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Ferram's Launch Vehicle Tutorial

NathanKell edited this page Feb 25, 2015 · 1 revision

First step, if you're not using FAR, my aerodynamics-fixing mod, get it; you're gonna need it if you want your launches to make any kind of sense. You also want Procedural Fairings for your un-aerodynamic payloads. You also want KER or MJ for dV readouts, and likely for data during the ascent so you can gauge how you're doing. Other part mods like KW, AIES and Novapunch are also good, just keep the engines and get rid of the rest. You'll need a large selection of engines for this.

So once you get into actually designing and building rockets for RSS is that what you've learned in stock KSP with regards to launching and designing launch vehicles is pretty much garbage. The very basics are correct, but the details are wrong.

Basic Rocket For RSS

To start off, you're going to build a simple, two- or two-and-a-half-stage rocket to launch a Stayputnik with a battery and 4 antenna into orbit; you're starting with Sputnik 1 and nothing more complicated than that to make designing things easier so you don't need a ton of fuel.

You're going to build your second stage, which is going to have 4.7 km/s of dV. Attach a payload fairing base underneath your satellite and then attach a stretchy tank underneath that. Find a smallish engine for your upper stage; you're looking for somewhere between 200 and 450 kN for this. Attach it and fill the stretchy tank up by selecting it in the action groups menu and then filling it with the proper mixture for that engine. Then, scale up the tank until KER or MJ says it has 4.5 km/s of dV. Then check the TWR; make sure that it's somewhere around 0.7; higher will cause g-force problems near burnout, while lower will cause you to be unable to reach orbit.

Once you have that, use a procedural fairing adapter to make an interstage; make sure to attach the top floating node of the fairing to the bottom of the engine. Scale up the adapter and the size of the fairings at the second stage connection; attach fairing parts and put the adapter and the fairings into a single stage.

Then for your first stage, attach a stretchy tank below the interstage and find a high-thrust engine; make sure that it has good Isp at sea level so you're not grabbing a heavy-duty upper stage engine. Attach it to the bottom and fill up the tank with that engine's particular fuel mixture and scale as before, until that stage also makes about 4.5 km/s of dV. Check what the initial TWR of the stage is; if it's somewhere around 1.4, you're done; launch it. If not, slap a pair of decouplers on the sides, attach stretchySRBs to them.

If you're attaching SRBs, set them to have a burn time of ~80 - 100s, and then scale them up so that they bring the initial TWR to about 1.5. Remember to select them in the action groups menu and give them better-than-default Isp. Make sure that the TWR of the rocket once the SRBs are gone is somewhere around 1; if not, increase the burn time of the SRBs and then rescale them to bring the thrust up again. Attach nosecones to the top of the SRBs and add sep motors. Adjust your staging.

Basic Gravity Turn and Ascent Profile for RSS

You don't want to go up to 10km and then turn over; that wastes a lot of fuel. You also don't want to throttle down, since that ends up wasting fuel as well (due to increasing gravity losses). Instead, go full throttle the whole way, starting your gravity turn when you're going ~50 m/s (for an initial TWR of ~1.7) or ~100 m/s (for an initial TWR of ~1.3) and aim surface prograde until you're out of the atmosphere. If you've done it correctly, you'll be flying down 45 degrees at ~1 km/s.

You're not going to coast to apoapsis and then burn to circularize; you don't have the time for that kind of a burn. Instead you're going to have to keep burning the engines until you're in orbit. Once you're out of the lower atmosphere (~60 km) aim the rocket slightly above prograde to delay reaching apoapsis. For lower orbits you might have to circularize after apoapsis (burn up a lot to compensate). If you think you're not going to be able to circularize before you fall back into the atmosphere, you're not going to be able to; angle the rocket up to delay reaching apoapsis, and do that as soon as you can to reduce steering losses.

It'll take some practice to get it, but you'll manage it after a while. You can achieve a stable orbit at 105 km if you want (atmospheric drag stops there), but to give you time to finish the ascent you're better off going to ~300 - 400 km to start; gives you time to fall a bit.

Real Fuels Intro

Engine fuel mixtures can be changed by selecting the engine in the Action Groups menu and using the window that pops up in the bottom left. There are three basic types of fuel mixtures, and they all have their benefits / disadvantages:

*Kerosene + LOX: simple, fairly dense, high-thrust combination. Isps range from 260s at sea level for lower-stage motors to 350s in vacuum for upper stage motors. LOX is mildly cryogenic and will boiloff for any mission that lasts more than a few days.

*LH2 + LOX: highly cryogenic, not dense, but really high Isp. Isp range from 360s at sea level for lower-stage engines to 470s in vacuum for upper-stage motors. LH2 is highly cryogenic and noticeable boiloff will occur even after a few hours, though if you bring enough it can last a few days. Tanks end up being huge for this stuff, which means aerodynamic instability can result if it's not accounted for.

*Hypergolic mixtures; UDMH + N2O4, MMH + N2O4, Aerozine + N2O4: very dense, not cryogenic, good thrust, poor Isp. Isp ranges from 245s at sea level for lower stage motors to 320s in vacuum for upper stage motors. These won't boiloff at all, so they're the standard choice for long-duration flights. Also, they react instantly on contact and don't require a separate igniter, which is useful if you're working with the engine igniter mod.

You need a proper config for the engines in order for them to be set up to do any of these things. Engine configs can be found in the RealFuels thread, setting up stock-alike engines, real-life engines or realistic, but not real-life engines.

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