Onwards to Uranus and Neptune! Options

[briv1016]

Here's the entire slide for context and volume considerations.

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[imipak]

50 mT to Mars! _wow_. That would open up a whole new vista for the fun game of Fantasy UMSF.

[ugordan]

Yes, except for two inconvenient facts:

1) should the "classic" Ares V ever materialize, it will be expensive as hell
2) Spacecraft are usually many times more expensive than the launch vehicle. It's not a problem of launching a largish spacecraft into space, it's a problem of funding it in the first place. See JWST.

I would put my hand in the fire that no unmanned spacecraft would ever be launched on a dedicated Ares V launch.

[nprev]

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.

[Greg Hullender]

Spacecraft are usually many times more expensive than the launch vehicle.

Then why is it so much more expensive to have a Neptune orbiter than a Jupiter orbiter? This is a poinit that puzzles me a lot. If SpaceX really succeeds in reducing launch costs by a factor of 10, does that suddenly enable lots of interesting outer planet missions or not?

--Greg

[dvandorn]

I think a 5-fold to 10-fold reduction in launch costs would result in more twin-probe or multi-probe missions. It costs maybe half-again more to produce two new probes than a single probe (if you take the MER project as an example). A third probe adds quite a bit less to your costs, as does a fourth, etc.

So -- if you can design four probes that are nearly identical (with perhaps some variation in their scientific payloads) and launch them to the same destination for the same cost as what you'd pay today for a single flagship mission, you can get a potential for a lot more bang for your buck. I can envision flying a really solid NetLander mission to Mars this way, or sending a flotilla of four to six Jupiter-system probes, each with its own unique program to execute, and each perhaps half as capable as a flagship probe.

You're still talking flagship mission funding, of course, and so only looking at seeing such multi-probe missions once a decade or so. Even so, in the meantime, you'd at least be able to spend a little more on Discovery-class mission spacecraft and a little less on their launch costs. In other words, reduction in launch costs is always a good thing, but I think it'll have more of an impact on flagship-class planning than on intermediate-class missions.

-the other Doug

[ugordan]

If SpaceX really succeeds in reducing launch costs by a factor of 10, does that suddenly enable lots of interesting outer planet missions or not?

There are two schools of thought to this. One would be that scientists would still want to build expensive spacecraft, but launch them more cheaply, but it's not a really high net saving. Let's take Cassini: $3 billion + $400 million Titan IV launcher. If the launcher was switched to a 2 or 3 times cheaper one, you see that's still very expensive in total program cost.

Another school of thought says that if you suddenly have cheap launchers that can launch a lot of mass to space, you can build spacecraft that are more massive and rugged, while maintaining the same science capability as before. Essentially less science per kg, but you can ease up on expensive structural and environment testing and just beef up systems to make them more rugged. This would be an enabler for cheaper missions, but there is always the siren call of putting as much as we can on it and it ends up expensive again.

[imipak]

Yes, except for two inconvenient facts:
[...]
I would put my hand in the fire that no unmanned spacecraft would ever be launched on a dedicated Ares V launch.

I'm sure you're right, but that's why it's _fantasy_ UMSF! My daydreams were of a multi-vehicle MSR project, for instance. An MSL-scale sample-collection rover, with a separate lander carrying nothing but an ascent stage to receive the samples, with sufficient mass still in orbit for the return leg to earth, with the remaining orbiting infrastructure left in place for communications and data relay. Using a hand-waving assumption that cost scales with mass, roughly ten times the mass of Cassini-Huygens would cost well over $30B, so I shan't be holding my breath for an announcement any time soon, though.

[tasp]

A little levity this Sunday morning as we enjoy our coffee and doughnuts:


the mighty Saturn V did manage on 2 occasions to loft craft appropriate for discussion here. PFS-1 and PFS-2 were used to study fields and particles from lunar orbit back in the seventies. Each were under 40 kilograms and probably set the record for the largest launcher used on the smallest spacecraft.

[Guest_Enceladus75_*]

Then why is it so much more expensive to have a Neptune orbiter than a Jupiter orbiter? This is a poinit that puzzles me a lot. If SpaceX really succeeds in reducing launch costs by a factor of 10, does that suddenly enable lots of interesting outer planet missions or not?

--Greg

I think part of the difficulty in a Uranus or Neptune orbiter is not the mass or the design of the spacecraft, but getting the spacecraft into orbit around these planets in the first place. With conventional rockets, the fuel tank required for the fuel to brake into orbit would be prohibitely expensive. I think some type of aerobraking/aerocapture would be required with all the related dangers that this approach would involve.

[Greg Hullender]

With conventional rockets, the fuel tank required for the fuel to brake into orbit would be prohibitely expensive.

By that logic, both Gallileo and Cassini were impossible, so that can't be it. Although I'm a big fan of aerobraking, it's clearly not required. As far as I can tell, everything flows from the exponential in the rocket equation. For a given weight of payload, the fuel cost goes up exponentially with the delta-V divided by exhaust velocity. Since chemical rockets have exhaust velocity of about 3 kps, this means an extra factor of e (multiplied!) for every 3 kps of delta-V. Outer planet missions need more delta-V, so they have higher fuel costs.

I think a Neptune mission using a Jupiter gravity assist COULD be no more expensive than a regular Jupiter mission, since the chemical rocket only needs to get the probe as far as Jupiter, and it only has to slow it down for Neptune -- easier than slowing it down for Jupiter. I think the extra delta-V requirement comes from us wanting to get the probe there in a decade or so -- not wait 20 or 30 years. That makes sense to me, but (again) it suggests that the lion's share of the cost of a Neptune or Uranus probe should be fuel, and so a 10x reduction in cost-to-LEO should result in a 10x cost reduction (almost) of the Uranus/Neptune mission itself.

There's probably a hole in this logic somewhere, but I'm not seeing it -- I've even considered whether the main cost might be salaries (which would be more on a long mission) but based on how small the incremental costs of running the MERs has been, I find that hard to credit.

So I remain convinced that sharply lower launch costs should be a HUGE enabler for Uranus/Neptune missions, but I'd love it if one of the real rocket scientists could give me a clear answer; even if only to show me where I'm wrong. :-)

--Greg

[Holder of the Tw...]

... it only has to slow it down for Neptune -- easier than slowing it down for Jupiter.

Not necessarily, depends on what you mean. The deeper the gravity well, the more effect you have firing off a rocket at the bottom of that well after falling into it. It can be a lot easier getting into orbit around Jupiter than an asteroid. The excess velocity the spacecraft carries with it on appoach, say a few miles a second, will end up being just a fraction of that speed above orbital velocity at close approach, making only a small burn necessary to achieve an elongated orbit. For an asteroid target at the same distance as Jupiter, and with an identical trajectory, you have to cancel out all of the relative speed. The asteroid's gravity doesn't offer much help.

But, if you're talking about rocketing directly into a close circular orbit around Neptune, as opposed to Jupiter, well then yeah, Neptune will be a lot easier. Except that with current technology both would be impossible.

[ugordan]

The deeper the gravity well, the more effect you have firing off a rocket at the bottom of that well after falling into it.

Right, and this is also known as the Oberth effect.

[nprev]

The other thing to remember is that a putative Neptune orbiter would be approaching the planet with a great deal of excess velocity to shed (assuming that it's to make a reasonable transit time, say 15 years from launch to beginning of mission).

So, unless aerobraking or some other exotic deceleration method is used, you're looking at sending a good-sized engine + lots of fuel along in addition to the spacecraft, sort of like keeping the Apollo CSM + LM attached to the S-IVB all the way to lunar orbit (bad analogy, but you hopefully see what I mean.) Bottom line is that the total throw weight (and cost) would go way, way up, which would constrain the actual payload considerably.

[stevesliva]

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?)