Model rocket retropropulsive landing - Falcon 9

[sevenperforce]

My kids love watching rocket launches, so I got them some model rockets and we were launching them. I had also made my oldest a 1:80 scale model of the Falcon 9 with pop-out grid fins and landing legs. He kept asking me when we could launch his Falcon 9 and make it land on a boat.

So I got to thinking...how hard would it be to build a hobby rocket with propulsive landing capability?

Proper timing for a true suicide burn is hard enough with a liquid-fueled rocket; it would be even more difficult with a solid-fueled rocket, even if you had a laser rangefinder. Instead of trying to do a true suicide burn, then, you could build your model rocket with over-engineered shock-absorbing landing legs. Then, with slightly oversized grid-fin-style airbrakes, the terminal velocity would be fairly low. You could get away with a TWR < 1, since you would only need to decrease terminal velocity, not zero it out entirely. Your landing legs would catch you regardless of whether the motor burned out a few feet above the ground or was still firing on landing.

Here's a 1:40 scale model, reproduced in SketchUp. Two stage rocket. Eight motors for ascent; central motor is reserved for landing.

The octaweb at the base holds the launch and landing motors, mounts the landing legs, and holds the leg deployment channels (they are deployed by the landing-motor ejection charge). It is either aluminum or dense resin.

The legs are likely either aluminum or PVC. They snap into place at the top and are deployed by springs (not shown).

Main body (C) is light plastic to allow it to be launched without a license. Fairing halves (A) and (B) are plastic. Upper stage (C) is dense 3D-printed plastic; interstage (D) and "grid fin" air brakes (E) are either aluminum or dense resin. The landing ignition wire guide (F) is also light plastic. 

Detail view of interstage and upper stage:

On ascent, the landing legs serve as a sort of shuttlecock to maintain guidance, with the upper stage providing sufficient counterweight. Return airbrakes on the lower stage and guidance brakes on the upper stage both pop out on springs at stage separation, which also closes a circuit to ignite the upper stage motor.

On upper stage burnout, fairing halves pop out to deploy parachutes; these are tethered to the second stage (not shown) and allow for recovery.

Lower stage descent and landing:

 

[Rakaydos]

Interesting, but I'd worry about the launch acceleration pulling out the landing engine igniter.

 

Also, make sure your "Grid fins" have a higher drag than your "landing legs", or you may be coming down the wrong way and accordian.

[Motokid600]

You don't have any gimbals and I'm guessing the grid fins will be static as well. Meaning there's nothing to stop the rocket from... Rocking back and forth as it falls. I'm wondering if you'd need a streamer/drouge chute pop out the top just so it can fall straight down and remain stable. Because once the landing motors ignite if the rocket isn't perfectly vertical your gonna have major issues.

[sevenperforce]

19 minutes ago, Rakaydos said:

Interesting, but I'd worry about the launch acceleration pulling out the landing engine igniter.

Also, make sure your "Grid fins" have a higher drag than your "landing legs", or you may be coming down the wrong way and accordian.

The landing engine would be recessed in the octaweb with the landing engine igniter glued into place.

Landing leg deployment will really mess up aerodynamics; that's why I'd time it as late as possible. Perhaps I could use an automated servo with an ultrasonic rangefinder to deploy at 3 meters or so.

9 minutes ago, Motokid600 said:

You don't have any gimbals and I'm guessing the grid fins will be static as well. Meaning there's nothing to stop the rocket from... Rocking back and forth as it falls. I'm wondering if you'd need a streamer/drouge chute pop out the top just so it can fall straight down and remain stable. Because once the landing motors ignite if the rocket isn't perfectly vertical your gonna have major issues.

With a landing motor TWR < 1, the rocket should still descend shuttlecock-style on the grid fin airbrakes even at full throttle.

[cantab]

For a totally different approach, build a quadcopter into the top of the booster, with the rotors disguised as the grid fins.

[sevenperforce]

55 minutes ago, cantab said:

For a totally different approach, build a quadcopter into the top of the booster, with the rotors disguised as the grid fins.

That would be...awesome. Assuming, of course, that you can engineer a foldaway design.

Put the rotors in a square cowling and you'd never know. As an added bonus, the rotors will function as true grid fins while extended even without being powered. You could even use a ninth central motor with a TWR << 1 to give the appearance of retropropulsive landing.

You know, come to think of it, that might actually work for orbital-class rockets.

 

[Dman979]

1 minute ago, sevenperforce said:

You know, come to think of it, that might actually work for orbital-class rockets.

Seems pretty complicated for a large-scale launcher, but for models it might work well.

[HebaruSan]

Sorry to be a killjoy, but this idea sounds unsafe. For the launch phase of a model rocket, you can clear the area and warn people, but you won't know exactly where it's going to fall, so having an engine ignite on the way down with no abort switch means potentially harming persons or property.

[Thor Wotansen]

The quadrotor gridfins would be the answer to easy retropropulsive landings as well as safety concerns about where things are landing.  You could hack an Inductrix or similar small quadrotor for the motors and electronics and if you need more power there are upgrade kits available for those things with more powerful motors and batteries.  It would also be easy to counter for the < twr 1 by reducing throttle inputs to the rotors.

[sevenperforce]

15 minutes ago, Dman979 said:

Seems pretty complicated for a large-scale launcher, but for models it might work well.

Lemme think about this.

Each of the Falcon 9 grid fins have a surface area of 20 square feet, for a combined surface area of 80 square feet or 7.43 square meters. The Falcon 9 masses 22 tonnes dry. Electrical induction motors wouldn't mass too much; the power output of the Merlin turbopumps should be enough to hover it.

23 minutes ago, HebaruSan said:

Sorry to be a killjoy, but this idea sounds unsafe. For the launch phase of a model rocket, you can clear the area and warn people, but you won't know exactly where it's going to fall, so having an engine ignite on the way down with no abort switch means potentially harming persons or property.

You'd want to launch in an unpopulated area after a rain.

17 minutes ago, Thor Wotansen said:

The quadrotor gridfins would be the answer to easy retropropulsive landings as well as safety concerns about where things are landing.  You could hack an Inductrix or similar small quadrotor for the motors and electronics and if you need more power there are upgrade kits available for those things with more powerful motors and batteries.  It would also be easy to counter for the < twr 1 by reducing throttle inputs to the rotors.

Then you just need a servo for pop-out shock-absorbing landing legs.

[BagelRabbit]

As someone who has built and flown plenty of rockets IRL, you'll need to consider some things before proceeding:

Making a rocket with a plastic airframe is a good way to have it warp during or after flight. There is no way that rocket would be stable on the way up without additional fins on the bottom. Lighting eight motors at the same time is harder than one might imagine, and if one or more motors don't light, your rocket will veer off, flip, and/or crash spectacularly.

No sane range safety officer would allow a rocket to land like this, due to fire risk. Trying to ignite things at a precise height above the ground is hard with a small rocket probably traveling at 30 to 50 mph. Too much igniter delay, which is a factor you cannot control, WILL result in the rocket crashing and then lighting itself on fire. If the rocket lands while still firing its engine, it WILL tip and then thrash about on the ground under power.

Is this idea possible? Yes! But it would best be executed by rocketeers with many years of experience. See if you can scout out a local rocketry club and ask them to help. They'll probably be willing to help you refine, build, and test this idea.

[Motokid600]

3 hours ago, sevenperforce said:

 

With a landing motor TWR < 1, the rocket should still descend shuttlecock-style on the grid fin airbrakes even at full throttle.

Yes, but even a shuttlecock rocks a bit back and forth in its trajectory. A quad copter with a small rocket motor for effect may be your best bet I like that idea alot. More control and much safer. I just can't for the life of me picture the rocket falling perfectly vertical. If the motor fires and it's even a degree off vertical it'll hit the ground with excessive horizontal velocity.

[Wjolcz]

I really can't do math and never launched a rocket IRL, but I can tell you how I do it in KSP.

What I do is first drop something and check what's its speed when it hits the ground. Then add enough solid fuel so the dV matches the speed it hit the ground with and adjust the burn time (using KER). The tricky part is to fire it at the right altitude.

So you would probably need to know your rocket's terminal velocity first (drop a mock-up or something). Then adjust the fuel amount to have the same dV to match the terminal velocity (I'm assuming it would fly high enough to achieve it before hitting the ground). Then it would be only the matter of firing it at the right time, I guess.

[Camacha]

Oversize the grid fins and make them draggy as anything. That way you can use them as a disguised parachute, both considerably slowing and stabilizing your descending rocket.

[Red Iron Crown]

On 6/28/2016 at 3:35 PM, cantab said:

For a totally different approach, build a quadcopter into the top of the booster, with the rotors disguised as the grid fins.

Or bottom. (Props to @technicalfool for showing me that.)

[sevenperforce]

1 hour ago, Red Iron Crown said:

Or bottom. (Props to @technicalfool for showing me that.)

Yeah, I saw that. Can't really disguise them as grid fins, though. You could do a combination landing, where the grid fin rotors and the landing motor each provide a TWR of 0.7 or so and are used in sync for a soft landing.