Landing on Mars

[james]

Well firstly I know this section is ment for Apollo, but I wasn't sure where else to post this as there is no 'general space' section. A mod can feel free to move this thread to where ever they see fit.

I have a question about landing on Mars. I'm working on a little, just for fun project, for a trip to Mars. I've been Mulling over in my head, what would be the best way to land on the surface. I'm trying to keep it fairly realistic as best I can. Basically the plan is having the main ship in a LMO (Low Mars Orbit) and the lander would leave from there. The two design options I see are a LM styled lander. Something that has landing legs and all that, but of course it would need a heat shield though. The other would be a winged lander. Something that could glide within the Mars atmosphere. It would need some decent sized wings and control surfaces since the Atmosphere is fairly thin. It would also have to be able to land and take off vertically as well. I envision the design to be able to carry 4 people, some equipment and supplies and a small two manned rover to drive around in. I'm just not sure what design would be best.

Another part of my question is about the atmosphere itself. Since the Martian atmosphere is around .7kPa. Would that mean one wouldn't need that much heat shielding? Or if one had a heat shield that was designed to handle an Earth re-entry, would it allow one to do multiple re-entries on Mars?

That's all I can think to ask for now. I'm trying to figure out as much as I can on my own, but I'm no engineer, so I'm sure I'll have more questions.

[Bob B.]

I got involved in a pretty lengthy discussion about this at thespacerace.com forum a couple months ago. Here is the thread:

www.thespacerace.com/forum/index.php?topic=1227.0

We investigated several methods for landing on Mars and concluded the following would be the most efficient:

There would be one surface habitat module, one space habitat module, one surface access module, and one Earth reentry module. There would also be two propulsion units, one for TMI (trans-mars injection) and the other for MOI and TEI (mars orbit insertion and trans-earth injection).

After TMI the first propulsion unit would be jettisoned and the remaining stack would coast to Mars. Upon arrival, the surface habitat module would separate and perform a direct entry and descent to the surface. The other components would be inserted into an orbit around Mars. The crew would transfer to the surface access module (similar to a LM but with a heat shield), descend to the surface, and land near the habitat. After spending over a year on Mars, they would takeoff in the surface access module’s ascent stage and rendezvous with the orbiting habitat. The ascent stage would be jettisoned and the habitat injected into a trajectory back to Earth. After TEI the propulsion unit is jettisoned. Upon arrival at Earth, the crew transfers to the reentry module and performs a direct reentry, descent and landing (no orbit insertion). The space habitat either flies by Earth into solar orbit or harmlessly disintegrates over the ocean.

Regarding the heat shield… The heat shield not only protects the vehicle from the heat of atmospheric entry, it is also the surface that generates the drag needed to slow down the vehicle. Most of the vehicle’s velocity is lost to drag, or at least that’s what we want. Since Mars’ atmosphere is so much thinner than Earth’s, we require a heat shield with a much larger surface area to generate the required drag. If we used a small heat shield like that on the Apollo capsule, we’d lose very little velocity to drag and we’d end up crashing.

I suspect the most likely landing sequence is to: (1) use a large heat shield to slow us down as much as possible to permit the use of parachutes, (2) deploy parachutes to increase drag and slow us down as much as the thin atmosphere will allow (to about 100 m/s), and (3) use rocket propulsion for the final descent to the surface.

[echnaton]

These are my thoughts and an actual landing craft.

To me the requirements for landing on Mars call to mind a cylindrical ship with crew habitat areas around the outside. Like the LM, the ship must serve as the launch pad for the return trip. That return rocket would need to be more like an earth launched rocket and be mounted in the center of the cylinder. It would house the astronauts in cramped quarters while landing, serve as the abort system, and have the ability to control the landing and navigate back to the orbiter. The outer portion would be accessible through a hatch from the return rocket and contain the landing and maneuvering systems, as well as the habitat, equipment, shielding and power needed for a stay on the surface.

Landing would be made by using a heat shield to get into the atmosphere then parachutes to make the landing. Since the wind on mars can blow quite fast the parachute will need to be steerable and some forms of thrusters could be used to further maneuvers the ship to a landing spot and null out the effects of the wind while getting close to landing. The touchdown would use a Russian dry land technique. I seems likely that any landing area would have to be preselected based on onground observations by a robotic lander with a guiding beacon in place.

[Bob B.]

echnaton said:

Landing would be made by using a heat shield to get into the atmosphere then parachutes to make the landing.

I believe you'll need to use some sort of propulsion for the final part of the descent. I'm pretty sure the air is too thin for parachutes alone to sufficiently slow the craft for a soft landing.

[echnaton]

bob said:

echnaton said:

Landing would be made by using a heat shield to get into the atmosphere then parachutes to make the landing.

I believe you'll need to use some sort of propulsion for the final part of the descent. I'm pretty sure the air is too thin for parachutes alone to sufficiently slow the craft for a soft landing.

I wasn’t clear on that, but it is what I meant by a Russian dry land technique. Using rockets to slow down for the final touch down.

[Bob B.]

echnaton said:

I wasn’t clear on that, but it is what I meant by a Russian dry land technique. Using rockets to slow down for the final touch down.

Do you mean like on the Russian Soyuz? Those rockets fire for only a second or so within the last couple meters before touchdown. I don't know how fast the capsule is moving when the rockets fire, but I doubt it is more than 10 m/s. I don't think parachutes on Mars can realistically slow a vehicle down that much.

I was referring to something more like the technique used by the Viking landers. I don't remember the numbers exactly, but I believe the parachutes slowed the vehicle to about 100 m/s at an altitude of around 1,500 m, at which point the engines ignited. Slowing from 100 m/s to 0 m/s over a distance of 1,500 m requires an acceleration of 3.33 m/s² for 30 seconds, plus we must counteract gravity at 3.73 m/s², so the total acceleration is 7.06 m/s² for 30 seconds. This means the propulsion system has to provide a delta-v of at least 212 m/s.

[echnaton]

As to the required altitude of any maneuvers, I haven’t a clue. I will certainly accept the conclusion you draw from your math. It sounds quite reasonable. I see why the parachute would have to be ejected well before landing. Otherwise as the rocket slowed the chute would either fall and cover the lander or worse catch a side wind an drag the lander off center.

[Bob B.]

echnaton said:

These are my thoughts and an actual landing craft.

To me the requirements for landing on Mars call to mind a cylindrical ship with crew habitat areas around the outside. Like the LM, the ship must serve as the launch pad for the return trip. That return rocket would need to be more like an earth launched rocket and be mounted in the center of the cylinder. It would house the astronauts in cramped quarters while landing, serve as the abort system, and have the ability to control the landing and navigate back to the orbiter. The outer portion would be accessible through a hatch from the return rocket and contain the landing and maneuvering systems, as well as the habitat, equipment, shielding and power needed for a stay on the surface.

In the thread I referred to in my first post, several people discussed different scenarios for landing on Mars. We found that how to best configure the various modules depended largely on whether we planned to perform a direct descent or go into orbit first. Two simple rules dominated the decision making: (1) don’t land on the surface anything needed only in space, and (2) don’t put into orbit anything needed only on the surface. Although these rules may seem self-evident, it is sometimes easy to lose sight of them when kicking around different possibilities. Even seemingly small violations of the rules can result in large consequences.

Echnaton, if you plan to land your habitat and crew together via direct entry and descent (without first going into orbit), then the module you propose is, in my opinion, definitely the best way to go. The only thing you need on the surface is the habitat and the ascent module, so don’t waste the fuel to put it into orbit. Put your space habitat*, earth reentry capsule, and propulsion unit into orbit since none of that is needed on the surface (it is cheaper to go into orbit than it is to launch from the surface). You can save much propellant by initially placing these components into a long elliptical orbit and then slowly aerobraking to lower orbit while the crew explores the surface.

* Unless you plan to manufacture propellant on the surface, it is definitely more efficient to have separate space and surface habitats. Having a single habitat means you must boost it from the Martian surface back into space. This more than negates the savings of having just one habitat.

Those of us in the other thread had some reservations about landing the crew via direct descent. You have only one shot at a high-velocity entry with no opportunity for postponement. Maybe this is not cause for concern, but we were a little frightened by the prospect and therefore decided to put the crew into orbit before attempting the landing. This necessitates some violations of my previous rules, and is therefore less efficient than a direct crew landing, however we thought it gave some margin for safety and better abort options.

One idea was to land the surface habitat and ascent module, similar to echnaton description, unmanned. The crew would go into orbit with the addition of a small descent module that would take them to the surface. The problem with this, however, was that the descent module would only have enough propellant to land, thus there was no abort option.

We thought it better to land via direct descent the surface habitat only. The crew would then go into orbit with the addition of a LM-style descent/ascent vehicle (but with a heat shield and parachutes, of course). This method doesn’t have its disadvantages though. First, we have to carry enough extra propellant to put this descent/ascent module into orbit (violating rule #2). Second, we have to perform an extremely precise landing within range of the habitat.

We tossed around other scenarios as well but decided this one gave the best combination of efficiency and safety. Of course none of us were real rocket scientists so you have to consider our opinions accordingly. Let me also note that all the scenarios we considered assumed propellant would be carried from Earth. If propellant is to be manufactured partly from the Martian atmosphere, then all bets are off and we have to reconsider all possible scenarios.

[freon]

Has anyone ever considered a helium balloon technique to slow the final moments of the descent. Or is that just too crazy. I always wondered why they haven't encorperated balloons/derigable technologies in the crafts sent to mars. Could inflate and deflate to ascent and descend to cover more ground and views at different altitudes.

[Bob B.]

freon said:

Has anyone ever considered a helium balloon technique to slow the final moments of the descent.

The martian air is so thin I doubt a balloon would be very effective. I suspect a ballon might be able to float a small instrument package after landing, but as a means for slowing the descent of a lander, I don't think it would work. When I get a chance I'll check the math on this.

[Al Johnston]

I seem to recall a Russian plan for a balloon to explore Venus, but I'd think that the Martian atmosphere would be a bit thin for ballooning: particularly as some of the more interesting features, like Olympus Mons, are above what atmosphere there is.

[gwiz]

alasdhair said:

I seem to recall a Russian plan for a balloon to explore Venus.

More than a plan. They deployed two of them in March 1986. They were battery powered and lasted the intended two days, in which time they were blown thousands of km round the planet.