As shuttles and especially of hybrid shuttle/airships go, one needs to become somewhat flexible in their thinking outside the box, as well as seriously focused upon the most difficult of challenges we've ever encountered, not to mention the anti-everything flak associated with the anti-Venus crowd, as they're by far in the majority and, at this point they're not happy campers.
1) think truly big
2) think relatively fat
3) pray a lot that someone whom engineered the damn thing is onboard.
Option A: I'm thinking somewhat along the lines of our creating a fat Albert hybrid shuttle/airship, except instead of common rocket engines, I'm rationalizing hybrid turbojets that can morph into turboprops. Obviously if need be, as for getting that shuttle back into LVO, so as to standby for recovering whatever's left of the away mission, as that's most likely something for utilizing solid fueled rockets, as many of such rocket modules can be sent ahead as orbiting somewhat like a fast moving refuling pit stop, except that no one is actually stopping at any pit unless that's VL2, as this entire mission is sort of on the run with a whole lot of hoping for the best.
Obviously this bloated shuttle needs it's aerodynamics altered for taking advantage of the complex Venus atmosphere(s), as well as being suited for being under a compression mode of perhaps 1 Bar differential, but I believe that's structurally a good thing, as alloy steels go and as engineering goes, as for being under compression mode is nearly always a ten fold advantage over being in tension mode. Thus strength wise, this shuttle/airship ott to be quite tough per tonne.
Option B: There's certainly the possibility of deploying a lessor sub-shuttle as for a single or perhaps of two crew members to decend, then if all goes well, they re-orbit and join up the our mother shuttle/airship which would proceed at getting everyone safely back to VL2, then eventually Earth.
As for the main decent/ascent fuel requirements; the bulk of which I'm thinking initially being C12H26 plus H2O2, then switching over to CO/O2 once we're sufficiently within the density of the Venus atmosphere (say 1+ bar @45 km) and able to start processing upon the CO2-->CO/O2 (how hard can that be?).
Once on the ground we're obviously at the mercy of whatever welcome mat that's presented. Our best defence might turn out being a flashlight and, our best gift or commodity for exchange would have to be H2O. A little ice might even be over the line and ice cream could turn out being horribly sacrilegious, where we could potentially be labeled as witches or demons, especially if we just drop in unannounced.
If we're sufficiently lucky (that's asking a lot), and by that I mean unless they should eat us, we should be capable of flying this shuttle/airship off the local tarmac and ascending to 100 km (actually I've learned that the 50/50 CO2/O2 waypoint is at roughly 150+km, which means we could possibly fly this craft to that elevation utilizing the hybrid C12H26/H2O2 and/or CO/O2 jet and turboprop engine format), at which point the propulsion would likely switch over to purely rocket thrust, of which I'm told the best performance is a specific impulse of CO/O2 being 260 until we obtain LVO (185+km) where theoretically we can expect to obtain a thrust performance of 280. That's still not terrific but, at least the fuel for all that thrust was acquired on location, were we didn't have to pack it along, which would have taken up all the space and then some of what would otherwise have been beer and pizza and, I wouldn't be all that surprised if this mission didn't demand a tonne of pot just to keep our crew menbers from killing each other off, as we're talking about 6 months worth of to/from transporting plus whatever on location visitation.
At 1 bar or lessor interior pressure (presumably that's established after obtaining an exterior of 1+ bar (possibly that being at roughly 45 km and/or just below the bulk of those nighttime clouds), the idea being that our rigid shuttle/airship would remain under this compression mode when within the lower Venus atmosphere) displacing the sheer bulk of the interior with plain old H2, initially at 90% H2, 6% N2 and 4% O2, gradually increasing H2 upon final decent to 99% and O2 at 1%, always with the shell of this craft under external pressure as a differential of perhaps 1 bar until it's time to land at that the Venus interplanetary tarmac, in which case the standby compression of 0.1 bar should surface as to maintaining all of the H2 safely onboard (absolutely no chance of any H2 leakage if it's contained under a little external pressure and of mostly CO2 at that).
Lets say the available interior cavity of this craft, including wing and tail voids (excluding crew, machinery, fuel, beer and pizza) at 4000 m3, that which by formula of containing mostly H2, this would equate to 260,000 kg worth of buoyancy at ground elevation of the nighttime tarmac situated at roughly 5 km. That's 260 metric tonnes worth of buoyancy that just might offset 50% of the grand total of 520 t.
I'm not saying we need to construct such a massively hybrid shuttle/airship but, if push comes down to shove and, if there were a concerted effort made upon assaulting the Venus surface, surely there's a way of accomplishing this task (with or without any UN approvals). Placing that much machinery initially into LEO (say as to being assembled at ISS) is for one thing going to choke all of us back here on Earth with nearly 100 additional tonnes of newly created CO2 for every tonne placed in orbit and, if this entire mission were to involve 1000 t worth, that's obviously 100,000 t of choking CO2 left behind for you and me. Remember that it's not just of the launch contributing CO2 but, it's the overall industry involved in making all this happen that impacts by producing all sorts of CO2 so, unlike those NASA budgets that only have to include the launch phase of producing as little as 25 tonnes/tonne, realistically there's perhaps another 75% factor that needs to be accounted for, thus the 100 tonnes of CO2/tonne is being way more realistic.
Let us presume we (Earth) can manage with another scant 100,000 t worth of artificially created CO2 (what's another degree or so of global warming?)
Option C: Certainly a scaled down version should become more easily obtainable, such as a 100 tonne mission (10% of the previous) and thus involving fewer crew and lesser expectations. Utilizing a decent balloon format could improve our micro mission as to getting sufficiently below those nighttime clouds, buying us time so as to considering their options and final decent to the surface. This lesser hybrid shuttle/airship could then land and/or merely go about deploying an array of robotics to the surface, then re-orbit to VL2 where their return rockes are stashed.
Option D: Obviously, for all sorts of perfectly good and sane reasons, I'd much rather make the initial contacts and subsequently any number of intellectual exchanges by phone (basically via a local laser/packet calls) and, then go about placing our reservations for obtaining usage of one of their airships (sort of a version of Venus Enterprise Airship Rentals), as that way we'll only need to approach Venus where our crew could perhaps transfer over to their metro airship at the cruising altitude of 65 to 75 km (or of whatever's sufficiently above their nighttime cloud cover), as that's certainly going to be damn cold but, at least it's containing some measurable O2 and, presumably this is still fully nighttime at some lesser weather disturbance determined location, where there's not likely to being all that much turbulence, at least not until descending through those thick clouds. At that point we're merely passengers onboard one of their Venus metro airships, of what we should know actually works.
If it turns out there's no support from Venus or, perhaps they simply don't want us there, then we're going to have to rely upon processing sufficient amounts of that CO2-->CO/O2 in order to fill our "get away" fuel tanks with presumably several hundred tonnes of that CO/O2 stuff, as obviously we can't carry that much added weight down into the toasty surface environment of Venus and, ever expect as to having enough onboard energy as to re-launch and obtain our escape without a little help as well as another refueling pit stop at VL2, where presumably our return rocket boosters have been stashed.
This adventure is not for the light hearted, it pure Rambo stuff on steroids and, I'm not all that certain that we have what it takes. Perhaps a full blown nuclear rocket powered hybrid shuttle/airship craft could manage a two-way mission, however no one's got one of those and, I'm fairly certain that warlord Bush isn't going to pull out any stops on behalf of humanity because, if he ever did, I'd have to write a number of retractions but, somehow I'm fairly confident that'll never come to past (Apollo and all that cold-war stuff being a wee bit touchy and all).
So, once again, I've run myself seriously amuck, short of ideas that could work, not that my next round of therapy wont uncover some alternatives. Somehow I'd much rather hand this phase over to those encharge of such wizardly accomplishments (those mostly German ESA rocket scientist), as I'm fairly certain I've overlooked a few essentials as well as "what if's". As you should understand, I'm more into being on the other end of a biologically safe laser call than to be directly knocking upon those doors at GUTH Venus. Township or not, visitors may not be welcome, especially pathetically stupid and arrogant Earthy types that are perceived as not liking Cathars nor French cooking, as well as a long list of many axis of evil sorts which we seem undaunted at exterminating rather than setting an example of our doing good Cathar like things.