How much deltaV in mono propellant?

[Sleepy_Steve]

Lets say I have a ship with a 3 man capsule, it's got its own supply of RCS, and then 4 of those medium RCS tanks on it with 100 RCS each. And now lets put on two or three of those RCS engines.... How far could it go?

Hypothetically, if this ship were to run out of fuel coming back from say... Jool; how screwed are the occupants?

Edited September 15, 2014 by Sleepy_Steve

[ElWanderer]

Ordinary monopropellent thrusters have a vacuum Isp of 260, the O-10 has 290, so it depends which engines you mean.

A wet mass of 6.32t with 1.72t of monopropellent (based on just a pod and four tanks) gives 810m/s delta-v with the lower Isp value, or 900m/s with the higher value. That's quite a lot just for manoeuvring, do you know how much you need? The figures will be lower if you're carrying anything else with mass (goo, for example) or if I've cocked up my calculations.

[Red Iron Crown]

You can calculate it using the rocket equation.

delta V = Isp * 9.82 * ln(WetMass/DryMass)

The ship as described is 6.32 tons wet and 4.6 tons dry, with a vacuum Isp of 290. Plugging that into the formula above yields about 904m/s of delta-V.

If you have already made your Jool return burn, 900m/s should be plenty to do course adjustments to get into an aerobraking scenario for capture at Kerbin.

Edit: Ninja'd!

[capi3101]

El Wanderer and Red Iron Crown have got the delta-V down for you; not much I can add, except that you'll need to make sure and leave plenty time for your burn. The old-style RCS blocks have half the thrust of an ion engine and you'll have to do that burn manually (no physics warp); I pity you if that's all you've got available. Just make sure to do your burns as far out as you can; last thing you're going to want is a 700 m/s burn right there at Kerbin's SOI...your finger will get sore if nothing else. If you've got O-10s, no sweat.

[Drew Kerman]

Install the RCS Build Aid mod. Because some (most, really) thrusters are designed with angled ports, you're not actually getting full thrust in all directions. Plus, the tool gives you a delta V reading too

[KerikBalm]

I think those dV calculations were done assuming no parachute (since you didn't specify one)

Assuming the mk-16-XL parachute, it is 6.62 tons wet and 4.9 tons dry

290*9.8* ln (6.62/4.9)= 855 m/s for the O-10

260*9.8* ln (6.62/4.9)= 766.7 m/s for the RCS blocks/ports

If you have ANYTHING else that does not have physics significance set to 0, such as an empty liquid fuel tank and liquid fuel engine, you are going to get even less dV.

If it doesn't have a parachute...well, you could aerobrake and save some dV for circularizing at apoapsis, and then send up a shuttle to get your crew back.

[Rus-Evo]

Don't forget to have the ability to seperate from the rest of the ship!

So this is less efficient than a rocket emergency return setup right?

[Sleepy_Steve]

The full parts list is: Mk1-2 Pod, 2 radial parachutes, 1 smallest solar panel, 4 FL-R25 RCS Tank, and 3 docking ports. Engines are the 0-10's. So I'm seeing a full mass of 6.77 and a dry mass of 5.15 at an ISP of 290.... 777.3 m/s.

This is basically my emergency stage. I won't know how much of it I need to use till I'm done with the visit all the moons portion of the jool mission. But I do fully expect to run out of traditional fuel before I make it back to Kerbin.

How much of the ship I use to come back might be pretty minimal since there are a bunch of options with the way I designed the ship's different modules. My plan is to use whatever is the minimum possible to fit the remaining fuel into, but even that I expect to run out.

Edited September 12, 2014 by Sleepy_Steve

[tg626]

Mechjeb can tell you, but I think you have to manually add it to a window.

[SRV Ron]

By weight, mono prop has far less delta V then the fuel oxygen mix. You can test this out the Kerbal way by building a small rocket using KW mod parts, their service module engine, and a couple of monoprop cans. Launch it straight up and note how far it goes. Then rebuild that rocket using an LV-45 which has the same thrust as the KW service module engine, and an equivalent amount of fuel cans. You will see a big difference in performance.

You are better off to carry the extra fuel rather then the monoprop. If you do go the route of relying on monoprop thrusters as a backup, design the rocket so that you can stage the excess weight of the non functional rocket with its fuel cans.

[Red Iron Crown]

By weight, mono prop has far less delta V then the fuel oxygen mix. You can test this out the Kerbal way by building a small rocket using KW mod parts, their service module engine, and a couple of monoprop cans. Launch it straight up and note how far it goes. Then rebuild that rocket using an LV-45 which has the same thrust as the KW service module engine, and an equivalent amount of fuel cans. You will see a big difference in performance.

You are better off to carry the extra fuel rather then the monoprop. If you do go the route of relying on monoprop thrusters as a backup, design the rocket so that you can stage the excess weight of the non functional rocket with its fuel cans.

It's the mass ratio of the tanks and the lower Isp of the monoprop engines that does it. All the monoprop tanks have horrible dry masses compared to their fuel mass, except for the 2.5m tank which is comparable to the LFO tanks. Combine that with the generally lower Isp of monoprop engines and it'll have far less dV.

Really only good in specialized scenarios like propelling a pod only, pods have a tank built-in so a couple of massless monoprop thrusters are a "free" emergency propulsion system.

[KerikBalm]

and in that case, you want to use the lander cans, since they are lighter with more monoprop.

and you also want to use the single occupant pods/cans, because 3x single pods weigh less than 1x 3 man pod

[Rus-Evo]

Whilst mono prop will be a less delta v solution, maybe his idea makes sense if it uses monoprop that was originally on hand for docking etc and can be converted for emergency use.

Having dual use items is a good idea. I have used backup mini landers to get kerbs back home when a mothership has fallen apart interplanetary before

[sdj64]

By weight, mono prop has far less delta V then the fuel oxygen mix. You can test this out the Kerbal way by building a small rocket using KW mod parts, their service module engine, and a couple of monoprop cans. Launch it straight up and note how far it goes. Then rebuild that rocket using an LV-45 which has the same thrust as the KW service module engine, and an equivalent amount of fuel cans. You will see a big difference in performance.

You are better off to carry the extra fuel rather then the monoprop. If you do go the route of relying on monoprop thrusters as a backup, design the rocket so that you can stage the excess weight of the non functional rocket with its fuel cans.

As Red Iron Crown said, monoprop tanks have much worse fuel/dry mass ratios than LFO tanks. Especially the KW ones, which are even worse than most of the stock tanks. Also, the KW service module engine has low ISP in atmosphere. If built with the stock 2.5m mono tanks and flown in vacuum, it would go farther.

For my two cents, fully monoprop fueled landers are best when space is more of a concern than weight. For example if you have a plane or sci-fi ship you want to look sleek and store the lander in a cargo bay, you can make a really small lander with O-10s or mod engines such as RLA stockalike. It also makes a good emergency system if you used it for docking and you can take the pod off your ship and fly it home using its RCS. But usually there's not enough for a whole return burn, just corrections.

[Sleepy_Steve]

Yeah, the mono prop is there from docking requirements on the Jool mission I'm doing... It's basically a flotilla of 7 different vessels sent on a large single ship. The idea is that when the mission's landings are finished, that I'll consolidate all remaining fuel and RCS into the return ship and see how far it can make it home.

Would it make more sense to burn off the extra RCS first and ditch those tanks before using the last of my fuel? Even though I'd be pushing a much larger mass with the RCS?