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The project is to build a model rocket to take off and land vertically. Max height perhaps 100 feet.

Looking for a suitable engine which can provide variable thrust control and thrust vectoring which could interface to something like a Pi Nano or Arduino GPIO.

Was thinking liquid fuel but have seen other threads that suggest it is too difficult.

Any supplier suggestions welcome.

Please note I am NOT asking for instructions on how to build my own engine, I am asking if such an engine is commercially available and, if not, what alternatives are there? Therefore I respectfully submit that this question is not off topic and should not have been closed.

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    $\begingroup$ Estes model rockets which cost < $50 can reach altitudes far in excess of 100 ft. No guidance required, see your local hobby store. They land vertically, but not under power. $\endgroup$
    – Organic Marble
    Commented Aug 8, 2023 at 12:02
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    $\begingroup$ @OrganicMarble - many moons ago (~50 years) I had a poorly-glued fin come off an Estes rocket. It landed vertically, under power. The parachute was helpfully expelled as the rocket lay on the ground... $\endgroup$
    – Jon Custer
    Commented Aug 8, 2023 at 13:49
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    $\begingroup$ @JonCuster nice! Back then mine were like this: "launches unstably, flies around in circles, then straightens out and flies directly at the most vulnerable object around." space.stackexchange.com/a/25859/6944 $\endgroup$
    – Organic Marble
    Commented Aug 8, 2023 at 14:01
  • $\begingroup$ I would think the comments that you came across about liquid fuel being very complicated are correct. Possibly something simple like a cold gas thruster would be feasible as you just need a valve to throttle it that your processor could control. I'm not sure however whether the weight and low efficiency of that type of engine would be too heavy to get it to 100 feet. Thrust vectoring is probably too complex for that small of an engine, maybe some thrust vanes in the exhaust like early rockets? $\endgroup$
    – Steve Pemberton
    Commented Aug 8, 2023 at 15:37
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    $\begingroup$ Even with cold gas thrusters, you quite easily start getting into the realm of some dangerous stuff -- starting with pressure vessel failures and tossing multi-kilogram gas cylinders around, since high pressures inevitably require heavy metal. While setting up a purchased liquid fuel engine is probably safer than designing it, they are not really commercially available and even a static test is solidly in the realm of dangerous expert experiments. While amateur liquid rocket research does exist, this is mostly in the realm of larger nonprofits and people who are literally Robert Goddard. $\endgroup$
    – ikrase
    Commented Aug 18, 2023 at 9:37

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The non helpful answer for buying an engine for a 100 foot flight is you go shopping for RCS engines with someone like Aerojet and look in the catalog, work out practicalities of ITAR or relevant laws for the country/s in question and then sort out whatever legal agreements the OEM requires to be allowed to buy and fire their engines without them becoming liable for the resulting crater. Then you need to actually build the system to use the engines, most likely involving some exciting chemicals and multiple expensive and/or dangerous failures.

For a hobby project you can instead and look at the work with solids, like youtuber BPS space who vertical landed a solid rocket using obstruction to exhaust flow, also used for some commercial systems.

Be aware you are building a guided weapon system so expect complications in seeking advice on the project, and interest from your local government agencies.

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  • $\begingroup$ As a note: BPS.space does sell a printed circuit board that can be used for controlling his rockets, or at least has in the past if I am not mistaken, but will not ship it out of the USA for legal reasons. $\endgroup$
    – ikrase
    Commented Aug 18, 2023 at 9:41
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    $\begingroup$ This is an extremely helpful answer, if only to explain the scope of the issues to me. It gives me several resources to consider and has helped my understanding considerably. Thank you. $\endgroup$
    – Andy Woolford
    Commented Aug 25, 2023 at 3:12
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For a hobby type system where you want variable and vectored thrust to develop control electronics, you don't want a rocket engine at all; you want an electric fan type engine as used in hobby drones.

A controlled-thrust rocket engine will generally require liquid fuel; and a liquid-fuel engine with variable thrust and thrust-vectoring nozzle will be multiple orders of magnitude more difficult to acquire (both cost and regulatory requirements) than a fan-type engine, which should suffice if what you are really trying to learn is control systems for vertical landing.

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  • $\begingroup$ That's a really good idea. A vehicle with a drone-style prop at the bottom and a weight (battery?) at the top would simulate a rocket for the purposes of control software without requiring the tricky fuel control problem. $\endgroup$
    – Darth Pseudonym
    Commented Aug 8, 2023 at 16:25
  • $\begingroup$ I feel like you are talking about building a toy helicopter looking like a rocket $\endgroup$
    – The Rocket fan
    Commented Aug 9, 2023 at 8:40
  • $\begingroup$ Interesting concept. I'll give this some serious thought. $\endgroup$
    – Andy Woolford
    Commented Aug 25, 2023 at 3:09
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What I recommend using is a waterrocket for safety reasons. You could launch a 1.5L bottle 100 meters in the air and deploy a parachute. After that you could deploy the wings as long landings legs and try to land it vertically. If that does not work, maybe you could have the landings legs and the wings as two separate items. However, it is important to try to keep the center of gravity at the right spot, otherwise you might have some issues with stability. It might take a few attempts, but a vertical landing should be possible.

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  • $\begingroup$ Unless you are using rather interestingly high gas pressures, pneumatically pressurized water rockets have such a small impulse that I don't think you could do much interesting work with throttling and thrust vector control. Additionally, water is not a "fuel" in such rockets, the compressed air is doing all the energy storage. $\endgroup$
    – ikrase
    Commented Aug 18, 2023 at 9:19
  • $\begingroup$ @ikrase Even with low Isp, there is a lot you can do. With a 1.5 L bottle you can easily launch your bottle up to 100 meters in the air. What I recommend was adding a parachute and then trying to use the fins to steer the rocket to make sure it is stabil. Landing legs could deploy making it easier to do a vertical landing. $\endgroup$
    – The Rocket fan
    Commented Aug 18, 2023 at 12:50
  • $\begingroup$ Thanks for the suggestion. For reference, I am not looking for rocket design solutions. The project is to develop a vectored control system for vertical landings, (a la Space X) so a parachute would sidestep the primary objective. But thanks for the creative suggestions of water thrust. As @ikrase points out, the project requires a continuous and sustained source of thrust to be controlled and a pressurised water system has too many limitations. That said, a small scale "desktop" model might work for proof of concept. $\endgroup$
    – Andy Woolford
    Commented Aug 25, 2023 at 3:07
  • $\begingroup$ @AndyWoolford Here are my suggestions: 1. Design some variant of quadcopter or similar VTOL aircraft controller as others have suggested. Lithium ion batteries and brushless motors are cheap and powerful these days. 2. Build a small benchtop model that flies with pressurized water and is supplied with pressurized water from a tether. $\endgroup$
    – ikrase
    Commented Aug 26, 2023 at 6:33

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