sevenperforce

Flying wing designs are very good for low-drag applications, but they're pretty slow. The ways of fixing instabilities in flying wing flight result in increased drag at supersonic speeds; no supersonic flying wing has ever been built. However, with Breaking Ground robotics, we could fix that. One of the coolest ideas NASA has played around with is the bi-directional flying wing: a flying wing that takes off along its short axis but then can fly supersonically along its long axis: Here's a rendering from a NASA study that shows it in supersonic flight: Ostensibly, the engines are mounted at or very near the vehicle center of mass and are able to rotate by up to 90 degrees to switch between the two different flight configurations. I'm guessing it also needs control surfaces that can be switched on and off. Your challenge, should you choose to accept it, is to build and fly an aircraft which is able to take off with a prograde cockpit facing one direction, then transition to flying with a prograde cockpit at a 90 degree different direction. Additional challenges: Reuse Me! Not only can your aircraft take off and transition to stable flight at the new orientation, but it can also transition back and land safely in its original configuration. Look Ma, No Wheels! Build an aircraft which doesn't use reaction wheels or RCS and relies solely on aerodynamic control through all modes of flight. Double Trouble! Instead of using rotating engines, try having completely separate engines for forward and sideways flight. To Infinity! Exceed the speed of sound in your "sideways" flight orientation. And Beyond! Make it to low Kerbin orbit.

It's not (particularly) a problem of interaction between the stabilizers and the wing surface, although that can come into play. Rather, it's more an instance of one thing leading to the other. Because vertical stabilizers are so close to the center of mass, they don't have a very good control moment over yaw. Because they don't have a very good control moment, they must be very large in order to effectively damp yaw. Because they are much larger than you would need on a conventional aircraft, they create much more drag (all by themselves) than the vertical stabilizers of a conventional aircraft. You can. But at this point you basically just have an ordinary plane.

Another note -- we like to yammer a lot about SSTOs, but for science fiction purposes the more useful vehicle is the SSFO -- Single Stage From Orbit. You really want a vehicle which can leave low planetary orbit, re-enter and land, perhaps ferry itself between a couple of destinations, and then return to orbit again, taking off and landing vertically each time but remaining in a horizontal attitude throughout.

The coolest spaceplane I know is the Serenity from Firefly. Unfortunately, having two engines on a swivel each opposite the center of mass gives you massive amounts of roll control, but no pitch control (and yaw control only by differential thrust). So that's not as pretty as you might think. The "ultimate" spaceplane (to me) is one that can re-enter, land vertically in a horizontal attitude, take off vertically in a horizontal attitude, deliver cargo or pick up passengers, fly to somewhere else, then land vertically once again. That vertical takeoff means it doesn't need a prepared runway, and the self-ferry capability is what will make that so useful.

As @Beccab said, yes it does. There are twelve "canted" Draco engines which are used to control pitch/roll/yaw as well as perform short-distance translational maneuvers for docking, and then there are four main Draco engines under the nose cone which perform all major burns. The deorbit burn takes about 10 minutes, if I recall correctly, and expends about 100 m/s worth of dV. When I say that the other 12 engines are "canted" I mean that they point at odd angles. This allows them to be used in pulses to rotate the vehicle around its center of mass. But it also means that firing them together is not an efficient way of giving the spacecraft more forward momentum because even though their thrust CAN balance, it ends up losing efficiency.

SuperDraco doesn't **need** to operate in the atmosphere; it operates just fine in a vacuum. It is just wildly overpowered for use in space. For additional reference, a single SuperDraco is three times as powerful as a RocketLab Rutherford engine. It doesn't operate at particularly low pressures, either. The difference in specific impulse is because the SuperDraco has a truncated nozzle to allow deep throttling since it was originally intended to support hovering landings on Earth or Mars. It does not have a turbopump; it is entirely pressure-fed just like the SuperDraco. Because Dragon doesn't need to carry a bunch of dV, it can afford to have very beefy high-pressure tanks. SuperDraco's increased complexity is partly from the fact that it is regeneratively cooled using the hypergolic fuel, allowing sustained firing (which, again, really wasn't as necessary as originally planned). It is 3D-printed, though, which helps make the regenerative cooling a little simpler. The SuperDracos can still be throttled despite using burst discs for startup, and since they can be throttled they can also be shut down. They have to be shut down because the capsule can still use the remaining propellant for maneuvering/pointing via the Dracos. I believe the difference (with respect to the burst disc) is that the SuperDracos receive the full tank pressure rather than the step-down pressure that the Dracos use. **performs brief "of course I was right" dance**. The forward translational thrusters can't be fired with the Dragon nose cone closed. Obviously. I confess I was taken aback by how large Dragon is compared to Hubble. For some reason I had it in my head that Hubble was bigger (e.g., Skylab sized) but I guess if it was carried in the Shuttle docking bay then this makes sense. You are correct. The docking adapter soft capture ring on Hubble is not at all compatible with the standard forward docking adapter on Dragon. They will need to do a telescoping bespoke adapter in the trunk. There would be a pretty nasty limitation on the physical dimensions of any such trunk airlock. There's just not that much room in the trunk. Doing an extending trunk or fairing would require re-rating the whole vehicle due to the OML changes. And they'd still be left unable to perform a boost burn. Here's a better render of what the reboost would look like in real-time, from an NSF member: The soft capture ring on Hubble can't be used directly for the official "grip" because it's, well, soft. Hubble has hard "towel bars" that can be gripped by a bespoke system for hard capture. However, if the adapter is extensible already, then it's possible that they wouldn't need the towel bars at all. They could extend the adapter, execute soft capture, and then retract the adapter, pulling the base of Hubble flush against the trunk. The trunk handles all gee-loading on Dragon during launch compressively, so the trunk will easily be able to transfer the comparatively much lower forces directly into Hubble's structure.

That is the PAF and upper bulkhead of the second stage propellant tank. Note that while this is the cutaway/schematic for the v1 cargo dragon, which had berthing rather than docking, the dimensions are approximately the same for Dragon 2. The Draco engine is a very small bipropellant engine used for RCS and in-space maneuvers. It has a vacuum-expanded nozzle and gets about 300 seconds of specific impulse in short bursts. With each burst you get a short puff at about 90 pounds of thrust, which is enough to rotate or realign the capsule but otherwise isn't particularly significant. The SuperDracos are much, much larger bipropellant engines. They run on the same propellant as the Dracos but they are intended only for abort and they produce an immense amount of thrust for their size: up to eight short tons of thrust per engine. They are significantly more powerful than the Kestrel engine used on Falcon 1's upper stage. With all eight SuperDracos at full throttle, the abort thrust of Crew Dragon is 30% greater than the full thrust of the SLS Block 1B Exploration Upper Stage and more than half as powerful as the J-2 vacuum engine on the Saturn V third stage that propelled the Apollo astronauts to the moon. Although the SuperDracos and the Dracos both draw from the same propellant tanks, the SuperDracos are single-use and are activated with burst disks rather than standard valves. They also have nozzles sized for sea level so their specific impulse is extremely low. They are not at all suitable for use in space.

So, the reason we get the Perseids and the Leonids and so forth is not because we are in any kind of sync with the comets that created them, but because those comets regularly shed portions of their mass each time they go around the sun, which scatters material around the course of their orbit. Such a thing could be possible for the galaxy. However, it would be on a much larger scale. Rather than a comet shedding little pieces of ice and rock, think about a globular cluster getting twisted and stretched into a long orbiting string of stars by tidal forces over billions of years. That’s more likely. Research “stellar streams” if you want to learn more about what can happen to globular clusters.

One other cool possibility: SpaceX could use a Dragon XL spacecraft with an APAS docking adapter on the tail end and a hatch on the side. The XL could perform all the reboosts using its main forward thrusters and it could also allow Polaris II to dock with it and use it as an airlock to test Hubble servicing.

I don't believe so. Modifications to allow them to be restarted would invalidate human-rating. Also they are less efficient than the Dracos, both in terms of vacuum specific impulse and cosine losses. I think the gee-loading could potentially be within limits, although it would be iffy. The SuperDracos are not only canted out but are also canted at an angle to allow roll control by differential throttling, so you'd have to fire a minimum of four. Even at 20% (minimum throttle), that's 58.4 kN, reduced by cosine losses to 56.4 kN. Crew Dragon masses about 12.5 tonnes and Hubble is 11 tonnes and so that's about 0.25 gees which is probably just on the edge of what Hubble and the docking system can handle. I'm sure that adapting the docking software to allow "back-in parking" is a shorter pole than major hardware redesigns.

Keep in mind that everything in the galaxy is orbiting the galaxy (except the pressure wave arms themselves) and so there's no meaningful astronomical alignment that would happen once per orbit.

Someone over on NSF pointed out that these are probably ellipsoidal caps, not spherical caps, so I recalculated. While the lower (assumed CH4) booster tank volume somewhat intuitively remains the same, the upper (assumed LOX) booster tank volume goes up from 238.3 cubic meters to 255.4 cubic meters, bringing the apparent mixture ratio up from 2.99 to to 3.2 which is still surprisingly fuel-rich but not as severely as before. Booster propellant mass goes up, from 363 tonnes to 382 tonnes, and upper stage prop mass goes up from 84 to 90 tonnes. If the 1.27:1 TWR holds then we are looking at a total vehicle dry mass percentage of 11.1% which makes sense.

So this is a flyback booster with a side-slung expendable upper stage? Kinda giving me Rockwell 1969 vibes but that's cool. Not sure I would want to launch humans on anything side-slung after STS, though.

Yes, but then you have no way to reboost Hubble, because the main thrusters are under the nosecone. Cosine losses on the aft translational thrusters would be prohibitive.

They confirmed that they contemplate a direct docking with the existing soft-capture mechanism, without the need for a grapple arm. Hubble's current velocity at apogee is 7.593 km/s and its perigee is 333.7 miles. To flip the perigee and apogee and bring the new apogee up to the 375 miles mentioned on the call, Hubble's velocity would have to increase to 7.611 km/s. So it only needs a change in velocity of about 18 m/s. Crew Dragon carries 1,388 kg of propellant and its main thrusters get 300 s of specific impulse in vacuum. Crew Dragon has a mass of approximately 12.5 tonnes on orbit and Hubble has a mass of approximately 11 tonnes. It would need 143 kg of propellants to develop 18 m/s of dV, which would be a burn of around 77.7 seconds. My guess, then, is that Crew Dragon (assuming a good insertion by Falcon 9) has enough capability to fully circularize Hubble's orbit at ~375 miles and return to LEO and then deorbit. They just said they are looking at pulling off up to 70 km of reboost which would bring it to almost 380 miles. I'm guessing that using the onboard propellant is the limiting factor, then. If they were docking nose-first and putting new thrusters in the trunk then they could put much more propellant back there. That mode would make it impossible to EVA unless they used the ground access hatch.

Not really, no. You'd have to have a complete redesign. And there's no way to pass through the heat shield so getting back to the trunk would require an EVA.

Something like this? Crewed flights can absolutely carry stuff in the trunk, but it just can't be very large. See, here's where the top of the F9US sits: AND YES they are doing a feasibility study for reboosting Hubble with SpaceX!

Another possibility -- place an extensible adapter in the trunk of Crew Dragon and have it rendezvous tail-first, with crew on EVA available to troubleshoot any problems attaching properly to Hubble. Remember that the main thrusters on Crew Dragon are under the nosecone so if you want to reboost Hubble efficiently you'd want to dock tail-first.

Remember that humans are space orcs. Compared to virtually all other terrestrial life, our resilience, endurance, healing factor, and capacity for accepting diverse sources of energy is utterly unmatched (in the aggregate, not on every individual level).

A quick check shows flights from LAX to Sydney run between 14 and 23 hours (with stops or nonstop). The flight with Starship would be what, under 30 minutes? That's a lot of flight opportunities during the 14+ hour flight the old-fashioned way. Such a system would be hub-based (again, I find the idea incredibly unlikely, I'm just steel-manning the concept). Get people to a distant hub, then they can take a plane, or possibly another rocket I suppose (to aggregate travelers). Dubai likely has ~0 weather scrubs. Most cities would have more weather issues, obviously, but if you wait a few hours to leave, then fly to the opposite side of the world in 30 min, it's still a short trip. I, for one, can't stand long flights. I agree that the market for it is extremely speculative. But we'll see. They're building Starship anyway for orbital purposes, so if the market exists, there's no barrier to access, just to execution.

Yes, that's genuinely very impressive.

It’s based on an image stack which I believe was already linked upthread.

Someone on Twitter overlaid all the images to get the maximum possible resolution, and so I used that to put together an animation of the last five seconds prior to impact in real-time at the blazing speed of ten frames per second (which is at least enough to get a sense of motion). I think I'll do an animation of the full impact but double the speed every five seconds, so it's 1x speed from T-5 to T-0, 2x speed from T-15 to T-5, 4x speed from T-35 to T-15, 8x speed from T-75 to T-35, and so forth. That should be nice for viewing.

It's accurate in the sense of a head-on collision. However, it is flipped relative to ecliptic north. This should be readily apparent because Dimorphos orbits Didymos retrograde to the solar orbit. I'm not qualified to duel numbers with you - but this strikes me as wrong. I googled a bit and I wonder if your number is classic newtonian? "If the impactor has pushed a mass equal to its own mass at this speed, its whole momentum has been transferred to the mass in front of it and the impactor will be stopped. For a cylindrical impactor, by the time it stops, it will have penetrated to a depth that is equal to its own length times its relative density with respect to the target material. This approach is only valid for a narrow range of velocities less than the speed of sound within the target or impactor material. If the impact velocity is greater than the speed of sound within the target or impactor material, impact shock causes the material fracture, and a higher velocities to behave like a gas, causing rapid ejection of target and impactor material and the formation of a crater. The depth of the crater depends on the material properties of impactor and target, as well as the velocity of impact. Typically, greater impact velocity means greater crater depth. You're absolutely correct, but that's why I characterized it as a point impactor. For a hypervelocity impact like this one, the crater depth is not a function of penetration depth, but a function of the energy delivered to the substrate via the impact shock. Basically you can ignore the size and physical characteristics of the impactor and just treat it as an energy source emanating from a single point. To your ballistic examples from earlier...imagine firing a plastic BB at ballistic gel at such terrific speeds that the BB completely disintegrates on impact. Ordinarily, temporary cavity formation is approximately cylindrical because it is formed by the conical shockwave coming off the bullet as it penetrates the gel. Here, however, there is no penetration, and so the cavity formation is hemispheric from the point of impact. It makes sense, though. Sure, there's a massive amount of energy compared to the gravitational binding energy of the moon, but there's no way to transfer that energy uniformly throughout the moon. You could almost liken it to the Liedenfrost effect.

But the system is near periapsis of a more elliptical orbit, so it could still be going faster than Earth. I took exception to this earlier, but having reviewed it in more detail, I was completely wrong. The velocity of Didymos at periapsis is 34.8 km/s, significantly faster than Earth, and DART was not "catching up" to it; rather, it was catching up to DART. In other words, DART's solar-orbital velocity at impact was lower than Didymos's, not higher. This also explains why the view on approach looked the way it did. Per NASA, the images shown are mirrored on the x-axis (due to the design of DRACO's camera) and show the ecliptic north toward the bottom. The actual approach image, if corrected for how we would intuit it should be viewed, would look like this: Dimorphos has a retrograde orbit, so since DART was coming in "against" the orbital direction, it needed to impact Dimorphos while it was on the sunward side of its orbit. This also explains why the right-hand side is illuminated.