Many of the proposals for Venus sample return that I've seen involve liftoff from the surface by balloon to an altitude of about 66 km, then a rocket to put the sample into Venus orbit (example):
Ascent
Venus Ascent Vehicle designs were simulated with a variety of stage combinations and guidance schemes. A successful rocket ascent was simulated for a three-stage combination of off-the-shelf solid rockets, a Star 24C, a Star 17A, and a Star 13A
Return to Earth is proposed via solar-electric propulsion:
Trans-Earth injection is very demanding because of Venus’ size. A comparison between conventional chemical propulsion and solar electric propulsion (SEP) showed a large mass advantage to SEP - more than a 30% reduction in total system mass leaving Earth at the beginning of the mission. This does not include the difference in the mass of the aerocapture ballute. In contrast to the use of SEP for the delivery of the spacecraft to Venus, the use of SEP for the return trip takes advantage of the closer proximity of the Sun and the lower mass of the returning vehicle, which both imply a smaller, less costly SEP system. The SEP system designed here consists of one advanced NSTAR thruster (under development) and a 2.5 kW GaAs solar array (capacity measured at 1 AU, end of life).
The SEP system would spiral the orbiter out of Venus orbit into heliocentric space over the course of 437 days beginning on September 29, 2005. Arrival at Earth would occur on September 29, 2008 after a heliocentric transfer taking 536 days and with a final hyperbolic approach velocity of 3.2 Ms. Figure 9 shows the heliocentric transfer using SEP
Now that is spaceflight: Launch from Earth directly from the surface into trans-venus injection, aerocapture at Venus, lander descent to the surface, surface operations, ascent above the bulk of the atmosphere by balloon, a three stage SRB puts the sample into orbit, docking with the return vehicle in Venus orbit, a >400 day ion spiral out of Venus orbit and to trans-Earth Injection, followed by a high velocity return from interplanetary transfer at earth.
-------- Original Message --------
On Tue, Jan 30, 2018 at 01:24:13PM +1300, Rupert Boleyn wrote:
On 30Jan2018 1314, Thomas Jones-Low wrote:
The low orbit to Venus surface requirement looks off - it's gas
giant level.
Venus does have a very thick atmosphere. Reading back over the Reddit
thread where the calculation was done, the assumption was using the
Goddard solution from the surface with reasonable drag parameters.
The problem is that the lower levels of Venus's atmosphere have
densities more comparable to soup than to the air we breathe, and
consequently very strong drag at even modest speeds.
I'd take it with a big boulder of salt, because if we ever do get
something to take off from Venus's surface, we certainly won't try to
get through the first 70 km or so with a conventional rocket.
The launch delta-V's for actual gas giants in the table were
calculated from the Earthlike density levels in their atmosphere. If
their entries in the table used the same take off from "surface" using
a rocket like Venus, their delta-V's would be even more insane.
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