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Saturday, March 15, 2014

Moho? More Like MoFo You Mean!

The next mission I've set for the Kerbals is to land a probe on Moho and return it to Kerbin. This is no easy feat. In fact, it is harder than getting to Jool and even Eeloo. Jool has a large sphere of influence (SOI) and is hard to miss. It also has a very thick atmosphere that lends itself to the best aerocapture and aerobraking in the system. If you're even half way decent at those maneuvers, you'll get into orbit. Eeloo has no atmosphere and requires retrograde thrust to get into orbit. Eeloo's gravity is only slightly more than Mun's, so it is relatively easy to modify a Mun lander to work on Eeloo. And Eeloo orbits Kerbol at just over 4000 meters per second making it the easiest planet to "sneak" up on and get into orbit around. Eeloo orbits Kerbol at just over 4000 m/s. That makes all your velocities, both there and back, at least manageable.

Now let's compare that with the speed of Moho. Moho whips around Kerbol at over 12,000 m/s at apoapsis. At periapsis it's traveling at a blistering 18,000+ m/s. Any probe hoping to have any chance of catching Moho must travel at those speeds. That'll peel the special space paint right off your parts! It's a hard mission no doubt and I hope the Kerbals are up to it.

To begin this mission, the Kerbals designed a new advanced probe. The old stacked design had a couple of issues. It had a high center of gravity and needed a heavy base to be stable. It also had a bad habit of coming apart at the joints when the landing parachutes fully deployed. There is nothing more frustrating than a successful mission undone in the last few minutes because your probe doesn't survive the sudden jolt needed to bring it home safely. So with this in mind, the Kerbal scientists began designing a more squat probe with a better landing system. Here's what they came up with.

[caption id="attachment_1945" align="alignleft" width="300"]Advanced Probe Advanced Probe[/caption]

This probe has an inline RC-L01 Remote Guidance Unit as it's brain, powered by 4000 kilowatt Z-4K inline battery bank, which in turn is charged by two Radioisotope Thermo-electric Generators.  It has 200 units of mono-propellant for it's RCS maneuvering system. The RV-105 thruster blocks are installed at the probe's center of gravity for maximum efficiency. Total mass is 4.395 tons.

To address the issue with sudden deceleration causing structural failure, there are four Mk25 "Drogue"  parachutes. These open at a higher altitude than standard parachutes and have less drag fully deployed lessening the negative g-forces on the probe at deployment. If you look at the staging for the probe, you'll see we've programmed the parachutes to open in three stages. We've tested this system many times and have found it does quite well. We deploy the first stage chute just below 10,000 meters so it can aid in slowing the probe. It fully deploys at 2500 meters. At deployment, the probe's velocity is just under 120 m/s and drops to around 40 m/s once the chute is fully deployed. It is then safe to deploy the second chute. This will decrease vertical velocity to around 14 m/s. This is still too fast for most parts to withstand landing however. Therefore we have two more chutes we deploy below 500 meters. This drops the velocity below 8 m/s. Most parts tolerate this well. If you want to land at an even slower velocity, you can make your last two chutes standard chutes without risking deceleration damage.

[caption id="attachment_1946" align="alignright" width="300"]Moho Lander Moho Lander[/caption]

We can mount the probe on a variety of lander modules. For instance, here's a lander/return stage we figure would get the probe down to Moho and back into orbit. It has a total of 3144 m/s delta-v in vacuum. The escape velocity for Moho is approximately 1200 m/s. Double that to get a quick and dirty idea of the delta-v needed to land and take off again. That leaves us with a comfortable margin of just over 700 m/s. Of course, that in no way gives the probe enough delta-v to affect a transfer back to Kerbin. But we are still in the planning stages here, running simulations and seeing what is possible. We'll worry about a return stage later. It's the easy part.

Let's discuss the hard part, the transfer from Kerbin to Moho. As I mentioned above, Moho is fast. Damn fast.  That makes it difficult to catch. According to the KSP Wiki, it takes 1676 m/s delta-v from a low Kerbin orbit (70 km) just to get into the vicinity of Moho. But that's far from all it takes. Moho has a very small SOI. It's a little bullet going very fast. Unlike Jool, it is easy to miss. As I've mentioned before, it's best to complete a refinement burn while on the way to any other planet in the Kerbol system. This will ensure you arrive within the SOI of your target. To the left is the refinement burn for Moho according to my simulations.

[caption id="attachment_1948" align="alignleft" width="300"]Moho Approach Burn Moho Approach Burn[/caption]

That's a LOT of delta-v! It's three quarters as much as we needed to transfer. But without that burn, we'd miss Moho by a Kerbal country mile. The final Moho periapsis of the new trajectory is 50 km. Before it was in the millions of kilometers. Our transfer stage now has to have at least 2970 m/s delta-v. But wait, there's more!

[caption id="attachment_1950" align="alignright" width="300"]Moho Orbital Insertion Moho Orbital Insertion[/caption]

Take a look at how fast the probe is travelling in this simulation. It will easily top 12,000 m/s by the time it gets to intercept, and probably considerably more. So what's it going to take to get into a 50 km orbit of Moho from that velocity? Here's the answer for you (look right.) It will take another 3660 m/s delta-v to get into orbit. That's way more than needed just to get this far. It's more than needed to land and take off from Moho.

So what can deliver the total delta-v needed, all 6630 m/s, in an economical package? Many (most?) people would immediately shout, "Nuclear Engines!"  The Jool mission had a nuclear transfer stage of 4403 m/s delta-v. We can certainly design a nuclear transfer stage with enough delta-v. But, I'd quietly reply, "Think again." Remember what I said about Moho being a very small, fast bullet with a dinky SOI? At 12,000 m/s that SOI is even smaller than you think. How long does it take a nuclear engine to affect a 3660 m/s change in delta-v. The answer is too long. Have a look at the other picture to the right. See what happens when you use nuclear engines for an orbital insertion of Moho the normal way (half delta-v before and half after the maneuver node?) The probe goes right the hell out of your insertion window and ends up with a horribly useless "orbit."

[caption id="attachment_1952" align="alignright" width="300"]Nuclear Engine Moho Insertion Nuclear Engine Moho Insertion[/caption]

So we'll have to use rockets to get into orbit, there's no two ways about it. And that means more fuel. More fuel means a bigger lift stage just to get it into orbit around Kerbin. We've been working on the lift stage. We've got one that will get the entire payload into a 75 km orbit of Kerbin. It isn't enough. I personally like to start planetary missions in a 650 km orbit for reasons of time dilation and OCD, if I must be completely honest about it. Such a lift stage is dauntingly huge. In fact, it's probably too huge.

That leaves us with a few decisions to make. We'll need to take the mission up piecemeal and assemble the transfer, orbit and lander stages in space. We may have to fuel those sections in space and that means docking practice. That will take time, but it is technologically feasible now. While we were testing the new Advanced Probe, the clever Kerbal scientists were also collecting data with it's full suite of instruments. "You know, just to make certain they worked," I was told. The result is the Kerbals have now researched every stock technology possible. I guess I owe them that weekend long party after all. It took them only a quarter century. Those guys are good!

[caption id="attachment_1954" align="aligncenter" width="2048"]All Stock Science Researched All Stock Science Researched[/caption]

1 comment:

  1. […] my last post about Kerbal Space Program (KSP) titled Moho? More Like MoFo You Mean!, the Kerbals and I have been trying to work out the capabilities needed for a ship sent to Moho […]

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