The payload mass factor challange

[cmfarsight]

We all know that efficiency is king ("yes Jeb SRB's are king to") and higher is just better because well its higher. I therefore propose a challenge to create a vessel that can get at least a 20 tonne payload to a 200km orbit with the highest payload mass factor possible.

Payload mass factor is the starting mass of the vessel divided the final mass in orbit, to find these out there a load of mods that will do it ie mech jeb

Rules are very simple

Minimum payload is 20 tonnes ie the vessel in orbit must weigh at least 20 tonnes

Orbit must be a 200km circular orbit.

No modded engines or fuel tanks

To enter post a pic on the ground and one in orbit along with their respective weights and payload mass factor

Winner will be whoever has the highest mass factor, draws will be settled with who has the heaviest vessel

I will keep a running leader board here assuming this doesn't die in the next 5 min

Here is a craft to start us off

so a starting weight of 251.76 tonnes

Final weight of 57.72 tonnes

Giving a pay load mass factor of

57.72/251.76= 22.9%

Edited May 16, 2013 by cmfarsight

[tavert]

Actually higher is worse because of the Oberth effect, there's no reason to ever orbit higher than 70-75 km (with the exception of mapsats).

There was a big discussion about this in the previous payload fraction challenge, but technically any engine or fuel tank that gets used during the ascent should not count towards payload. Otherwise you can take a big spaceplane and easily get to orbit using only 10% of its takeoff mass in fuel.

[Reavermyst]

I actually launched half of my station into a 150km orbit earlier, I need to check and see how much that weighed.

Eh it's not that significant

Edited May 17, 2013 by Reavermyst

[Plur303]

There was a big discussion about this in the previous payload fraction challenge, but technically any engine or fuel tank that gets used during the ascent should not count towards payload.

I agree with this. The payload should not be any part the ship itself involved in achieving orbit. It should be dead weight. Perhaps you can decide on some standard payload that everybody must deliver into orbit. Otherwise people will just make the "payload" consist of useful mass like spent fuel tanks and engines.

[tavert]

Jets can get 70% or higher payload fraction with plenty of intakes: http://imgur.com/a/87DPK

Also low-drag parts like RCS blocks, wings, cockpits, etc can inflate the number. With a standard-drag payload and no jets, payload fraction for a highly-optimized design maxes out at around 18%.

[cmfarsight]

I agree with this. The payload should not be any part the ship itself involved in achieving orbit. It should be dead weight. Perhaps you can decide on some standard payload that everybody must deliver into orbit. Otherwise people will just make the "payload" consist of useful mass like spent fuel tanks and engines.

while I agree that that is not an actual pay load it gives a true representation of what the lifter is capable of as at the end of the day mass is mass, you can swap out fuel for probe parts but its still mass. assuming I am not missing something that is.

[tavert]

No, not all mass is the same here. A quantity of mass cannot fairly be called "payload" unless it could be replaced with an equal amount of mass (assuming the same drag coefficient) made of any other arbitrary components and the rocket would still get to orbit in basically the same way. For example in your rocket above, you can't replace your 2.5 ton poodle or the 2-ton dry mass of the fuel tank you started using with equivalent structural components and still get that orange tank, command module, etc to orbit.

[Francesco]

I agree with tavert, you have to separate the payload mass from the launch vehicle mass.

in real life, a rocket's payload to LEO doesn't include the last stage of the rocket itself.

Edited May 17, 2013 by Francesco

[nyrath]

Well, unusually you have

propellant mass

structural mass

power plant mass

propulsion system mass

payload mass

Generally the astronauts and the habitat module are considered part of the payload mass.

The total of everything is the "wet mass", the total minus the propellant mass is the "dry mass"

[NeilC]

Actually higher is worse because of the Oberth effect, there's no reason to ever orbit higher than 70-75 km (with the exception of mapsats).

Interesting, can you elaborate a bit tavert? Are you saying that, assuming the payload needs to maneuver after a circular LKO insertion, the best spot to do that burn is as low as possible due to the Oberth effect? How significant is the delta-V penalty for burning at, say, 150x150km instead of 70x70km?

I agree 70x70 is a better starting orbit from a pure efficiency point of view, but I think there are some reasons to break this guideline. For instance, I use a 100km parking orbit for stations and the first launch of multi-launch craft that need to rendez-vouz. I'm rarely bang-on with my launch phase angles, particularly with spaceplanes, so it's valuable to have a "catch up" band below the target's orbit. This way if I'm slightly trailing my 100x100 target in a 70x70, I can complete 2-3 orbits and catch up to the rendezvous instead of needing to burn to ~150x100km and wait 10-20 orbits to fall back to the rendez-vous.

Doesn't it also take less dV to change orbital inclination at a high apo than a low peri? I know you mentioned mapsat as an exception, did you mean just to get them higher for a wide field of view or because they should be launched into a high elliptlcal before changing to a polar orbit?

[tavert]

If you consider the total cost of getting to the parking orbit in the first place plus getting to wherever you're going, then yes you want your parking orbit to be low. The exact delta-V penalty depends where you're going, and also how much more delta-V you expended to get to the high orbit vs if you had gone into a low orbit. Since that depends on your rocket and ascent trajectory, that's hard to predict in general.

For a mostly-fair comparison example, let's take going to the Mun.

First burn of Hohmann transfer from 70 km to 11400 km: 864 m/s

Hohmann transfer from 70 km to 150 km (I think this should be a lower bound on the increased cost of going straight into the higher orbit): 63.8 m/s burn at 70 km + 62 m/s burn at 150 km

First burn of Hohmann transfer from 150 km to 11400 km: 807.2 m/s

So the difference is at least 69 m/s. (Your relative velocity when you get to the Mun will be roughly 10 m/s different, but I'm ignoring that here)

If you're doing an orbital rendezvous and want a lower phasing orbit, then I suppose it makes sense to have your station a little higher. I was referring to the wide field of view rather than changing inclination. You are correct about inclination changes being cheaper from high apoapsis, but for a mapsat around Kerbin I would rather just make my gravity turn to the north or south and go straight into a polar orbit from the surface. If you launch a bunch of mapsats simultaneously and only one is for mapping Kerbin, it's cheapest to use a gravity assist from the Mun to get from equatorial into polar Kerbin orbit.

Edited May 18, 2013 by tavert