Hacker News Reader

Comment by dogma1138 | Hacker News Reader

I’m pretty sure that your problem isn’t going to be the software but rather the engine and the rocket itself.

Hobby rockets use simple solid fuel engines you’ll need to build an actual liquid rocket engine with controlled thrust that can be reignited the rocket it self also need to be large enough to be aerodynamically controlled and hobby rockets tend to be too light so they’ll simply tumble in the wind.

While you don’t need to build something the size of a falcon 9 for propulsive landing you’ll need something way larger than any hobbiest rocket I know of for sure.

If you just want the control part then simulating it is likely going to be a better use of your time, you can even build your own auto-pilot for something like Kerbal Space Program it won’t be the real thing but it would at least be achievable.

View on HN

You don't necessarily need a liquid rocket engine if doing something like the hoverslam, because you'd just time the burn correctly and fire engine at full power.

The problem is that you can't stop a solid fuel engine as it has the oxidizer mixed in it, unless it requires some sort of a catalyst that you can control.

Hybrid engines which use solid fuel and a liquid/gaseous oxidizer can be controlled better but these are often very hard to reignite.

So this means you'll have to have additional engines that are used just for the slam landing, technically possible but again might be harder to construct than a liquid engine since solid fuel engine ignition is also fairly inconsistent.

The Soyuz capsule uses rockets to soften the landing but these technically aren't propulsive landing and the landing forces tend to vary quite a bit between landings.

Overall when I think about this problem as a hobby the software isn't going to be the biggest blocker here especially considering the speeds/altitudes that we are talking about as well as the overall mass of the rocket, in fact since these rockets are going to be considerably smaller and would travel considerably slower you could absorb much more of their relative energy via hydraulics so the amount of precision needed for the slam/hover landing isn't as great.

It’s undesirable for many reasons but could a small rocket be made to work if you made it really heavy? Of course one would be just pissing away efficiency but would a small heavy software-controlled model rocket work?

Define small, you still need a rocket with some sort of liquid fuel engine with some sort of throttle control and I'm not sure how small can you actually scale those to be.

You might be able to do it with a hybrid rocket engine where the oxidizer is liquid or gaseous but the fuel is solid but I'm not sure.

As for the weight if you make it small but heavy then you are likely to lose aerodynamic control since it essentially will be a free falling brick.

You need something that is massive enough to resist the wind but have a large enough surface area to be aerodynamically stable and controllable, I'm not saying it's not possible I'm just saying that the time and money that you'll have to invest in it is going to be quite likely astronomical and while I don't doubt that there might be individuals that might be able to do it on their own this likely would require a pretty large team of dedicated people to solve.

If you are capable of building a rocket engine I would actually focus on simply building a hovering rocket rather than trying to land it, or at least you better off trying to build a hobbyist grasshopper https://en.wikipedia.org/wiki/Grasshopper_(rocket) than a Falcon 9.

But realistically even for something like this you should be better prepared to spend the better part of a decade building it and likely at least a few $100K's in funding since none of this is going to be cheap to produce.

Couldn't you develop some sort of servo-controlled thrust deflector that allowed you to effectively reduce the net thrust without throttling the engine itself? It would be similar to thrust reversal on jet engines, but you would only do it partially since you're aiming for thrust reduction instead of reversal.

Could be but since your burn time is very short and the rocket gets lighter as it burns I’m not sure that would work either.

If you go this route you might as well just use an EDF engine instead of a rocket engine for the landing.

Hybrid motors using solid fuels (plastic, rubber, cardboard, even sausage) and liquid oxidizer (typically nitrous oxide) are pretty common in hobby rocketry. These can be trivially made throttleable within a range by metering the NO2 flow. With the proper fuel core, they can be kept "smoldering" for shutdown / relight by maintaining a trickle of oxidizer to keep the fuel surface at combustion temperature.

Reply on news.ycombinator.com