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dodecahedron
2009-Jan-28, 06:26 PM
I was wondering how feasible it would be to actually have a weekend and vacation project, over the course of a decade, where someone makes a probe and sends it to Mars. I figure the hardest bit would be getting a launch vehicle and trying to put ten pounds of manure into a five pound bag.

mugaliens
2009-Jan-28, 07:44 PM
What's the mission? Fall planting?

stktos
2009-Jan-29, 01:30 AM
Say someone did do that. Make a rocket and do all the math to get a probe or something to Mars.
I would not think NASA or the government would like it very much.
Probably stop you before the launch.
You would have to keep it crazy secret I would think.

dodecahedron
2009-Jan-29, 02:43 AM
I would not think NASA or the government would like it very much.

Why?


What's the mission? Fall planting?

Eh, hard landing, take a few digital pics and post them on the interbutt for bragging rights.

stktos
2009-Jan-29, 03:06 AM
Why?

All the trouble NASA takes to make sure they don't seed Mars with organisms from Earth. For example.
They would not want someone just sending up a home built rocket/probe to Mars and not realizing they just sent up some bacteria, or other organism, that would take a foothold on the planet.

I'm not saying that it wouldn't be awesome. I would love to see someone do it!
I just don't think NASA/gov. would be to nice to the idea.

astromark
2009-Jan-29, 08:52 AM
I am old enough to remember those early lunar probes. The camera sending back a stream of data until impact. They were not big. They were relatively basic. Were they clean ? In light of todays technology,. No. We now know better. A Mars prob or lander would need to be sterile.
If other nations send space probes to Mars we need to get over ourselves and trust the technical ability of those responsible. After all I still take issue with the close fly by of a comet and return to Earth of 'Samples' from it... Yes I trust those who know... but, how safe was that.?
Next question would be...Is this possible ? Could a small low budget team get a small lightweight prob to Mars. and Yes would be the answer. The words Low budget might have been replaced with...Hidiously expensive. If the want is strong the can do will be done. NASA does not have the right to space. China, Japan, India. Might all be lining up to challenge the boys from Huston. This could get interesting.

dodecahedron
2009-Jan-29, 01:41 PM
All the trouble NASA takes to make sure they don't seed Mars with organisms from Earth.

I say it's a roll of the dice. Either the organisms will survive the rigors of interplanetary space then weather the tribulations of planetfall and should some hardy beast, lol waterbears, actually reach the surface alive then it has to deal with a potentially hazardous environment that is unforgiving to unprepared earth life.

My odds are on Mars keeping itself sterile of Earth.

marsbug
2009-Jan-29, 02:07 PM
Yes it probably could be done, and bragging rights is as good a reason as any. Still even getting one tiny camera to crash land on mars is fraught with less than obvious traps: making sure the camera will still work after months at near zero kelvin, making sure it warms up at the right rate to not be damaged, will it work under space radiation ,which is nearly impossible to simulate from earth, doing the same for the power systems. And you have to convince people that you really did make it and aren't just faking your fuzzy grainy pictures.

Cheap is a relative term, it probably would be cheap next to say MSL, but still cost more than most people could cough up, even over ten years!
I'd love to see it done though.

IsaacKuo
2009-Jan-29, 02:32 PM
To get some idea of the cost involved, perhaps we could compare/contrast with Google's lunar X prize? I'd guess that unless you're a millionaire, you can't even think about sending a rocket to the Moon or Mars.

I wouldn't worry about sending a camera or anything, I'd think in terms of sending a bit of reflective foil--something with sufficient reflection of sunlight that someone can track the thing with telescopes. Just get the thing on an Earth escape trajectory that's vaguely in the direction of Mars, and I think that's sufficient bragging rights. And if it's sufficiently well aimed to actually hit Mars? So much the better!

Hmm...maybe you can get the last one or two km/s with some sort of "gun". The payload is a lightweight coil of wire, which uncoils after leaving the muzzle. That still leaves the first ten km/s to deal with...

marsbug
2009-Jan-29, 04:19 PM
I think the most realistic way would be to go along the cubesat route: build a cubesat to do some sort of experiment in mars orbit and then piggyback it there on something else. There might actually be funding from somewhere to do that!

If you actually wanted to do the whole shebang yourself that would mean building you're own miniature launch vehcle and interplanetary propulsion. I think that might make things much more difficult.

If you've just got the first pictures from a cubesat orbiting mars you don't need anything extra to brag about in my opinion.

IsaacKuo
2009-Jan-29, 11:37 PM
Hmm...a cubesat would start off in Earth orbit, so you can afford to use a low thrust propulsion system. One propulsion system which might work out is a solar powered steam thruster.

The main components are:

1) solar cells
2) A capacitor
3) A water tank with a hypodermic needle
4) A second needle with its tip near the first needle for a spark gap
5) A plastic conical "nozzle" which is pierced by the two needles

The idea is to use the solar cell to charge up a capacitor. After the capacitor reaches sufficient voltage, it sparks across the spark gap. This heats up a bit of water at the tip of the needle to steam, producing thrust as it leaves the nozzle.

I think this sort of thruster might be something that a lone experimenter could develop. It's not too much more complex than a spud gun.

stktos
2009-Jan-30, 01:17 AM
Hmm...a cubesat would start off in Earth orbit, so you can afford to use a low thrust propulsion system. One propulsion system which might work out is a solar powered steam thruster.

The main components are:

1) solar cells
2) A capacitor
3) A water tank with a hypodermic needle
4) A second needle with its tip near the first needle for a spark gap
5) A plastic conical "nozzle" which is pierced by the two needles



COOL! I'm going to go build one now!
JK.
But that idea would be plausible. It would take a great deal of time to get up to speed but could be done cheaply. Of course you still have to get it into orbit. And would have to have a way to control when the capacitor would release its charge. And possibly to different nozzles. So you could have it go were you wanted. If not it would just thrust in the direction its pointed whenever the capacitor reaches its charge.

IsaacKuo
2009-Jan-30, 12:20 PM
I'm mainly thinking in terms of the development effort that a hobbyist could plausibly deal with. I think this might rule out fancy fabrication of an efficient miniature chemical rocket, so that leaves some sort of electric rocket. Developing a fancy ion thruster seems harder than a simple spark gap electrothermal thruster (although maybe not).

A hobbyist could start off trying to develop the thruster in open air using off-the-shelf components (medical syringes, simple prototyped electronics). If that works, then he could invest in a vacuum chamber so it works in a vacuum.

Attitude control is an important question. Ideally, you'd want to have two solar panels which were aimed at the sun--but this requires an attitude control system that you're confident of. Without access to zero gee, how do you test your attitude control system? The answer might be that you don't. So maybe you coat all sides of your probe with solar panels. And maybe you have several thrusters pointed in different directions so you switch the electricity to a thruster when it is pointed more or less in the desired direction.

stktos
2009-Jan-30, 07:22 PM
Yes I agree IsaacKuo, A hobbyist could in fact build something of the sort without having to spend a fortune.

And I agree with you answer to your question. Coat all sides would be a safer bet. But you could have larger panels if you use panels with a attitude control system. Give and take relationship for simplicity there.

astromark
2009-Jan-30, 08:16 PM
All of that looks far more complex at every corner... No. The use of solid burn thrusters has worked in the past to attain that escape velocity. A military launch vehicle insertion into orbit and a solid fuel thrust into Mars impact track.
Regardless of how this shot at Mars is undertaken its going to cost a fortune... Getting off of Planet Earth is not easy or cheap. Find a better way, yes. Until you do, leave it to the lads from mission control NASA. Remembering that that is your money too. All of this is doable... It is just about the money.
Convince them that a endless supply of petroleum is available and we will be gone by lunchtime...:)

cjameshuff
2009-Jan-30, 08:28 PM
I'm mainly thinking in terms of the development effort that a hobbyist could plausibly deal with. I think this might rule out fancy fabrication of an efficient miniature chemical rocket, so that leaves some sort of electric rocket. Developing a fancy ion thruster seems harder than a simple spark gap electrothermal thruster (although maybe not).

http://myelectricengine.com/projects/mpdthruster/mpdthruster.html

Orientation might be maintained with reaction wheels...much easier to construct and control than thrusters, though they might accumulate angular momentum from tidal forces while in orbit around Mars, or from uneven absorption/reflection/emission of light. A "tail" using light pressure from the sun might be a simple way to bleed off the latter, don't know about the former, and it'd greatly complicate deployment.

There's also the issue of communication. You'd likely be relaying through the other probes there, and be taking bandwidth that could be used by more sophisticated probes.

IsaacKuo
2009-Jan-30, 11:35 PM
That electric thruster is attempting to use the rather sophisticated technique of magnetohydrodynamic thrust (and it's not clear that it achieves that goal). I'm not asking for such a sophisticated thruster.

However, I think the basic performance of a hobbyist level solar electric thruster may simply be insufficient. Some sort of small monoprop thruster would be ideal, but it needs to be something easy/non-toxic to deal with...a spark gap might still be used for "ignition", but with some sort of energetic fuel so it doesn't need to provide all of the "oomph".

I've figured out an elegant method of attitude control and efficient thrusting using just one fixed thruster.

First, imagine the cube has four thrusters on one face. This is a cluster of thrusters. If you fire up all of them, they produce a straight "burn". If you fire up only one of them, then you get a turning motion--a "turn". This lets you maneuver the probe using classic "turn and burn" style maneuvering.

Now, imagine that the cube is rotating along the thrust axis. The same principle still works, of course. Now here's the clever bit--erase all but one of the thrusters. You have just one fixed thruster remaining. If you apply constant thrust, it will provide a straight "burn". If you apply cyclic thrust, however, it will prove a "turn". This gives you all of the maneuverability of a cluster of thrusters, but using only one thruster!

So how do you get the probe spinning in the first place? Well, you start by thrusting the one thruster a little bit. This will set you rotating--but this rotation will be "end over end". It's not the direction of rotation you want. But you can convert any rotation into rotation aligned with the principle rotation axis by concentrating the probe's mass all along the central plane. Sloshing of the fuel will do the rest.

My first thoughts had to do with reaction wheels, because all you need is a cheap electric motor (you don't even need to attach anything extra to the axle). But then it gets annoying.

For communication, I think this probe should have a very basic transmit "ping", that can hopefully be used to track the probe. Other than that, the probe needs a GPS receiver antenna so it can tell where it is and what thrust maneuvers to take. I don't know how far up GPS works, but it certainly won't work near Mars. So therefore, you'll do all of your maneuvering in LEO. It's just a straight shot toward Mars, and then the probe is a dumb ballistic bullet. Maybe it has a solar powered "ping" transmitter, but it won't have any idea where it is.

BTW, maybe a mission to Venus is an easier idea. No pesky worries about conforming to biological contamination guidelines.

cjameshuff
2009-Jan-31, 10:26 PM
That electric thruster is attempting to use the rather sophisticated technique of magnetohydrodynamic thrust (and it's not clear that it achieves that goal). I'm not asking for such a sophisticated thruster.

It was merely an example of an attempt to build one of the more sophisticated thrusters by a hobbyist that I'd happened to come across a few days ago. I don't think it's quite as difficult as you make it out to be, given the existing R&D that's already been done. The hardest part may be plumbing that won't leak the fuel away over time in space, and getting access to a sizeable vacuum chamber. And power...



Now, imagine that the cube is rotating along the thrust axis. The same principle still works, of course. Now here's the clever bit--erase all but one of the thrusters. You have just one fixed thruster remaining. If you apply constant thrust, it will provide a straight "burn". If you apply cyclic thrust, however, it will prove a "turn". This gives you all of the maneuverability of a cluster of thrusters, but using only one thruster!

Sounds like a control nightmare. You'd need extremely good knowledge of the thrust profile as the thruster ramps up and down...you need this for all systems using thrusters for orientation control, and this system would exaggerate the issue. The characteristics would also change over time as fuel was exhausted. This is why I suggested reaction wheels.



My first thoughts had to do with reaction wheels, because all you need is a cheap electric motor (you don't even need to attach anything extra to the axle). But then it gets annoying.

How so? They do require constant power, but tiny in comparison to an ion drive, and their effects are easy to quantify and extremely repeatable...they are about as easy to engineer as you can expect to get.

The biggest problems with reaction wheels that come to mind is that if there's a glitch that causes loss of orientation, resulting in loss of sufficient power to counter drag in the wheels, the probe will start spinning as they spin down, and second, any misalignment of the main thruster will add angular momentum that they'll have to absorb, spinning them up to their limits over time. If the thrust can be vectored slightly (maybe using segmented electrodes), the momentum wheels could be used to both do pointing without consuming fuel and calibrate the thrust vector to point it through the center of mass.



BTW, maybe a mission to Venus is an easier idea. No pesky worries about conforming to biological contamination guidelines.

Better solar power for the same panel size, too (particularly important if ion thrusters are used). Rather more demanding thermal and radiation environment, though. A rotating probe would help equalize temperature, and the increase in light would help compensate for the fact that photovoltaic cells around its equator would be facing away from the sun half the time.

IsaacKuo
2009-Jan-31, 11:47 PM
It was merely an example of an attempt to build one of the more sophisticated thrusters by a hobbyist that I'd happened to come across a few days ago. I don't think it's quite as difficult as you make it out to be, given the existing R&D that's already been done. The hardest part may be plumbing that won't leak the fuel away over time in space, and getting access to a sizeable vacuum chamber. And power...
I don't think the R&D has been done at all. Did you see the size of the pulsed power banks? This vehicle needs to be very small and lightweight.

The more I think about it, the more I think some sort of chemical rocket power is required. You might still use some sort of electrical power to modulate thrust.

The basic problem with an electric thruster is that the low thrust complicates navigation. I figure GPS can be used if your altitude is sufficiently low, but this means you need to do all of your thrusting at LEO altitudes. This is good for maximizing the Oberth effect, but it does mean you need to have a reasonably good acceleration capability.

So, some sort of small chemical rocket may be the best option.

BTW, the electric "steam rocket" I was thinking of wouldn't be an "ion" rocket. Ionization would consume even more energy than merely vaporizing water.

Sounds like a control nightmare. You'd need extremely good knowledge of the thrust profile as the thruster ramps up and down...you need this for all systems using thrusters for orientation control, and this system would exaggerate the issue. The characteristics would also change over time as fuel was exhausted. This is why I suggested reaction wheels.
It's not a control nightmare. For your main burns, you just apply constant thrust. You only need cyclic thrust in order to "turn". You don't need sophisticated thrust control. A simple "on/off" capability is sufficient. A simple "full/low" capability is also sufficient. The nice thing is that the probe is spinning rapidly along the main thrust axis. This gives you a good gyroscopic effect that keeps you pointed in the current direction, and it means that your rotation adjustments are "steady".

By "steady", I mean that the axis stops turning as soon as you stop applying rotational thrust. Control is inherently stable and simple. This may mean you can get away with simpler more restricted ground control tests.

How so? They do require constant power, but tiny in comparison to an ion drive, and their effects are easy to quantify and extremely repeatable...they are about as easy to engineer as you can expect to get.
Accumulation of rotational inertia needs to be dumped somehow, and that's an annoying complication. I'm also concerned with how you ground test the thing. You need to somehow allow your probe to freely rotate without friction in any direction.

My proposed system also has issues with ground testing, but I think you can get a useful ground test by suspending the vehicle from a string.

cjameshuff
2009-Feb-01, 02:43 AM
I don't think the R&D has been done at all. Did you see the size of the pulsed power banks? This vehicle needs to be very small and lightweight.

It was also operating at higher-than-atmospheric pressure and at rather insane flow rates, and with a power supply designed for simplicity of design and construction and low cost rather than for use on a spacecraft. There is a great deal of information out there about far more suitable power supplies, but what I was referring to was R&D on the engine itself...good design parameters, electrode geometries and materials, various fuels, etc.



The basic problem with an electric thruster is that the low thrust complicates navigation. I figure GPS can be used if your altitude is sufficiently low, but this means you need to do all of your thrusting at LEO altitudes. This is good for maximizing the Oberth effect, but it does mean you need to have a reasonably good acceleration capability.

More than that, you need extremely fine control of your acceleration. An agile engine with rapid throttle response. The Oberth effect means you get the most out of your delta-v, but it also means any error is magnified by the same degree.

GPS may be useful for determining initial orbit parameters, but the motion of the craft will be an issue (it will constantly be hunting for new satellites), as will be the update rate. MEMS accelerometers are tiny and not that expensive, you should be able to use inertial navigation far more effectively than GPS.



BTW, the electric "steam rocket" I was thinking of wouldn't be an "ion" rocket. Ionization would consume even more energy than merely vaporizing water.

Resistojets, huh? Steam will not give very good performance, and more, will cause corrosion issues. Hydrazine is toxic, an issue for both testing and getting the thing launched. Ammonia? It's reasonably safe, easy to store in liquid form, won't erode the filaments (not as badly as most other candidates, anyway), requires less energy to vaporize, is a gram lighter per mole...



It's not a control nightmare. For your main burns, you just apply constant thrust. You only need cyclic thrust in order to "turn". You don't need sophisticated thrust control. A simple "on/off" capability is sufficient.

The issue I was trying to point out was that real thrusters don't simply turn on and off. You're also relying on fuel slosh in a tank that is getting progressively more empty.



Accumulation of rotational inertia needs to be dumped somehow, and that's an annoying complication.

I'm aware of this. Mostly it will be because the thruster won't be aimed precisely through the center of mass...tidal effects won't matter for a flyby or impactor probe. Segmented electrodes might give enough asymmetry in the thrust to allow this to be canceled by the main engine. Or allow the engine to be physically moved between a few discrete positions...you don't need much, just enough to be sure of the three positions forming the points of a triangle enclosing the center of mass. The engine could be on a spring suspension, with magnetic latches holding it in each stable position and solenoids switching those positions...minimal added weight, no continuous power required, and simple construction.



I'm also concerned with how you ground test the thing. You need to somehow allow your probe to freely rotate without friction in any direction.

For some reason, I had a sudden image of the probe rolling around a lab floor in a gerbil-ball...

Seriously, gimbal mounts can be made with extremely low friction bearings. If a vacuum chamber is unavailable for long term tests, compressed air jets could be rigged up to simulate the vectoring.



My proposed system also has issues with ground testing, but I think you can get a useful ground test by suspending the vehicle from a string.

I...don't really think that'll give any useful results. No more useful than it would with the momentum wheels and vectored thrust.

IsaacKuo
2009-Feb-01, 06:49 AM
More than that, you need extremely fine control of your acceleration. An agile engine with rapid throttle response. The Oberth effect means you get the most out of your delta-v, but it also means any error is magnified by the same degree.
The goal isn't to do some precision maneuver but just to get into the general direction of Mars (or Venus). It's okay to miss the target by millions of kilometers--that's still vaguely close enough.

GPS may be useful for determining initial orbit parameters, but the motion of the craft will be an issue (it will constantly be hunting for new satellites), as will be the update rate. MEMS accelerometers are tiny and not that expensive, you should be able to use inertial navigation far more effectively than GPS.
How will that work? The probe is in freefall, so how is an accelerometer going to help you determine your location?

I do have an alternative idea of how to do navigation. You could use a small webcam to track the position/size of Earth and Moon, as well as the positions of Venus and Jupiter (the Sun might be too bright for the webcam if it's in the field of view). This gives the probe a good measurement of its attitude and a somewhat good idea of its position. It could passively take in readings for several orbits until it has a good fix on its orbital parameters. From there, it determines the appropriate time and angle to do its maneuver burn.

Resistojets, huh? Steam will not give very good performance, and more, will cause corrosion issues.
Steam offers a specific impulse of around 190, which may be good enough. As for corrosion...huh? What corrosion issues? The amount of solar power available to a 10cm cube is going to be orders of magnitude less powerful than a microwave oven, and you can use a simple plastic container to boil water in a microwave. Just make your rocket nozzle out of the same plastic.

Oh--you're wondering about resistojet "filaments", right? I was thinking of using a spark gap instead of some sort of filament. The problem with a filament is that you'd need a lot of power to get a filament hot and keep it hot. Given the small amounts of solar power available, I think a periodic spark using a capacitor may be more realistic.

Hydrazine is toxic, an issue for both testing and getting the thing launched. Ammonia? It's reasonably safe, easy to store in liquid form, won't erode the filaments (not as badly as most other candidates, anyway), requires less energy to vaporize, is a gram lighter per mole...
Ammonia could be better than water.

How about hydrogen peroxide, or a bi-prop mix of hydrogen peroxide and alcohol? The spark doesn't provide much of the energy, but rather it's used to ignite the mixture. For such a small scale rocket, it might be more practical to burn in small pulses than continuously.

The issue I was trying to point out was that real thrusters don't simply turn on and off.
This depends on the specific thruster design.

You're also relying on fuel slosh in a tank that is getting progressively more empty.
No, I'm not relying on fuel slosh as it gets more empty. The only time fuel slosh is used is during the initial transition from "coin flipping" style rotation to "rotating wheel" style rotation. After that, fuel slosh neither helps nor hurts the functioning of the spacecraft.

I'm aware of this. Mostly it will be because the thruster won't be aimed precisely through the center of mass...tidal effects won't matter for a flyby or impactor probe. Segmented electrodes might give enough asymmetry in the thrust to allow this to be canceled by the main engine. Or allow the engine to be physically moved between a few discrete positions...you don't need much, just enough to be sure of the three positions forming the points of a triangle enclosing the center of mass. The engine could be on a spring suspension, with magnetic latches holding it in each stable position and solenoids switching those positions...minimal added weight, no continuous power required, and simple construction.
If you implement any of those ideas, then you can use them directly for attitude control. You don't need the reaction wheels to consume additional mass/space.

For some reason, I had a sudden image of the probe rolling around a lab floor in a gerbil-ball...

Seriously, gimbal mounts can be made with extremely low friction bearings. If a vacuum chamber is unavailable for long term tests, compressed air jets could be rigged up to simulate the vectoring.
It's not just the friction, but the balancing. If the probe is slightly unbalanced, then this gives an artificial impression of stability.

I...don't really think that'll give any useful results. No more useful than it would with the momentum wheels and vectored thrust.
If the probe is designed to be spinning, then it has an inherent amount of gyroscopic stability. So, you only really need to test small angle effects.