Follow on Pluto orbiter/lander/rover?

[Bubbinski]

After New Horizons stunned the world with its first Pluto pics, I've been wondering what a follow on mission would look like.  NH couldn't go into orbit around Pluto as it was going much too fast and did not have nearly enough fuel to go into orbit.  What propulsion tech could enable a Pluto orbiter?  Could a Cassini copy do the job or would something more advanced be needed? 

Also what gravity assist trajectories are available in the future (and when)?  And when this mission does finally happen I would hope it would include a lander and rover. 

[MATTBLAK]

A Pluto orbiter should be doable, although even a half-ton spacecraft like NH would have required a lot of propellant to slow into Plutonian orbit. A one ton spacecraft - including a couple of powerful hypergolic braking engines, but not including propellant - would need how much delta-vee and propellants? Are we talking a 'Cassini-Lite' design with a really big high gain antenna and two or three RTG power units? Would Falcon Heavy be powerful enough to send it direct, via a Jupiter flyby? Would you need SLS or would 2x Vulcan launch be necessary? Or an architecture with 1x Vulcan and 1x Falcon Heavy? That is, Vulcan launches the spacecraft and Falcon Heavy launches the Earth Departure stage, they dock in LEO and then depart.

[MP99]

A bit of an overestimate, but to bring a 1 ton spacecraft to a complete stop from 13.78 km/s with 300s Isp would need 108 tons of propellant.

Of course, this assumes a magical tank that can hold the prop without adding any mass.

Actually, the tank would be heavier than the spacecraft, so you'd need a lot more prop to also brake that mass. So you need a lot more prop. The page I went to couldn't calculate a mass without getting an arithmetic overflow. (I was probably using it wrong, but still...)

Cheers, Martin

[MATTBLAK]

Heh! What about an RTG powered Hall thruster? Would such a thing be doable, or is that too low powered to be feasible? I know Cassini was traveling a fair bit slower than NH, but how much storable propellant did it need for Saturn orbital capture? Bearing in mind of course that Pluto only puts out about 12% of a G.

[Star One]

The amount of fuel to brake is probably unworkable so you're probably looking at alternatives such as an advanced heat shield that was disposable along with some kind of advanced electric propulsion.

[MP99]

The amount of fuel to brake is probably unworkable so you're probably looking at alternatives such as an advanced heat shield that was disposable along with some kind of advanced electric propulsion.

There's very little atmosphere.

Could try lithobraking? But, crashing into the surface at that speed would need a very advanced heatshield. ;-)

Cheers, Martin

[MATTBLAK]

As much as I'd dearly love to live long enough to see a follow-up probe, I doubt it will happen, budgets being what they are. New Horizons may truly be a once in a lifetime deal!   Though, maybe later in the century there could be a nuclear-electric propelled probe option that could visit several places out there.

For the next twenty years, I'd like to see a dual spacecraft launch - twins of a same design - that are approximately Cassini-class that would go into orbit around Uranus and Neptune respectively and survey them and their moons at 4k resolution or better. And a dirigible & submarine combo for Titan, not to mention Europa and Enceladus landers...

[tea monster]

I'd say some kind of nuclear electric was a given for this mission. The problem with that is that I've seen the power of NH as about that of two 100 w lightbulbs! You are not going to get much ooomph from that.

So maybe a much more advanced nuclear power source is required.

... unless someone quickly develops a workable solar sail.

Ideally what you would want would be some something like the original JIMO, but with better engines, such as X3 Halls or ELF thrusters. You could have a bus spacecraft that carries several different probes. The bus tootles out to pluto, drops off a lander and and orbiter, zooms out to Eris, drops a lander and orbiter, etc., etc. The chances of that are, of course, nil.

[Dalhousie]

There's very little atmosphere.


Cheers, Martin

At present there is about as much as Mars, probably good for quite a substantial amount (SWAG up to 8 km/s).  But the atmosphere is in the process of freezing out so there won't be much by the time we next send a lander.

[Zed_Noir]

Why try to develop a Pluto orbiter concept.

Instead use a stream of small flyby probes with roughly the same or less instrument suite as the New Horizon. Like a new probe every Pluto half day for several days. In effect getting several orbital encounters from the probe stream.

Of course the propose bigger launch vehicles for the near future have to show up along with a supply of Pu²³⁸ for the RTGs.

Presuming building a batch of near identical small flyby probes will be cheaper than a single large customized orbiter.

[Oli]

There is a Pluto Orbiter design from ESA in the pdf attached.

Doesn't look too difficult actually, just needs a lot of time to get there (~17 years).

[nadreck]

There is a Pluto Orbiter design from ESA in the pdf attached.

Doesn't look too difficult actually, just needs a lot of time to get there (~17 years).

And a fair bit of plutonium.

[cartman]

How much can we improve on this mission with current thruster technology? or maybe a SEP stage until Jupiter.

[Bob Shaw]

There is a Pluto Orbiter design from ESA in the pdf attached.

Doesn't look too difficult actually, just needs a lot of time to get there (~17 years).

Yup, time or speed, you takes your choice.

I would hope that a small 'affordable' spacecraft (better still, two - one insurance clone vehicle launched several years after the other and with non-Pluto targets pencilled in, giving the best of both worlds should vehicle A be healthy) would be the outcome rather than a multi-planet Battlestar, complete with entry probes and the like. That's just low-hanging fruit waiting for cancellation.

[Bubbinski]

Thank you all for your replies and insights.  So if a Pluto orbiter would take a boatload of propellant, could a penetrator/lander only be workable?  Have a hardened penetrator attached to a fuel tank and engine that could slow down the combo to a survivable velocity before the penetrator digs in to the surface.  After the "landing" the top of the penetrator would use a telescoping rod to get itself to the surface and send signals to the earth while the pointy part stays dug in.  Can this work or would the velocities involved be too much?

Of course another alternative to Pluto follow up would be one or more flyby probes targeting different areas of the surface as someone else mentioned.  If that is done, would like to see a flyby of Uranus or Neptune added in along visits to Eris, Sedna, etc if feasible.  (It's been a long time since Uranus or Neptune were visited).

[Burninate]

Sample mission outline, all ion thrust, powered by solar panels and lots of RTGs:

GPHS-RTG gets about 5.4W/kg, in units of 300We/4400Wt with mass 58kg.

One NEXT thruster pushes ~236mN from 6.9KW at 58kg thruster mass (coincidence) mass plus 5.76mg/s (182kg/yr) of propellant expelled at 4190s Isp.  It's rated for ~5 years of burn, and has been tested to 5.5.

23 units of GPHS-RTG (we would have to spin back up some Cold War reactor capacity to generate this, or maybe raid the Sellafield Stockpile for Pu-238 reprocessing) power one NEXT thruster.  One extra GPHS-RTG powers electronics.  Give the thing six individually gimballed NEXT thrusters for some redundancy.  We're at a total mass of 1740kg on thrusters and power.  Add 260kg of solar array for inner-system operations, or 52KW at 200W/kg @ 1AU, so if you're near the sun you can turn on more than one thruster, and you have a 2 ton power/propulsion section.

Assume a 5 ton payload.  Start the mission in GTO (to save electronics some time in the inner Van Allen belt).  Falcon Heavy will launch 20T to GTO;  That's 7 tons on payload and propulsion, fill the other 13 tons with xenon/krypton (neglecting tankage).

Mission delta V starting in GTO is now 25.4km/s.

It will be around ~1.5km/s to circularize from GTO (complies with high-thrust apogee burn math) and another ~1.5km/s to escape Earth orbit on a spiral trajectory (low-thrust math approximating 2 * high-thrust), at which point you're in a heliocentric 1AU orbit around the Sun with 22.4km/s left to go.  For one benefit: This approach gives an indefinite amount of buffer time to wait in GSO before any orbital conjunction or assist you desire to reach, so there are long launch windows.

How long will it be to reach Pluto with those sort of numbers?

[nadreck]

Consider this:

If you have a propellant depot in LEO and you provide enough propellant to a one or two stage vehicle launching from LEO able to provide a ΔV of 11km/s, you would have an earth departure velocity of just under 16km/s that if it tangential to Earth's orbit would leave you with a Solar departure velocity of about 17km/s. However the velocity relative to the Sun would only be about 47km/s if that were all you were doing and the flight was another flyby the transit time would be about 5 years.  But let's say that was a probe that would thrust steadily all the way to Pluto to arrive at Pluto in roughly the same orbit that Pluto itself has around the Sun.  This would require approximately 15km/s of ΔV and nearly triple the transit time.  So to impart roughly 1km/s per year of ΔV would require 32mN of thrust per 1000kg of vehicle weight.

Note you need an impulse of just over 8km/s from LEO to get into a Hohmann transfer orbit to Pluto(without using any gravity assist anywhere).

[a_langwich]

A bit of an overestimate, but to bring a 1 ton spacecraft to a complete stop from 13.78 km/s with 300s Isp would need 108 tons of propellant.

Of course, this assumes a magical tank that can hold the prop without adding any mass.

Actually, the tank would be heavier than the spacecraft, so you'd need a lot more prop to also brake that mass. So you need a lot more prop. The page I went to couldn't calculate a mass without getting an arithmetic overflow. (I was probably using it wrong, but still...)

Cheers, Martin

Using the same calculations I believe, if you increase the ISP to 450 s, you drop from > 100 tons of propellant down to 23 tons.  If you increase the ISP to 500s, the propellant drops to <17 tons.

(Same caveats apply, this is a simplified calculation, so it's actually saying the entire spacecraft would weight 17 tons at the start, 1 ton at the finish, with all of the difference being fuel burned at 500s ISP.)

The conclusion I would like to advance is that ISP and minimizing dry weight are the crucial design points for the kick stage. 

You don't really want or need an RL-10 sized engine, you would actually like to be exploring the design space (yes, I'm still talking about chemical liquid rocket engines at least partially) back toward EP:  _less_ thrust, higher ISP.  The problem is I don't know of any LREs smaller than the RL-10 with a higher ISP.  There aren't many with a higher ISP at ANY thrust level.  That's why I asked in the rocket Q&A about small thrusters with high ISP.

As a thought experiment, consider a Centaur upper stage with IVF, but replace the RL-10 with a much smaller engine not much larger than what IVF is burning for ullage.  Ideally perhaps, a high ISP (450s or better) liquid engine of tens or hundreds of pounds of thrust.  Or, perhaps you could build a LRE/EP hybrid, where an EM field increases the exhaust velocity of the LRE (possibly also forming an extended nozzle, possibly also confining exhaust to allow hotter combustion temps).  Or, worst case, just use your Centaur with IVF as your generator, and put a modest set of EP engines on it.

Except these EP engines wouldn't be off-the-shelf electric grid engines.  They should be compatible with accelerating hydrogen and oxygen, which means field generation external to the flow.  Perhaps if heavier ions are desired for the EP, something like iodine could be injected into the exhaust.

Once IVF is applied to Centaur/ACES, I don't know what would limit the stage life if there's no complicated spin-up/chilldown process for the RL-10.  Beyond Mars, boiloff would be modest.

Distributed launch capability implicitly brings automated fuel transfer techniques, which means you could design and build fuel tanks that launched empty and deflated, and were filled on orbit.  That could potentially open up large increases in stage mass fraction, but it would require innovation in load paths and in tanking/engine pressurization and so on.


The point is, all of these techniques are far more within reach than the not-financially-available nuclear options, just as going to Home Depot and buying a Honda generator is far more practical than financing a small nuclear fission plant.

RTGs will be necessary for power on the final science platform (probe, orbiter, lander, etc), but I think the financial reality is producing enough Pu-238 for fistfuls of RTGs is beyond NASA's budget, certainly beyond NASA SMD's budget.  I think the current reality, where each couple of hundred watts of RTG is painstakingly rationed between all the hungry open mouths per decade, is likely to continue.  If NASA had the budget and the will to produce more Pu-238 (that is, once they actually do fully re-start production), they would also have the budget and will to finish up ASRG, which magnifies the power produced by each kg.

If I were the science czar, I would charge Mars missions proposing to use RTGs the full cost of producing and encapsulating their amount of Pu-238, maybe double, since there are other power options at Mars and since Mars gets so much more of the NASA budget anyway.  And since Mars' community isn't much interested in RTGs, which increases the resistance for funding more Pu-238 generation.

[hop]

Thank you all for your replies and insights.  So if a Pluto orbiter would take a boatload of propellant, could a penetrator/lander only be workable?  Have a hardened penetrator attached to a fuel tank and engine that could slow down the combo to a survivable velocity before the penetrator digs in to the surface.

NH flew past at ~13.8 km/s. AFAIK, the "high velocity" penetrator mission concepts that have been studied involve velocities well below 1 km/s (typically, a few hundred m/s like http://www.bbc.com/news/science-environment-23281423). So it really doesn't save much, and if you have the dV to get from 13 km/s to a few hundred, you might as well go for orbit.

[A_M_Swallow]

{snip}
RTGs will be necessary for power on the final science platform (probe, orbiter, lander, etc), but I think the financial reality is producing enough Pu-238 for fistfuls of RTGs is beyond NASA's budget, certainly beyond NASA SMD's budget.  I think the current reality, where each couple of hundred watts of RTG is painstakingly rationed between all the hungry open mouths per decade, is likely to continue.  If NASA had the budget and the will to produce more Pu-238 (that is, once they actually do fully re-start production), they would also have the budget and will to finish up ASRG, which magnifies the power produced by each kg.

If I were the science czar, I would charge Mars missions proposing to use RTGs the full cost of producing and encapsulating their amount of Pu-238, maybe double, since there are other power options at Mars and since Mars gets so much more of the NASA budget anyway.  And since Mars' community isn't much interested in RTGs, which increases the resistance for funding more Pu-238 generation.

The KiloPower system can use uranium–235. Leave the long lived Pu-238 for long lasting deep space missions.