, until aerodynamic forces broke it up, and then the pieces would burn up. The larger the surface area you can expose, the better. Going in nose-first would produce far more heating and a much nastier deceleration. EDIT: I kept the thrusters behind the landing panels to protect them from re-entry -- I can't imagine that plasma is healthy for engines -- but you never know. There might be a way to have them exposed but not directly in the plasma stream, which would allow them to be used for RCS control as well. EDIT 2: Note that with the first stage, the landing thrusters can be used to give an extra kick to TWR off the pad. They won't have as good of specific impulse as the main engines, but for very large payloads it might be a good tradeoff. EDIT 3: You can also use a larger cluster of dev Raptors on the first stage in place of the two full-size SL Raptors and maybe be able to land on the center one, but they won't have as good of a TWR or isp. I like the idea of using a smaller version of the same engine on the upper stage. It should be noted that the dry mass of the first stage is actually significantly lower than the dry mass of the Falcon 9 first stage, due to the use of a slightly greater diameter to hold more propellant for less mass, and the use of composites. It would be very easy to move and transport and still within road-transport size.
  12. Actually, the biconic entry is far, far more mass-efficient. A larger surface area will encounter drag at a greater altitude and will be able to use hypersonic lift, dramatically reducing g forces and peak heating. Fuel is cheaper, mass-wise.
  13. Thanks! Right now it would cannibalize Falcon 9 terribly, but it might be something they'd look at in the future. I'm really a sucker for horizontal, Star Wars style VTVL, so someone else might not have come up with something like this. Here's an underside render so it makes a little more sense:
  14. If SpaceX's plans for the Mars come to fruition, SpaceX would eventually want to transition Falcon 9 and Falcon Heavy payloads to a Raptor-based architecture, to enable access to space for payloads smaller than the full ITS/BFR/BFS capacity. On the flip side, if the Mars colonization plans never quite pan out, SpaceX will still need a use for their methalox engines. So either way, we need a Raptor-derived fully-reusable TSTO with payloads roughly equivalent to the Falcon family. It will need to be man-rated, too, since there will surely be a need for sending passengers to orbit in numbers lower than the 100+ capacity of the ITS/BFR/BFS system. One problem with this is that a single-engine upper stage has a TWR too high to use for propulsive landing, even if it wasn't overexpanded at sea level. Thus, it needs auxiliary landing thrusters. Another problem is re-entry; an unmanned stage can come back using a heat shield on its nose, but that's not much fun for passengers, and I have a strong preference for a "true" TSTO where the crew cabin is integrated. You need biconic re-entry a la ITS. But if you're already using auxiliary thrusters and biconic re-entry, you don't necessarily have to align the aux thrusters with the main engine vector. That's where things get...interesting. Here is a line drawing and a very very rough sketch of my concept: Looks a bit like the ITS, doesn't it? Same basic principle (composite monocoque tanks, etc), except the diameter is only four meters, making it roughly the same size as the Falcon 9 but slightly wider. The first stage has two full-size SL Raptor engines for launch and boostback and six methalox hot gas SL thrusters for landing, along with four landing legs: The first stage has a dry mass of 17 tonnes and a propellant capacity of 421 tonnes; it delivers the upper stage at a notional staging velocity between 1.5 and 2.5 km/s and executes a boostback RTLS landing. Minimum initial TWR for the boostback burn is 2.7:1 with both Raptors at minimum throttle; maximum landing TWR on thrusters alone is 3:1 but it can easily hover. I've factored in the masses of the thrusters and everything else. The upper stage is where it gets really interesting. Rather than using Raptor engines, which would be way oversized, it uses a pair of the Raptor Development engines (1,000 kN SL thrust) with vacuum nozzle extensions. I'm estimating their mass at 638 kg each. Total stage vacuum thrust is 2,292 kN. Dry mass is 6.6 tonnes and propellant capacity is 141 tonnes. Because the vacuum engines cannot be used at sea level, I gave the upper stage eight SL-expanded methalox thrusters in addition to its vacuum-optimized RCS thrusters, with a combined SL thrust of 688 kN. But I didn't want to cluster them around the devRaptors in the tail, both for space considerations and because of damage to the engine bells. See the wing extensions shown in the above line drawing? The landing thrusters are placed underneath the wing extensions, pointing down. For re-entry, the upper stage enters biconically, on its belly. It then glides/falls to the landing site before hydraulically-actuated panels open up underneath the wing extensions, both exposing the landing thrusters and providing rear "legs" for the vehicle to land on, so it lands vertically but in a horizontal attitude, eliminating the risk of tip-over. The landing would look like something out of Star Wars, because it drops, winglets open, and it lands on the wingtips with rocket propulsion. Based on my simulations using this calculator, the launch system could deliver up to 6.8 tonnes to GTO with full reuse or up to 24 tonnes to LEO with full reuse. For LEO launches, the upper stage can also recover up to 30 tonnes of downmass. This is obviously plenty of margin to have a crewed version, which would use the same tank and body as the rest of the orbiter but have a crew cabin in place of the cargo bay. Payload capacity is high enough that the crew cabin could carry at least a dozen crew members plus unpressurized cargo and still have independent LES and re-entry capability (lifeboat). Know what else is great? Due to the vertically-oriented thrusters, the upper stage could both land on and take off from the Moon or from the surface of Mars without needing a launch pad. On Mars, it would need to be refueled on the Martian surface; the lower gravity means that the thrusters have enough thrust to lift it off the ground so the main engines could be fired up. For lunar missions, simply being refueled once in LEO would give it ample dV to fly to the moon, land, SSTO, and return to LEO.
  15. Yes, that's entirely possible. But my point was that collecting propellant on Mars via ISRU and transferring it back to Earth orbit in a tanker might be more efficient than simply launching it on Earth.