Mudskipper: To Mars and Beyond

There has been a lot of talk in recent weeks and months about various schemes to get people to Mars. Some are suggestions for an orbital flight while others talk of landing and/or building a colony of between a few people and several thousand. As visitors to this site are aware, I have also thought about this — ostensibly for a novel but also practically. If we had the resources (i.e. money and will) to do it, what could we do relatively quickly, frugally (best bang for the buck, not necessarily inexpensive), and with a high expectation of mission success? My solution was the Mudskipper.

Two years ago, I posted an article wondering about this (A Spacecraft For the Future) problem. I even posted a quick whiteboard sketch. Not long afterward, I started giving the matter a lot more serious thought — and by that I mean math. Pages and pages of math. I worked on propulsion, mass, EDL (entry-descent-landing) issues, environmental protection, medical issues, etc. I read papers on the (lack of) success of growing food in zero-G. I looked at space suit design. Researched spacecraft and technologies that were at least in design stages. In short: it was a lot of fun.

After a few weeks work of research and number-crunching, I came up with a preliminary design that I called the “Mudskipper”. I didn’t post anything about it until about a year later with the preliminary article, “Mudskipper: A Mars Colony Ship“.

Since then, it’s just pretty much been sitting there while others have ramped up the rhetoric positing what form a ship to Mars will take and whether it would be worth the cost. I think it’s not only doable but likely sooner than you’d expect. To that end, let’s look at the ship.

The Mudskipper

Overview

The ship is meant to not only serve as a colonization vessel, but also a long-duration vessel capable of visiting the outer planets before finding a permanent home — presumably on Mars. It is essentially a space station that can move around the solar system. It has six modules that would be the foundation of a colony as well as two entry-capable nuclear reactors that supply an excess of electricity to the ship. It’s this excess that allows for the long-duration viability of the ship. The surplus power allows for recycling and manufacturing that have been, up to this point, very limited on the International Space Station as well as other space-based laboratories. Power provides freedom.

The ship is also designed with a high level of redundancy and the idea of “as simple as possible, but no simpler” in terms of design. The ship is also designed with the safety of the crew as tantamount. The kinds and location of materials are designed to provide the crew of 10-12 with as much advantage to survive as possible.

Of course, if you are going to offer this level of protection, it’s going to require a bit of size. Here is Mudskipper in comparison to other large space vehicles:

At first glance, Mudskipper looks huge — almost as long as a Saturn V was high, and a bit longer than the ISS is wide. Even so, much of the Mudskipper’s length is due to the long truss that makes up the tail section. Take that and the nuclear reactor shielding away, and the primary part of the ship is much more in line to what we often think of with a modular deep space craft.

Front Section

The front section is the main storage and food-growing part of the spaceship as well as the storage area for the EDL systems.

The core of the section are the central, six-sided Main Module as well as the five habitation modules (HABs) placed on five of the Main Module’s faces. The top, sixth face of the Main Module is a work area for spacewalkers.

The Main Module is a solid module that gains extensive shielding with the modules on its perimeter providing significant additional protection. While in space, it is the “lifeboat” where crew will gather during a major space-environment event (e.g. coronal mass ejections) and will also serve as the initial base of a landed colony.

Because of its shielding, this module also hosts the primary computer(s) as well as communications equipment. It is built primarily for maximum functionality and protection with long-term full-crew comfort pretty much ignored.

The Main Module also provides the skeleton to which all other parts of the front section attach. Extending from its vertices are radiator panels/shields which provide protection to the HABs. From these panels project out spars which hold both the in-transit nose shield as well as the EDL heat shields.

Between the modules and the heat shields are descent modules that will provide for a period of powered descent to the modules. These can be either LEM-style modules or Curiosity-styled skycranes, though LEM modules are the simpler solution.

Ringing the tops of the HABs are the EDL parachute systems.

The Habitation Modules are heavily influenced by the inflatable modules produced and preliminarily tested in-orbit by Bigelow Aerospace.

Each of the HAB modules is 7 m (22.9 ft) long and has a 3.25 m (10.7 ft) radius — giving a maximum volume of about 232 m³. In practice, once the shell is lined with water, the usable interior volume will be 184 m³. Overall, the module is about 6 m shorter than the Bigelow BA 330, with the inevitable reduction in volume. The main structural support is an axial, central hexagonal truss cage that is about 1.7 m (5.7 ft) across. The truss is attached on either end of the module to identical double hatches. A near-semicircular EDL/tool storage compartment arcs around the inboard hatch.

The shell fabric will be situationally more durable than the Bigelow shells. They need to be able to allow the contents (protected with expanded foam) to survive Mars EDL though don’t necessarily need to survive completely intact — just well enough to avoid catastrophic burn-through.

The interior will be lined with two layers of 30 cm long sealed tubes of water. This serves as both long-term water storage as well as radiation shielding. The small tube size is designed to minimize water loss due to puncture.

The expected full-up configuration will have three HABs fully stocked with supplies — emergency consumables as well as raw materials. The other two HABs will be primarily tasked with growing food: yeasts, algae, possibly insects, as well as some fruits and vegetables. As a side benefit, human waste will be partially processed by the biologic systems.

The HABs all have limited self-contained air processing to support a small number of the crew possible trapped in the modules should a pressure differential result in the hatches closing automatically. These systems and optionally others will be attached to the central truss, with the central area kept free for human access.

The Mid Deck

The middle section of the Mudskipper is composed of three main elements connected by a central tunnel: next to the HABs is the Docking Petal which connects the HABs with the ship; the Work Module where most manufacturing/repair is done as well as main water storage tanks and other raw material storage; the Rotating Section with Crew Pods, and the Aft Airlock and suit storage.

The Docking Petal provides a spoke of passages which connect all of the HABs to each other and which feed into the central tunnel that runs from the Main Module through the other sections until reaching the Aft Airlock. It’s designed for access as well as to provide means to close off HABs when necessary.

The Work Module contains the 3-d printers and chemical reclamation/manufacture areas as well as airlocks which open to the top of the Work Module as well as onto the deck on the top of the Main Module. As people who saw my preliminary specs can attest, 3-d printing was always meant to be the primary means of supply. The printers would be in small centrifuges, if necessary, to allow for proper functioning. This section also provides the creation of various fibers and cloths necessary for repairs of shells, clothing, cabling, and spacesuits — including loom and sewing stations. A protected area is also available for the machining of metals.

The bottom of the Work Module holds the extendable Observation Dome. This provides not only large viewports, but also a variety of telescopic/scientific instruments.

The Rotating Section is the most dramatic addition to the spacecraft’s design. It rotates at 4.5 rpm to provide a 1 G environment for the pods at the ends of its 45 meter spokes. The spokes are shielded tunnels with internal ladder rungs which are encased in a triangular truss structure. Each well-shielded pod is designed to nominally hold four crew member bunks. The rotating section also has a contra-rotating flywheel to help nullify the angular momentum of the spoked section.

In the interest of saving mass and reducing the stress on the spokes, the crew pods are as small as possible, each nominally holding only four crew members. The main reason for the crew pods is to provide a sleeping area where the crew can passively live in a 1-G environment for a significant period each day in order to help stave off various medical effects of prolonged life in an otherwise zero-G environment. It might be an option for some exercise equipment to be installed in a pod to take advantage of the 1-G boost.

At the rear is the Aft Airlock. This is a large spacewalk prep area, suit storage, isolation area, as well as a double-hatched airlock that can open into either the top or bottom sections of the tail truss. This section allows for decontamination of spacewalking crew, emergency medical needs, as well as supplies and sanitation should the spacewalkers be detained-in-place for a while.

The Tail Assembly

The back end of the Mudskipper is defined by its long truss as well as the engines and reactors and their shields at the far end of the ship.

The Main Truss is a structure of two triangular trusses, the interior of each centered on the hatches on both the Aft Airlock as well as the Engine Access and Shield Structure. Spheres of fuels, liquids, and liquified gases are attached down the length of the truss. Most of these spheres are 1.3 m (4-1/4 ft) in diameter — just small enough to fit through the ship’s hatches — though the fuel spheres are as large as practical. In practice, the truss and spheres are covered in an EMI-reflective, opaque fabric so that they are constantly shaded, thus reducing the need for refrigeration of the cryogenic fluids (those needing to be warmer can be warmed via waste heat from power generation).

At the end of the truss is the large engine housing that is flanked on either side by the large  receptacles that hold and shield the nuclear reactors. The reactors themselves are housed in their EDL shells.

Primary acceleration comes from a set of ten VASIMR-NG electro-propulsive engines. (These are a speculative next generation version of the VASIMR VX-200 which would up the thrust for each engine from 4 N up to 6.5 N.) Total thrust would be in the area of 65 N. More engines could be designed in.

The Nuclear Reactors would be of some variant of a traveling wave reactor or other fission reactor not requiring refueling or inconvenient cooling/containment methods. Each reactor should have a nominal output of at least 250 MW. Thus each reactor on its own is able to power the ship, including the engines, for a successful mission, but two reactors allows for redundancy, especially for the critical EDL phase when they are most at risk. The excess power is what increases the odds for mission success as it can be used for breaking down and assembling compounds necessary and/or useful for crew, ship, and colony long-term sustainability.

The VASIMR engines are supplemented by conventional chemical engines using hypergolic fuels. The chemical engines are used for emergencies as well as initial thrust and possibly orbital insertion. Because of limited fuel storage, they cannot be used for long, but are very useful for gross accelerations, especially when exiting a gravity well.

The engines and the reactors are accessible via the hatches in the Engine Access and Shield Structure. This is so the crew can operate on the equipment without unnecessarily risking falling out into open space (the Mudskipper isn’t nimble enough to come back and get you).

Entry, Descent, and Landing (EDL)

When it comes time to colonize, the ship will have to be dismantled as much as possible. Preparations will be done using the ship’s two robotic arms as well as several spacewalks to configure the modules destined to go planetside.

The first step, once in orbit, is to drop down a locator beacon  or two that all other components can home in on and, ideally, position themselves in relation to. All the colony modules also have beacons, so if everything goes as planned, all modules should land in reasonably safe proximity to each other. The goal is to have the HABs and Main Module land in a cluster while the reactors will be a safe distance away but still close enough to string cables without having to move the reactor.

If both reactors are working, one will be configured for EDL. This means disconnecting power lines, closing the EDL shell, separating the shell from the ship, and at the appropriate time engaging the EDL procedure. What happens afterward depends on the landing. If the reactor did not survive the skycrane-style landing, the crew must make a decision whether to risk the second reactor. Generally this would be “yes” if the chemical rocket fuel is sufficient for an emergency return to Earth should the second reactor not survive.

Before a second reactor is landed on the planet, two HABs will be prepared for landing: one supply and one food-generation (though all will have the supplies necessary to continue food growth). Water will be pumped into empty, non-toxic spheres and loaded into the HABs along with critical gases. When the HAB is packed full, the hatches will be closed and the voids filled with foam (similar to urethane packing foam, but non-toxic). The descent modules will be attached and those covered with heat shields. The descent parachutes will be attached and activated.

If neither a reactor or HABs have successfully landed at this point, it is likely the crew will return to Earth. Should they decide to continue anyway, they would now attempt to land the second reactor successfully. If unsuccessful, they will likely abort to Earth. Otherwise, the remaining HABs will be EDL’ed to the general landing zone. Finally, the crew will attach the large descent module and heat shield to the Main Module and proceed with EDL.

If the crew successfully lands they will assess their best location to establish the colony. This largely depends on the relative distance of the Main Module to the nearest working reactor and how well the modules fit into the ideal landing goal. If the Main Module is the outlier of all the modules in regards to the reactors, it is likely that it will be dismantled and moved to a better locale. Otherwise, the nearest working reactor will be towed/trailered if necessary, via rover, to a place close enough to the Main Module to supply power. The crew will then start constructing a permanent base using local materials for shielding while also gathering the supplies in the HABs, as well as towing in the other reactor if it is also working.

Additional Thoughts

First of all, yes, it looks very phallic and the reactors look very mammary. Of course I can see that. It wasn’t intentional. In fact, as you can see from the early sketch at the start of the story, I was trying to avoid it. Still, sometimes you have to allow for some adolescent giggles when making a design. Incurring more mass or reducing safety just so that it was aesthetically bland didn’t make much sense.  A functional change would be fine, however.

The idea of the Mudskipper is to allow for long-duration human survival without expectation of ever returning to Earth or even resupply. Instead of only being a colony ship, it is meant to be an explorer to at least one other planet en route to founding a settlement on Mars. To that end, there are redundancies and contingencies with all major systems. For the most part, you can essentially lose half the ship’s functionality and still complete the mission — but if you don’t lose that, the mission is more robust.

The key is that, because you have energy being pumped into the system, you can have a closed environment with maximum ability to recycle: air, water, nutrients, fabric, tools, food, chemicals, and even some medicines. If you have the time, you have the energy to power machines able to build at a nano- if not molecular-scale. This means much of the stored items are raw materials awaiting transformation into finished products on an as-needed basis. This increases supply density considerably.

The ship builds on a variety of current and emergent technologies. Looking through much of the recent proposals, you can see similar lines of thought. Clearly the inflatable module is central to many of these ideas. I think we need to tip our hats to Bigelow Aeronautics  for reminding us that it’s not bad to think outside the established norms. In fact, the polymers of his inflatable modules seem to be better suited to long term habitation than our traditional hard-bodied craft. We won’t know for certain until a Bigelow module is docked to the ISS for further testing, but for now it seems promising.

The estimated mass of the fully-loaded Mudskipper is 500 T (500,000 kg ≈ 1.1 million pounds — which is only slightly more than the ISS) and produces 65 N of thrust. In comparison, the ion-drive propelled Dawn space probe that has visited Vesta and is en route to Ceres has a mass of 1,240 kg and a thrust of 90 mN (i.e. if it were the same mass as the Mudskipper, it’s thrust would only be about 36 N). It’s about as maneuverable as the Titanic hauling cement anchors behind it, but given time, and a fair amount of math, it’ll take you were you want to go.

The reactors are the major stumbling block to the entire enterprise. So far as I can tell, we’ve never put fission reactors with that kind of power output into space. To power electric engines with enough thrust to push around a large colony ship around in deep space, the only option is a large power supply.  For the foreseeable future, that means some sort of fission reactor.

Obviously, this is meant to be a one-way trip. Once you are on Mars, you are there to stay. With a ship like Mudskipper, you have a system that enhances the odds of not only survival but thriving for decades to come. As designed, the reactors should give peak power for about thirty years with reducing amounts over time. The same can probably be said of the crew. As a technology platform, Mudskipper-class vessels could start to expand the reach of humans throughout the solar system. It’s something to consider. Sometimes the wisest choice is to skip the cautious small step and instead go for a confident stride.