Hypography X Prize Entry

[CraigD]

Originally Posted by Roadam

Well why do you all think that heat from the motors will be such a problem?

I think I may have brought up that concern in post #106. My concern is based mostly on experience with small (1:10 - 1:28 scale) model cars and trucks. These motors get very hot – > 100° C, I’d guess from the times I’ve singed my fingers on them – and perform best when attached to large finned heat sinks (which only work well with air flow over them). They also have tremendous power to mass ratios – a good brushless RC car or aircraft motor massing .052 kg can draw 200 W of power, and produce much (80%+) of it as mechanical output – and are can be very small – eg: a cylinder .027 m diameter by .032 m length.

If one of these little motors were run stalled (no movement) at full power in a vacuum, and it were a perfect back body radiator, it would reach thermal equilibrium at greater than 900 K, close to the melting point of aluminum. As these motors are not perfect black bodies, the actual temperature would be greater – in short, the motor would melt.

None of this is a show-stopper. It only means that the motors on a lunar rover can’t be as powerful and/or as compact as those on a terrestrial RC vehicle. For example, unstalled and running at 10% max power, they’re black body equilibrium temperature would be only 345 K – barely hot enough to burn exposed human skin, let alone damage metal or plastic. I don’t think a complicated radiator cooling system will be necessary for the <10 kg rovers we’re envisioning.

The 210 kg Apollo lunar rovers were powered by 4 motors about as powerful as the above examples (attached to 80:1 reduction gears), but much larger – I’d guess about .1 m diameter by .075 m length, which would have given them a full-power stalled black body thermal equilibrium temperature of about 500 K. They had thermostatic switches that would cut off power when the motors exceeded about 530 K. I don’t recall that this safety system ever tripped – I suspect the rovers were never run flat-out in a jammed condition for extended periods – or at all.

Having done these rough calculations, I’m now more worried about COTS remote control receiver electronics than about motors – and eager to try putting one of my several cheap, tiny toys into a decent vacuum chamber to see what happens to it. Back-of-the-envelope, or even detailed engineering calculations are nice, but there’s no substitute for testing!

Sources:

• cyclonBrushless,cyclon Brushless for examples of COTS RC model motors
• wikipedia article “Stefan-Boltzmann law” for the black-body temperature calculations
• this NASA’s Lunar Rover page for rover information (including a link to a cool scanned copy of the LRV operations handbook )

[Turtle]

Originally Posted by Jay-qu

Excellent thanks for the info Janus!

Assuming we go with the russian option.. 673kg to lunar orbit should be more than sufficient. How do we go about calculating the mass we could deliver to Moon surface? this will place our limit on the rover/lander mass.

Agreed & agreed. Pretty hard to calculate without particulars but we can make some guesses. Suppose 2/3 of the 673 kg in lunar orbit is structure other than rovers, this gives us 222 kilos of rovers, or 55 kilos per rover if we have 4. (Mars rovers are ~174 kilos for comparison) Gives us an order of magnitude in breathing room over BigDogs original proposition. If there are no smaller rockets available, then not using the rated load is a waste, yes/no?

Not only do we need launch windows for the best trajectory to lunar orbit (perihelion best?), we have a less than 2 week window to land once there as we likely need to land in the daylight. This may be one advantage to going into lunar orbit?

Originally Posted by freezter

The Moon's gravity is 1/6 that of Earth, so:
5kg*1/6=5/6kg=0.8333kg

Perfect! That is ~2.5 pounds which is the bare weight of the vehicle. I'll conduct some speed vs. terrain tests as well as battery life (the amp/hr rating is marked on them conveniently).

In reading at the link for the mars rover manual I gave (not an official site I now see), they gave very specific reasons for using the 6 wheel "Rocker-Bogie Mobility System Components" arrangement, i.e.

Originally Posted by Rupert Scammell

The RB system permits each wheel to independently conform to uneven terrain, allowing the rover to traverse obstacles twice the diameter of the rover's wheels.

MER Technical Data

We might consider copying the design for all the right reasons. Any body know a page with more specs on the mars machines?

[TheBigDog]

I am guessing that only about 10% of the lander mass will be available as rovers. Much of the mass sent into lunar orbit would be lost as fuel for the landing, and as the rocket system used for the landing.

Let us assume that we send 673 KG of mass into lunar orbit, how much mass is required to slow and land on the moon? How much structure is required for the lander and its communications equipment, computer, batteries, and solar cells? How much mass remains for the four rovers, including how they are attached to the lander, and deployed from the lander. Since we are having diversity in our rovers, we may as well have diversity of size on the table. It adds to the research aspect of the mission. Once we find out the total mass we can have, we then horse-trade among the rover teams for allotment of mass, and see how it drives our designs.

I think that Craig is on the money about needing to do real world testing (as opposed to real lunar testing). We should form an environmental testing team. Any volunteers for that?

Bill

[DougF]

Here is a site that has good information that you might be able to use.

Rover mission analysis and design - Wikiversity

[freeztar]

I found this site that deals with dust experiments as well as solar cell longevity:
NSSDC Master Catalog Display: Experiment

[Roadam]

It looks just like we hit the hard wall as no one has posted for several days.

Idont think that copying mars rover design is to any good for this project. For one thing it greatly complicates things, and maybe isnt really needed after all. One of the goals in this xprize is to snap photos of older manmade things on the moon. So why not try to land near one apollo lander which is standing on reglith plain. There would be many things to picture and as far as I saw from apollo pictures, things arent so rocky that our rover would have to climb over.

[TheBigDog]

Originally Posted by Roadam

It looks just like we hit the hard wall as no one has posted for several days.

Idont think that copying mars rover design is to any good for this project. For one thing it greatly complicates things, and maybe isnt really needed after all. One of the goals in this xprize is to snap photos of older manmade things on the moon. So why not try to land near one apollo lander which is standing on reglith plain. There would be many things to picture and as far as I saw from apollo pictures, things arent so rocky that our rover would have to climb over.

I wouldn't say we hit a wall; we are just at the beginning. The official rules were supposed to be released on Thursday, but I have not seen them posted on the official site yet. In the mean time we have so math to do. Janus had mentioned that we could get 673 KG to lunar orbit. Of that, how many KG could we softly land on the moon? That gives us the mass for the lander and rovers.

I am going to build a rover. I am using Radio Shack as my outlet for all parts if I can manage. It will plastic rather than aluminum, but it will be my prototype rover. Once I have a prototype that I am happy with I will begin the process of conversion to moon safe materials. I will document the process online.

Bill

[Janus]

Originally Posted by TheBigDog

I wouldn't say we hit a wall; we are just at the beginning. The official rules were supposed to be released on Thursday, but I have not seen them posted on the official site yet. In the mean time we have so math to do. Janus had mentioned that we could get 673 KG to lunar orbit. Of that, how many KG could we softly land on the moon? That gives us the mass for the lander and rovers.

I am going to build a rover. I am using Radio Shack as my outlet for all parts if I can manage. It will plastic rather than aluminum, but it will be my prototype rover. Once I have a prototype that I am happy with I will begin the process of conversion to moon safe materials. I will document the process online.

Bill

Doing some research and some numbers, here's some rough estimates.

Using "off the shelf" rocket motors from Thiokol, the people who produced the RAD rockets for the Pathfinder mission:

Using a STAR 20 engine, we kill most of the orbiter's orbital motion.
We discard the used engine (28 kg) and its superstructure, say, 50 kg.

This leaves us with 325 kg after accounting for the fuel used by the STAR 20.

From here we use the Pathfinder landing procedure sans parachute. At the proper altitude (using a radar altimeter), we lower the main landing package by a tether attached to the RAD (rocket assisted descent) section.

Inflate the air bags

Using 6 STAR 8 engines (pathfinder used 3, but we don't have a parachute to help), we kill the remaining velocity just a little bit above the surface.

We release the tether and let the landing package drop to the surface, keeping the impact velocity under 100 kph (about the limit for air bags.)

6 fueled STAR 8's mass 105 kg, asuume another 50 kg for mounting super-structure, and this leaves us with an estimated 170 kg soft landed, minus the mass of the airbag system.

[TheBigDog]

What if we go Apollo style instead of airbags and rocket straight to the surface? There is a randomness to the airbag landing that really bothers me.

Bill

[freeztar]

Originally Posted by TheBigDog

I am going to build a rover. I am using Radio Shack as my outlet for all parts if I can manage. It will plastic rather than aluminum, but it will be my prototype rover. Once I have a prototype that I am happy with I will begin the process of conversion to moon safe materials. I will document the process online.

Awesome Bill! I admire your enthusiasm!

I look forward to your progress.

As far as the weight issue is concerned, I think the first step is to calculate the amount of fuel and type of rockets needed for the LM to make a soft landing, If indeed we plan on using rockets to slow and stabilize the descent. Specifications for the type of transmitter we will use will also be helpful in figuring our weight requirements.