Curiosity Style Egg Drop Lander

[christheman200]

For one of my high school courses next year I will have to construct a lander capable of returning an egg safely to the ground.

I don't find this particularly interesting, so I'm going to be one upping the criteria and constructing an automated water rocket lander, sky-crane style, that will place an egg gently on the ground with no packaging to protect it. The criteria I have set out is as follows:

The egg must be placed on the ground intact.

The egg may not have any materials protecting it. Only a harness may be used to attach it to the sky-crane.

The egg must be initially positioned 50 meters above ground.

The egg must be returned via retro-water rockets, and a parachute may be used as long as it is not active upon touchdown.

The idea comes about from the Curiosity lander's unprotected nature upon touchdown. If NASA can land an unshielded 1 ton rover on Mars, why can't I land a simple egg on the ground?

For the automated descent I am thinking of using software from a quadrocopter to land softly, utilizing gyroscopic and altimeter sensors.

Their will be one or two pressure vessels, and 4 nozzles spaced out around the frame. Individual fast-response valves will control the thrust of each nozzle, in order to keep the pitch of the craft under control.

I will have access to a 3d printer with dual-extruders to print water soluble plastics, metal and wood CNC machines, and a machine shop once school starts back up. I can currently use SolidWorks, AutoCAD, and Inventor to model the craft, although I have access to just about every modelling software on the market.

I am looking for knowledgeable partners in this venture, especially in the areas of calculations and pressure tank materials. If you find this interesting please consider "joining the team"!

Since we are on a rocket simulator forum, I'm assuming there are a number of you who have a solid background in engineering and design.

[DerpenWolf]

We'll this sounds like an interesting project, though getting enough thrust out of even a high pressure water jet will be quite a challenge, mainly due to what sounds like a relatively large amount of weight the vessel will have. Either way I wish you luck. I remember years ago in one if my first high school classes I had to build a bottle rocket that had to return an egg safely, It was a lot of fun as I developed a separating capsule and a chute deploying system. The chute may have deployed late and the capsule hit concrete but the egg was only slightly cracked in the end, good times..

[christheman200]

Actually, the problem isn't the thrust, but the delta-v. Average water rockets have 1000+ newtons of thrust at launch and pull an average of 160Gs. They only sustain thrust for around 30-60 milliseconds however. I remember earlier this year the water rocket competition held in our technology class! I overloaded my rocket with water, but it still flew rather high since it was reinforced and capable of high pressures. The parachute deployed, but it did not unravel. The nose cone looked like the lid of a garbage can. Fun times!

[Ralathon]

Yea, as Chris says the problem will be delta V.

Let's say we have a fairly bulky craft with 4 liters of water and a pressurized tank at 30 bar (Any higher and you need to use steel canisters, bumping up the weight).

You can use the venturi nozzle calculation to figure out the exhaust velocity of such a system. Assuming perfect flow coefficients you get:

v_exhaust = sqrt(2*g*deltaP) =~ 25 m/s

As you can see this is a very low exhaust velocity when compared with liquid fueled rockets that regularly hit several thousands of m/s.

Plug this info into the tsiolkovsky rocket equation and assume 90% of the craft is propellant (So it weighs 440 grams empty weight, which is a very low estimate).

dV = v_exhaust * ln(9) =~ 55 m/s of dV.

You need about 10m/s per second to stay stationary in earth gravity, so this gives you just over 5 seconds for the entire maneuver. Less if you start plugging in somewhat more realistic numbers or if you need to cancel out some initial velocity.

It is theoretically doable, but you have to be ridiculously fast. You're looking at some very expensive components as well, quick response valves tend to be heavy and expensive.

[christheman200]

I will be in contact with some higher level members of the Canadian Space Agency in the near future, and am hoping that someone there will be able to guide me. I am hoping to receive support for the project from a few major local aerospace companies that I have connections with. I feel that the maneuver could be pulled off with around 6 seconds of fuel at 2Gs.

Is anyone here capable of calculating what size pressure tank would be needed to sustain a force of 2Gs for 6 seconds with a dry weight of 100 grams plus the weight of the tank? If no one goes for the challenge I will attempt to work on it when I wake up later today.

[Ralathon]

I will be in contact with some higher level members of the Canadian Space Agency in the near future, and am hoping that someone there will be able to guide me. I am hoping to receive support for the project from a few major local aerospace companies that I have connections with. I feel that the maneuver could be pulled off with around 6 seconds of fuel at 2Gs.

Is anyone here capable of calculating what size pressure tank would be needed to sustain a force of 2Gs for 6 seconds with a dry weight of 100 grams plus the weight of the tank? If no one goes for the challenge I will attempt to work on it when I wake up later today.

Why would you need to sustain 2Gs for 6 seconds? You only need 1G to remain stationary in earth gravity so you only ever need to go above that to slow yourself down. 2Gs over 6 seconds will slow you down from 60m/s, but from a starting height of 50 meter the maximum velocity you'll ever achieve is 32 m/s.

[christheman200]

It was just a very rough guess. I suppose 6 seconds at 1G is much more reasonable.

[Aghanim]

You could use modified sprinkler valve to achieve fast opening time: http://www.privatedata.com/byb/aircannons/Turbocharging_Sprinkler_Valves.html

If that is too big and heavy, you could use a piston valve: http://www.spudfiles.com/spud_wiki/index.php?title=Piston_valve

The problem is both of it are not throttleable

[QuesoExplosivo]

Why water and not solid fuel?

[astropapi1]

Why water and not solid fuel?

This is what I was thinking. Better exhaust velocity, faster acceleration, and a more reasonable burn time. If you use KNO3/Sugar, you can even design your own rockets to meet your expectations. As long as you don't do anything extremely stupid and stay within the boundaries of common sense, you should be fine.

[Ralathon]

This is what I was thinking. Better exhaust velocity, faster acceleration, and a more reasonable burn time. If you use KNO3/Sugar, you can even design your own rockets to meet your expectations. As long as you don't do anything extremely stupid and stay within the boundaries of common sense, you should be fine.

No throttle control on solid rockets though. You'd have to go with a hybrid engine or something to make a hovering platform out of it.

[Kinglet]

I would rather build a RC glider to land the egg.

[astropapi1]

No throttle control on solid rockets though. You'd have to go with a hybrid engine or something to make a hovering platform out of it.

Well, water rockets have no throttle control either, so he'd have to make some kind of adjustable throttle, and doing these things when your rocket is so small could really cut down on Dv.

I agree with PenguinsMeep. If using rockets isn't a specific requirement, using a RC quadcopter with a winch would be a cheaper and more reliable alternative.

[christheman200]

You'd have to go with a hybrid engine or something to make a hovering platform out of it.

I haven't thought of using hybrid engines! I can see way too many complications with them, but if working would provide lots of power.

As for valves, http://www.smcpneumatics.ca/en.aspx has a large supply of pneumatic valves which I believe are throttleable.

For those suggesting I take a different approach to the project, I will not be doing so. This is going to be my first real venture into rocketry, and even a failed attempt would look great on my resume.

[Nuke]

engines aren't your problem. control logic, thats your problem. you need to control your descent and then at the last moment, kill your momentum and soft land your egg, then take off and go land/crash somewhere. you can always use pulsed operation hybrid rockets. changing your duty cycle gives you control (pitch and roll, id just damp out any yaw with big fins). then you need to run an inertial measurement unit and some kalman filtering to give you a good enough reference to keep your ship level. you can use an ir or ultrasonic range finder to give you a good landing solution. an arduino would probibly give you sufficient computational power.

Edited July 1, 2014 by Nuke

[christheman200]

Building a hybrid rocket is still a large problem. Control logic is certainly a huge factor but first the rocket must be modelled.

Are the pulsed operation hybric rockets pulsed on and off quickly to control thrust?

[mardlamock]

Good luck building a hybrid rocket, you will probably die in the process. Unless you are willing to waste a lot of money on a simple egg you should go for something more simple. You need an engine that is capable of providing enough thrust to lift itself, the 3 kg of extra structure you ll probably need (water rocket with thrust control), develop an accurate enough dead reckoning system to feed to the control system and get all of the money for that. I myself am currently building an R candy rocket with an active stabilization system that will have to stabilize it at almost 900kmh by moving fins around, so im more than interested in whatever you can develop and would gladly appretiate if you sent me some of what you have thought of.

[Nuke]

you need solenoid valves to regulate oxidizer flow to each of your engines. cycling the valve open and closed will regulate the oxidizer flow to your combustion chamber which turns the engine on and off (you need an ignition source too). you do this at a fixed frequency but you vary the duty cycle (this is called pulse width modulation). the duty cycle is the ratio between on time and off time, this is what you change to control your throttle. but remember you are controlling a valve, and this will limit your frequency to the maximum cycle time for the valve, i figure tens to hundreds of hz. ignition is a problem. you could use like a glow plug or something like that, but you need an initial ignition source. you could also use a boost regulator to make a spark gap, running continuously during operation (good luck keeping your electrodes from vaporizing).

i think the hardest part would be machining the nozzel. of course thats because i dont have much shop experience.

an ardupilot board should work for the control side, though you are going to need to tweak the firmware to handle your engine control. they normally work with hobby servos and escs, but you are going to need different timing than your typical hobby servo to make it work. you are also going to need driver transistors because your mcu wont be able to push enough current for the valves, and you are going to need flyback diodes across your solenoid terminals, because inductive loads play havoc on semiconductor devices.

if you ask me it sounds like a lot of work and money for a high school egg drop.

Edited July 2, 2014 by Nuke

[christheman200]

Yes, constructing a hybrid rocket to land an egg on the ground does seem rather overkill.

Doing some math, I will only need a mass fraction of 3.8 for the rocket at 400psi. This is certainly achievable.

I am pondering the design of my own, lightweight, high pressure, throttleable valves at the moment.

Could anyone here give me some advice on the use of composite materials such as carbon fiber?

[K^2]

It's a nightmare of a challenge. Let me run through some options from worst to best, but honestly, if you aren't willing to "fake" thrusters with ducted fans, I wouldn't hold out a hope of actually getting it to work.

With water rockets, short delta-V means timing and valve precision must be perfect. Slight delay on valves opening, or misjudging pressure and not adjusting in time - splat. And that's assuming you have a radar and MCU that can handle precision/timing to begin with. Give me a $1k budget, and I'm still not sure I'd be able to build a working machine for this task.

Solid rockets would be your next best hope, but as people pointed out, no throttle control. With 4+ rockets firing, you could put them on servos and angle each rocket individually to get forces and torques to cancel on all axes. Algorithm for this one would be a mess. You still need a very good MCU, accelerometers, and really good servos. Being better with code and electronics than mechanics of the valve system, I'd go for this option and correct on the fly. I could build it cheaper this way, too. But it's comparable to the water rocket idea in complexity.

Liquid bi-prop rockets. Just a big no. Not on any sane budget.

Hybrid. Now we're getting into semi-realistic scenarios. Hybrids are pretty benign. You can throttle them a little. You can ignite them as needed. Radar + MCU + a bit of code and calibrated servos on the throttle needles, and you have yourself a working lander. Use single NOX tank with four solid rockets made out of something like dense rubber, and you have yourself a cheap-o hybrid sky crane. Of course, there will be explosions, fires, electrical problems, and you'll lose about a dozen prototypes before you get a working one. But if you know what you're doing, this is purely an exercise in patience.

Pulse jet. Lots of noise, not a lot of thrust, but if you can figure out ignition, it could make it. And you just need a small propane tank on board for fuel. Same requirements as above, but way less likely to blow up and spray corrosive substance and shrapnel all over the place. You will need some retro parachutes to slow it down before you fire the jets, however, and this is cheating a little, this sort of approach works only in earth's atmo. You might as well go for the fans at this point.

Liquid monoprop. Now we're cooking with gas. Or superheated steam, as the case may be. Naturally, getting your hands on some real space-worthy monoprop is difficult to put it mildly, but hydrogen peroxide is fairly accessible. You can order the concentrated stuff from a number of reagent stores. Highly unstable, super corrosive, and yet safe in comparison with hybrids. Also, doesn't require ignition, not likely to blow up, and makes for very easy to build engines. They won't last long due to oxidation, but they'd be easy to replace. Throttle control is as easy as turning the valve. The main drawback of H2O2 as monoprop is low delta-V, but that's compared to the good stuff. You'll have more than enough to land the egg.

So if I had to build a lander for this challenge using rocket thrust specifically, hydrogen peroxide would be my first choice.

That said, your best and easiest option is still ducted fans. Yes, it basically boils down (heh) to building a quadropter to land the egg safely after parachutes brought down the descend speed. On the plus side, you can make the whole thing super light using capacitors instead of batteries and only allowing the rotors to run for a few seconds, giving you performance similar to retro-rockets in almost every way.

Fans give you the cheapest, safest way to do this. As well as you'd be able to get a lot of help designing the thing, because a lot of people do work with quadropters these days. If you want a challenge you can actually finish, go with that.

Edited July 2, 2014 by K^2

[christheman200]

I generally agree with you, K^2, but I'm definitely going to stick with using rockets of some kind.

I have arguments with a few of your points on hybrid rocket engines, but they aren't worth going into detail for right now.

I did some rudimentary math and to land the egg with a bit of clearance room for small maneuvering errors, I will only need a craft with a mass fraction of 3.8 at 400psi (this is for a water rocket). At least in the theoretical physics part of the project, it's doable. I have a lot of time on my hands in between now and when the project will be due. And my teacher luckily will not take any marks off if I can't finish the project by then, having talked with him on the subject.

[Brotoro]

If I *had* to do this with rocket motors, I'd drop the device initially on a small parachute harness to keep the rig stabilized. Then a timer would ignite three short-burn solid propellant motors that have the correct thrust and total impulse to bring the rig to a near stop at under 3 meters height (and simultaneously release the parachute by having one of the rocket flames cut the line to the chute). Then a set of three long-burn solid propellant motors (chosen such that their thrust is a little less than the current weight of the rig) would allow it to slowly descend the rest of the way in a few seconds. When a switch on the egg's crane line senses that the egg has hit the ground (the egg's weight is no longer on the line), a single high-thrust solid propellant motor mounted on the centerline will be fired to simultaneously cut through the egg's line and kick the rig up and away. A backup timer would kick in the exit motor at the end of the less-than-hover motors' burn, just in case the egg hasn't quite touched down before the less-than-hover motors cut out (if you are landing on grass or dirt instead of concrete, a naked egg can survive a small fall).

[Ralathon]

If I *had* to do this with rocket motors, I'd drop the device initially on a small parachute harness to keep the rig stabilized. Then a timer would ignite three short-burn solid propellant motors that have the correct thrust and total impulse to bring the rig to a near stop at under 3 meters height (and simultaneously release the parachute by having one of the rocket flames cut the line to the chute). Then a set of three long-burn solid propellant motors (chosen such that their thrust is a little less than the current weight of the rig) would allow it to slowly descend the rest of the way in a few seconds. When a switch on the egg's crane line senses that the egg has hit the ground (the egg's weight is no longer on the line), a single high-thrust solid propellant motor mounted on the centerline will be fired to simultaneously cut through the egg's line and kick the rig up and away. A backup timer would kick in the exit motor at the end of the less-than-hover motors' burn, just in case the egg hasn't quite touched down before the less-than-hover motors cut out (if you are landing on grass or dirt instead of concrete, a naked egg can survive a small fall).

Sounds more like the landing profile of the

instead of the

[K^2]

400psi can give you a roughly sonic stream. Of course, it all depends on volume of the gas you have on board. Since you haven't specified that, I'm not entirely sure how you are doing your math. It's also a considerable amount of pressure, which can cause serious injuries in case of failure, even if you avoid any metal parts that could turn into shrapnel. Pressure wave alone could do it. If you can get a composite pressure chamber, it might be worth considering. Otherwise, it sounds kind of insane.

What sort of delta-V are you looking for, anyways? You'd be looking at something in the 30m/s range for impact speed. So that's minimal dV you need in suicide burn. Of course, acceleration isn't instant. In principle, an egg can survive up to about 50g, but with harness and safety margin, more than 10g would be pushing it. That gets you to stop in 0.3s at a cost of only about 3m/s more in dV. You'll also have to start braking within 4.5m of the ground.

Looking at some affordable sonar options for robotics, you can start getting updates from about 6.5m at 50ms intervals. That means your first ground detection will be within 5-6.5m. That's enough, but leaves almost no margin for error. You'll probably want some reserve thrust here. Furthermore, you can count on at best 5-6 position updates while braking. Worse, voltage readings will lag. These are just the sort of hardware limitations you'll have to live with.

So in terms of delta-V reserve, you don't have to worry about much. If you don't manage a soft landing in ~40m/s, you've already gone splat. And you can certainly do 40m/s with a water/air rocket. So it's all about control.

Control is the hard part. Lets forget valves and lag on thrust for a moment. I'm going to pretend that you manage to design your valves, and they have perfect response time. We are back to our worst case scenario. Your sonar is reading something between 5-6m, the error is about 10% due to the fact that voltage hasn't adjusted yet, and you have 4 more readings to estimate your velocity and time of impact. You can Kalman filter that data, and you will want to, along with accelerometer data once rockets kick in. You'll also manually set uncertainty on terminal velocity before you do your first Kalman pass. Hopefully, you can get to within 10% on that as well. Your goal is to be within safe stopping distance until you hit about 80cm from impact. There you can start using IR range finder as backup. You'll want to get at least a few readings off of that before impact as well. You won't have enough to make major corrections on velocity, but it will give you smaller errors on the range.

The actual control logic should assume worst case scenario. Take your velocity, go up a couple of sigmas, take distance and take away a couple of sigmas. You need to be able to stop within that. And you don't have direct thrust measurements. Only accelerometer data, which you'll also have only a few updates on. There is no time to PID this stuff. You'll need a look-up table of pre-calibrated values. Given x acceleration on each jet and y position on the valve, go to z position for correction. If you don't finish correction within 50ms between updates, your lander is probably already flipped over, and trying to correct further is moot.

Naturally, this is also where lag corrections will have to come in. If you have to adjust little, you can assume corrections to be instantaneous. If you have to correct a lot, you need to over-correct. That has to be part of the look-up table as well. Again, calibration is the way here, and there is nothing you can do about that.

Finally, MCU. You'll need six axis acceleration readings, at least two sonar readings, run Kalman in between, compute corrections, get valve servo positions, set them, and still have time for a break before the next 50ms wave of updates. I hope you are comfortable with assembly.

So that's what this project looks like on the control side of things, rockets aside. The reason you might want to look at high delta-V options is to start slowing down way in advance and do a gentle glide, which is "easy" on the control side. If you want to do it with water rockets, you have to suicide burn it, and then the actual delta-V isn't your problem.

[Creature]

This is a pretty cool idea. I still think you should go for a simpler rocket powered lander and forget the sky crane for now. It would give you a ton of error margin to work with and you'd still have fairly awesome eggmachine. Add in a robotic arm to lower the egg gently on the ground for dramatic effect.

If you go for the crane, you could cheat a little and give the egg a small rover-like cradle to land with. This would add a lot to your shock absorption without looking out of place.

But a simple lander is already really cool. You could name it Dragon's Egg.