Onwards to Uranus and Neptune! Options

[tedstryk]

It is useful for both. The difficulty with aerocapture comes from the fact that, a) it will take a great heat shield to hit the atmosphere that fast and be slowed down enough to go into orbit, and b ) the parameters of the atmosphere of Neptune are poorly known. If it goes to a depth where the atmospheric thickness is too great, it could end up being an entry probe, might end up being permanently disabled/rendered inoperable, or might be put in such a short orbit that it decays before the periapsis can be raised to a safe distance. If it doesn't go far enough into the atmosphere, it could end up flying by the planet never to return or in some five year orbit that lasts longer than the probe's designed lifetime.

[stevesliva]

Sad but true. The Saturn V was never used for UMSF for the same reason, even though the original Voyager Mars concept (which evolved into Viking) did envision two Saturn launches.

And was envisioned for some brutishly huge MSR missions. If you have two separate huge landers-- an MSL-class rover and a large direct-to-earth ascent module-- is there any benefit to one Ares V rather than two Titans? Or two Ariane Vs... or whatever. Is it more economical to launch two things going the same place in one big launcher?

[Juramike]

Here's a timely little tidbit about an inflatable (=potentially cheaper) heat shield in this space.com article.

But low cost still won't mitigate the risk of unknown atmospheric characteristics (like at Neptune or Uranus).

[nprev]

Is it more economical to launch two things going the same place in one big launcher?

Yeah, generally, although there is a risk tradeoff to consider...'all the eggs in one basket'.

If you recall, most of the missions from Voyager on back were pairs but on independent boosters, and at least part of the rationale for that strategy was ensure that the whole project wouldn't be sunk by a booster failure. I'm sure that applied to the MERs as well. This was wise, considering that Mariners 1, 3, and 8 all splashed.

[dvandorn]

Also, dual-probe missions have increased the science return from their given opportunities. Primarily, we've been able to target the two probes on dual-probe missions somewhat differently, using the same instrument suite to look at different locations. That was actually done on Mariners 6 and 7, Vikings 1 and 2, Pioneers 10 and 11, and Voyagers 1 and 2. There were also, of course, plans for the Mariner 3/4 and 8/9 missions to use the same instrument suites to look at different locations.

Dual- and multi-probe missions, though, also give you the opportunity to send probes with different instrument suites to the same location. I can imagine this kind of approach would work very, very well for outer planets missions, where your orbiters all have shots of their own at observing the primary planet and each of its moons. Once you finish your pre-planned primary mission, you use the results of one instrument suite to define how the rest of the instrument suites are used on each extended mission.

I'd think you'd have more flexibility, as well -- imagine if you had 4 mini-Cassini's at Saturn now, one with SAR and general imaging, one with high-resolution multi-spectral imaging, one fields-and-particles vehicle, and one with VIMS and additional spectroscopic analyzers to look at the chemistry of the planet, rings and moons.

Now, each of these guys would be on their own mission orbits, with the SAR probe staying out near Titan a lot. One suite would follow up the results from the other suites. Perhaps the vehicle with all the spectroscopic analyzers could be designed a little mechanically tougher than the others, so you could dive it through plumes and the vaporous edges of rings.

Figure that each of these spacecraft could share designs (and manufacturing, etc.) for a common physical bus, common attitude control and propulsion systems, common data handling and communications systems... You'd be developing maybe 20% more sensor systems for the various instrument suites, but your designs, fabrications, engineering, etc., can actually get into savings-of-volume. If you can spend maybe 50% more on your multiple spacecraft than you would on a single Flagship mission, but spend only 60% of what you'd currently spend on launch costs, then it tends to even out. You get more science, a more flexible mission, and the ability to follow up on discoveries by some instruments with detailed analyses by others, without hauling all of the rest of your instruments along on your detailed analyses.

Does that kind of mission architecture sound exciting to you? 'Cause it sure does to me!

-the other Doug

[stevesliva]

Yeah, generally, although there is a risk tradeoff to consider...'all the eggs in one basket'.

It's funny, though, when you think of the complicated MSR profiles, one basket might be best, rather than risking the whole thing with extra launches. If we wanted a rover to put samples in an MSR ascent vehicle, no point not having half the eggs.

[tedstryk]

With missions like MSL and Europa Orbiter, failure is a really scary thing. Remember when the quasi-flagship Mars Observer failed and Galileo's antenna didn't open? Then again, with those failures (in Galileo's case I mean antenna failure, not mission failure), lightning might have struck twice...I am especially thinking of Galileo. If they didn't lubricate one antenna, having a second spacecraft to deal with wouldn't have changed much. I would be interested in knowing how much it would cost to have a backup vehicle that could, if necessary, be made flight-worthy in the event the initial mission failed. You would at least save the second rocket cost of the first mission succeeded.

[vjkane]

If they didn't lubricate one antenna, having a second spacecraft to deal with wouldn't have changed much.

As I recall, the failure for Galileo is that the repeated trips across the U.S. by road allowed the dry lubricant to fall out. Ironically, if they'd flown the spacecraft back and forth (an option rejected for cost reasons), I read that the lubricant would have still been there. I also seem to recall that if they had been able to back up the antenna screw mechanism (a capability eliminated by cutting a wire so it would never be done in space and collapse the antenna), they could have worked around the problem.

Fundamentally, the problem was that they didn't retest the antenna mechanisms after several years of storage and thousands of miles of road travel. It's those little things that in hindsight seem so obvious that get you. Like the solid rocket booster plume on CONTOUR.

This is so hard that I am always amazed that so many missions work so incredibly well.

[vjkane]

It's funny, though, when you think of the complicated MSR profiles, one basket might be best, rather than risking the whole thing with extra launches. If we wanted a rover to put samples in an MSR ascent vehicle, no point not having half the eggs.

I've always thought that the risk in this mission is in the landing and launch. I'd put two sample collection craft down on the surface and then send one orbiter to collect the samples. If the orbiter fails, you can fly another one in a few years. The samples will still be in orbit.

[tedstryk]

Well yes, you are correct. What I am referring to is that despite a decade and the truck rides, no one checked on the condition of the lubricant before launch. My point is that had there been a pair, it is unlikely that this error could have been recognized and corrected for the second spacecraft since it would have likely been already in space when the problem was discovered.

[Greg Hullender]

That is his point, though, is it not? If huge launchers get 10x cheaper, you design the Neptune Orbiter you want and can also afford the huge detachable propulsion module for NOI for the same launcher price... maybe. And you get there in 10 years rather than 30. Or whatever.

(Isn't aerobraking useless for orbital insertion, anyways? It's for circularization, right?)

Yeah, it's always a clue that people didn't read your whole post when they offer supporting arguments as evidence to the contrary. :-) It does leave one hopeful that if SpaceX really does prove itself, we might see a slew of new Outer-planet mission proposals. (Of course, there's still that power-supply problem.)

An additional obvious problem with Aerobraking is that (by itself) it only lowers the apapsis, leaving you with a periapsis inside the planet's atmosphere, so you still have to burn fuel (when you do reach apapsis) to raise that high enough to avoid things like atmosphere, rings, radiation belts, etc. Supposedly that's why it's not attractive for planets like Jupiter -- you burn more fuel raising the periapsis than you gained from the aerobraking in the first place. (I haven't worked this out myself though.) In the case of Neptune, in addition to the uncertainty about the atmosphere, I suspect there might also be doubts as to just how high we'd need to raise the orbit in order to be safe.

--Greg

[ugordan]

An additional obvious problem with Aerobraking is that (by itself) it only lowers the apapsis, leaving you with a periapsis inside the planet's atmosphere, so you still have to burn fuel (when you do reach apapsis) to raise that high enough to avoid things like atmosphere, rings, radiation belts, etc.

I think the difference between aerocapture and aerobraking should be clarified. What you're describing is aerocapture, while all recent Mars probes for example used aerobraking to condition the orbit. Aerocapture gets you from the initial hyperbolic trajectory to a capture orbit (though with periapsis inside the atmosphere as you say), aerobraking typically bleeds off your capture orbit's eccentricity.

[tedstryk]

In the case of Jupiter, the radiation environment is also a factor.

[dmuller]

I think the only more or less realistic (partial) aerocapture scenario at the outer planets would be using Titan to assist a Saturn orbit insertion. I read that the Titan atmosphere is "quite good" for that - dense atmospere & low gravity, and known to quite a degree. It will still require orbit trims and possibly a retro-burn to "finish up" the orbit insertion, but still use much less fuel than without the slowdown at Titan (hopefully the fuel savings vastly outstrip the weight of the heatshield+related equipment). The day-or-so it will take the spacecraft from the Titan atmosphere encounter to periapse should give enough time to calculate the additional burn needed.

No such option exists at Uranus or Neptune AFAIK.

Even if you have a strong rocket, budget constraints will still force missions to go as low-weight and low-cost launch as possible.

[dvandorn]

The real problem is that, with outer planet missions, you play transit time against approach velocity. To get out to Uranus or Neptune in 10 to 15 years, you have to be traveling pretty fast relative to your destination by the time you arrive. You can design a trajectory that results in a much lower approach velocity, but such a trajectory will take 30 or more years to get out to the farther reaches of the System.

So, unless you want to launch a probe that will be managed by multiple generations of PIs, flight support personnel, etc., you have to deal with taking out a pretty hefty amount of velocity upon arrival. This will be the case right up until we can design a constant-acceleration propulsion system and we can accelerate for half of the outbound trip and decelerate the other half. When we eventually develop such a propulsion system, we'll be able to travel to Mars in weeks and the outer planets in months.

Until then, though -- you gotta slow down when you arrive, so you need to carry enough fuel and/or aerobraking equipment to do so. And, the film 2010's fictional flight planning aside, you gotta have enough fuel to raise your periapsis out of the atmosphere after a primary approach aerobrake, and the amount of fuel plus the mass of the aerobraking equipment required for the aerobrake vs. the amount of fuel you need to do a rocket-only insertion generally comes out in favor of the rocket-only option.

-the other Doug