sevenperforce

Heavy breathing, Heavy Falcon.

I see what you did there.

Can't take it!

AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH

One big advantage of using the Falcon US as a crasher for the lander is that you can replace the trunk-borne propulsion unit with payload for one-way cargo missions. Crasher stage to the surface, use the SuperDracos to land, and jettison your cargo. Could be consumables, a rover, a BEAM unit, whatever. The idea of using a drop-in propulsion unit attached to the payload adapter in the trunk has been suggested before -- it's been called a JPAP, or jettisonable propulsion assist pallet. One challenge to using a Draco-based system is that pressure-fed tanks tend to have poor mass ratios. An electrical turbopump might work, but then your dev costs skyrocket.

Let's see. I count two cameras, definitely: the one at the very top right, and the one to the Starman's left. I also see a pair of structures the same size as cameras (but probably not?) in mirror symmetry under the Tesla's rear wheels, and some sort of object hanging from the right of the rear bumper. Finally, there's an odd-looking disc and some sort of additional apparatus just above the front grille, with three lines running to it, but no idea what it is. Safe to say the Roadster will not be decoupled from the PAF.

The debris cloud was supposedly formed by a Russian missile strike on a defunct satellite.

Yeah, I was already assuming a hypergolic Draco-derived kick stage for the homeward journey.

**likes** TWR for the core would be higher than a single Merlin on the boosters at sep. However, as I said, I do not believe there is a plan to keep a single Merlin firing through separation. I'm sure each booster is programmed to follow its own trajectory, which keeps it far from the other one.

The flame will diffuse, yes, but not so much that welding would be impossible. An oxyacetylene torch will still deliver a nice high-energy plume in a vacuum, albeit a rather underexpanded one.

Another option, one with even more minimal modifications, would be to keep two of the standard eight SuperDracos on the modified Dragon 2 lander for the hover, landing, and initial takeoff, and use a lower-thrust Draco-based kick stage for the actual burn to orbit. This would require, in theory, zero engine modifications. Using the canted SuperDracos for landing and takeoff would prevent debris impingement, and their high thrust reduces the dry mass of the kick stage. The low isp of the SuperDracos isn't a problem because they are only delivering a few hundred m/s of dV, while the bulk of the impulse is provided by the much higher-efficiency Dracos. You'd still need landing legs on the trunk, and modifications for egress and ingress, but everything else remains virtually untouched apart from removing the ballast sled, the heat shield, the aeroshell, and the chutes.

Hmm. The vacuum-expanded Rutherford dials in at 343 s isp and 24 kN. LOX boil-off would still be a problem, but if that could be solved, a Rutherford-based drop-in kick stage for both the command-module Dragon and the lander would be just about the right size. Kerolox doesn't have quite the bulk density of hypergolics, but it still might work. And the kick stage could be tested on any Dragon 1 mission.

And it's a very intuitive thing, once you learn about it. My three-year-old and five-year-old can explain basic physical chemistry based on valence electron arrangement.

The pendulum fallacy is about guidance stability, not about structural stability. It's a fallacy to think that placing the engines at the top of a rocket will help keep it pointed "up", because both the thrust vector and the force of gravity act on the rocket as a whole and not differentially. Structural stability is a different matter. Placing the engines at the front of a very long vehicle with a high fineness ratio (i.e., very thin) improves structural stability because elements are in tension, rather than compression, and bending moments are damped by tension but multiplied by compression. You need a high level of rigidity if you want to push a long, thin object; you need little or no rigidity at all if you want to pull.

Still flying the mission.

One problem in going with a single SuperDraco rather than clusters of smaller engines is that your vacuum nozzle ends up taking a substantial amount of the volume in the trunk, which is a problem if you want a drop-in kick/landing stage. It also exposes your engine to debris impingement on landing, which is bad if you want to use your landing engine for ascent. If the Dracos got larger nozzles to bump up their isp and thrust, then you could get up to 320 s and 427 N, which would require 40 Dracos to get a local TWR of 1.6. Not the MOST efficient ascent, but closer to doable. That's only twice as many Dracos as a standard Dragon 1 carries.

The planned Altair lander ascent stage would have had a local TWR of 2.54. The Apollo ascent module had a local TWR of 2.1. Erring on the larger side, and assuming a 6.7-tonne ascent vehicle delivered to just above the surface by Falcon Heavy, that means you'd need 27.6 kN of thrust on ascent. You'd need almost 70 Dracos for that. Or, you know, half a SuperDraco. The SuperDraco would need a vacuum-expanded nozzle to kick up the isp, which would further increase the thrust. It can throttle deeply, but I don't know how deeply. Anyone have a guess as to the mass ratio of pressure-fed hypergolic tanks?

Star Wars is space fantasy, full stop. Last night, I watched Downsizing. Horrible movie. Really interesting premise, and I was willing to suspend disbelief about the science of it if they explored the premise, but they didn't; instead it just took a nosedive. In any case, shrinking humans to five inches high would make said humans virtually indestructible on their new scales. Able to jump dozens of times their body length, immensely strong, and so forth. However, stuff like the surface tension of water would become a problem. A slight summer afternoon sprinkle would kill you. On the flip side, I believe we have already roasted Ant-Man to oblivion on this forum.

Oh god, how have I missed this thread until now? Interstellar cracked me up, because they were all serious about how it was a REAL wormhole because it was a SPHERE and not a DISC, and how they consulted with REAL scientists to figure out how space would appear that close to a black hole, and yet they got so much basic stuff wrong. Black-hole-proximity time dilation will not make a "life signs" signal repeat infinitely. They "turned off" artificial gravity as they approached the wormhole because it was disorienting, but they did so while they were sitting in the Ranger, which was in the center of the spaceship and thus wouldn't have had any net artificial gravity. Not to mention an SSTO that can pass through a gravity well with time dilation so extreme that 10 years passes in a few hours. Twice. The rank absurdities in 2012 met their zenith when a freaking WINNEBAGO outran a seismic wave. Armageddon wins the award, for me, simply because it manages to cram in so very many mistakes. It's almost a work of art. "The gravity on the asteroid will be really low, so instead of training you to navigate in low gravity, we're going to give you RCS packs with 5 or 6 km/s of dV which will continuously fire upward...but when you're not in EVA suits, never mind." The SSMEs that fire a gentle blue flame without any fuel tanks, and the Shuttles that fly through a vacuum using control surfaces, like fighter jets, through a trail of asteroid debris. The meteor impacts which portend an impacting asteroid by months. The dwarf-planet-sized asteroid that they "simply hadn't noticed" prior, which is somehow split apart using a tiny nuke on its surface and which evidently contains a gigantic spring to shove the pieces apart so that they don't simply stay stuck together by gravity. In Lost, they remove the "core" of a nuke from the body, and later detonate it by bashing it with a rock. The resident weapons expert explains that this is possible because the SmartCar-sized body is "just the delivery system". I'll let that one sink in.

Of course, there's one other very, very interesting possibility. Recall that one way of reducing the size of the Dragon 2 kick stage was to modify the Falcon 9 upper stage for delayed restart, and use it for 95% of the lunar orbit insertion burn. IF those modifications could be made to the MVac.... Falcon Heavy, expendable, can deliver 16.8 tonnes to Mars transfer, a cost of around 4.3 km/s. This is what it'll be doing with the Roadster (fingers crossed) on Tuesday. The Falcon 9 upper stage has a dry mass of 4 tonnes and carries 107.5 tonnes of propellant. A bit of math tells us that in such a configuration, the upper stage burns 48 tonnes of propellant at an isp of 345 s to deliver that 16.8 tonnes of payload (plus 4 tonnes of dry mass) from LEO to trans-Martian injection. Reversing the equation, this tells us that the other 59.5 tonnes of propellant provide only 2.2 km/s of the dV for orbit, which places expendable staging velocity for a 16.8-tonne payload at around 5.6 km/s. If the core and boosters of an expendable Falcon Heavy can deliver 16.8 tonnes of payload, along with the upper stage, to 5.6 km/s, then they can surely deliver a smaller payload to the same staging velocity. Let us suppose that the upper stage provides the 2.2 km/s to get into orbit, plus the 2.7 km/s for TLI, and still has enough propellant left over for BOTH the lunar insertion AND for the almost-landing burn. That's a total of 8.13 km/s, from Earth ascent staging to lunar landing burn termination. Our handy-dandy rocket equation tells us that for a single stage at 345 s isp to deliver 8.13 km/s, it needs to have a 90.97% prop fraction. Given the 107.5 tonnes of prop in the upper stage, this tells us that a Falcon Heavy flying expendable could deliver 6.67 tonnes of payload to a zero velocity just above the lunar surface, simply by modifying the upper stage for extended-delay restarts. The Apollo LM ascent stage, again, was under 5 tonnes. How much, exactly, would a Dragon V2 mass if you removed the aeroshell, heat shield, Superdracos, and parachutes?

Falcon Heavy already has the capacity to send an unmodified Dragon 2 on TLI, which will (purportedly) be used to do a free-return. With a Draco-based kick stage mated to the payload adapter in the Dragon 2's trunk, SpaceX would have the ability to send crew to lunar orbit and back. So that takes care of the easy part. The hard part is the lander. We don't have a lander, or anything that could be easily modified to act as a lander. In theory, if we had a lander that was under 25 tonnes and less than 5 meters in diameter and could perform its own lunar-orbit insertion, then it could be dropped onto a Falcon Heavy and sent in parallel to the Dragon 2. The fairing on FH is big, but I'm not sure if it is big enough to hold a lunar lander AND an Earth Departure stage. Hydrogen is not known for its surpassing density. The one place you absolutely MUST have pressure-fed hypergolics is the lunar ascent engine. There's no alternative; trusting any ignition system other than hypergolics (or any kind of turbopump) with that one critical point is a non-starter. Another very Kerbal solution (though not one without precedent; see the N-1 and the Nova) is to use a single restartable hydrolox stage for the lander's Earth departure burn, lunar injection burn, AND lunar landing burn, but drop it before the actual landing. You then use your ascent engine(s) for the actual touchdown and, later, the ascent. It's not the most rigorously efficient approach (the engine would be underpowered for the TLI burn and overpowered for the landing, and you're dragging along a bit of superfluous dry mass), but it's breathtakingly simple, and you can completely dispense with the whole mass and volume and added complexity of a formal descent stage. Such a high-energy multipurpose stage would need between 3.2 and 5.9 km/s of dV, depending on how much of a periapsis kick the Falcon 9 upper is able to give toward TLI (the TLI costs 2.7 km/s). The RL-10 might be a good choice, but it's pricey as all hell; you could also choose a vacuum-expanded BE-3 though I don't know what the isp is like. With hydrolox, you need a 50-60% propellant fraction to get 3.2 km/s or a 70-75% prop fraction to get 5.9 km/s. Falcon Heavy's 60 tonne+ expendable capacity to LEO would allow a loaded ascent stage mass of up to 14 tonnes (though the size of the LH2/LOX stage would be prohibitive), whereas if the Falcon upper stage performed the TLI, the hydrolox stage would only need to do lunar injection and descent, and so your loaded ascent stage budget would still be a healthy 8.4 tonnes. For reference, the LM ascent stage was under 5 tonnes, wet. The much larger planned Altair ascent stage was around 10.8 tonnes.

Not sure separation will occur low enough for aerodynamic forces to have much effect. But if they did, they would most likely not cause significant rotation around the CoM, not enough to make a collision likely. Keeping the adjacent engine burning through separation would certainly do the trick, but I don't know that they'd attempt something that risky.

My baby grand tour ship has landed boosters on the pad, on the VAB, on the island runway, and my SSTO will eventually come back to the runway.

P.S. The reason the helium COPVs are a problem is because a pressure-fed engine system doesn't lend itself well to having alternate plumbing shunted into it. If SpaceX did use Dracos to build a drop-in hypergolic kick stage for Dragon 2 with a good 2.6 km/s of dV, it could be tested in a free-return lunar orbit mission for the cost of an expendable Falcon 9 or a full-recovery FH. The LV puts it into a high elliptical orbit, and then the kick stage fires and bumps it up into the free-return.

The SuperDracos are underexpanded even at sea level, to allow very good throttle response, so their ISP is very poor...on the order of 250 s in vacuum. Plus, they are canted 15 degrees off-axis, which means cosine losses. TWR is no problem, but you'd need an enormous amount of propellant -- like, twice as much as could fit in the whole trunk. Plus, trying to crossfeed the SuperDracos from an auxiliary tank would require a complete system redesign (the SuperDracos are pressure-fed hypergolics, which means helium COPVs). A better solution would be to simply cluster a few ordinary Dracos in the trunk with their own tanks. They are already vacuum-expanded (300 s) and while they're not super-thrusty, you don't really need a huge TWR for the lunar injection or return burns. The whole system could be attached to the payload adapter inside the trunk, so you have zero modifications to the Dragon 2 itself. You need 1.3 km/s each for the lunar-orbit insertion burn and the Earth-return burn. That's a 1.4:1 propellant fraction, though if SpaceX could add an expansion nozzle to the Dracos to boost the isp up to around 320 s, it would drop to 1.2:1. The standard Dragon 2 is 7.8 tonnes with onboard propellant; allowing another tonne for crew and consumables bumps this up to 8.8 tonnes. Assuming that tankage, extra engines, and extra structure increase the dry mass slightly, you'd be looking at around a 13-tonne kick stage. The density of the hypergols used by the Dracos is 1.2 g/cc, so the kick stage would occupy a volume of 11 cubic meters, well within the 14-cubic-meter unpressurized volume of the Dragon 2's trunk. The whole stack would mass 21.8 tonnes, which is just under an expendable Falcon Heavy's TLI capacity. If the Falcon upper stage could be modified to allow for extended restarts, you'd save about 8 tonnes of mass on the kick stage, which might be enough margin to allow for side-booster recovery (the MVac's higher isp would make up for the additional dry mass). All this assumes JLOR. If we want to try ELOR like Constellation, other possibilities open up, though the Dragon 2 is always going to need at least a 6-tonne hypergolic kick stage to get from low lunar orbit back to Earth.