How much mass does a Kerbal have?

[Just Jim]

So a couple days ago I build a very basic 2-seat sub-orbital rocket. Quite literally 2-seat:

But when I launched it, I encountered something really interesting... and frustrating.  When I tested it unmanned, it flew perfectly several times.  But when I put two Kerbals into the seats, at about 10km it back-flipped badly. And no matter how many times I launched it, it went into the same back-flip, until I was finally forced to hide a small R/W wheel in it to help keep it stable.

And I encountered the same issue with this little rocket-throne I made a few weeks ago.

 

So this brings up a question I've had for a long time... do Kerbals themselves have mass, and just how much?  The way these rockets behaved both manned and unmanned definitely seems to support the idea they do, in fact, have some mass, and it shifted the CoM of the rockets when I added the Kerbals.

But I'm not using MechJeb or Engineer or whatever those mods are that calculate numbers... so I have no real way to confirm this.

Does anyone know what the deal is with Kerbals and their mass, and how to factor it in when you're building something like this?

Edited August 5, 2016 by Just Jim

[VincentThacker]

The easiest way to fix this is just to add a bit of weight below the rocket (opposite the seats) and try again. Yes, Kerbals do have mass. From the wiki,

Quote

A Kerbal in an EVAsuit has a mass of 93.75 kilograms (206.68 lb.), which is 0.09375 in-game Mass units. However, this only counts toward the mass of a craft if seated in an EAS-1 External Command Seat; Kerbals add no mass when in acommand pod or PPD-10 Hitchhiker Storage Container or the Mobile Processing Lab MPL-LG-2.

 

[MiniMatt]

from the wiki (which is sometimes out of date, but I think this one remains true): "A seated Kerbal adds 0.09375 tons of mass to a vessel and may cause problems when steering small rockets or rovers."

Drag is going to be the other issue now, those big green heads stuck out in the air stream are likely causing some of your flipping issues.

[Just Jim]

3 minutes ago, MiniMatt said:

from the wiki (which is sometimes out of date, but I think this one remains true): "A seated Kerbal adds 0.09375 tons of mass to a vessel and may cause problems when steering small rockets or rovers."

Drag is going to be the other issue now, those big green heads stuck out in the air stream are likely causing some of your flipping issues.

Interesting!  That could help explain it.

I also considered drag, but it occurred to me if it was drag issue, then wouldn't it be worse below 10km, in the really thick soup, so to speak, and not above it?

[Tex]

What @MiniMatt said- it looks more like a drag issue than a mass issue. But of course, like you mention, while it might be more prevalent before 10km, also remember that many rockets that aren't designed just so tend to flip in that area as well. Probably something to do with CoM changes due to fuel flow or some such combination of things. Snowball effect!

[Just Jim]

1 minute ago, Maximus97 said:

What @MiniMatt said- it looks more like a drag issue than a mass issue. But of course, like you mention, while it might be more prevalent before 10km, also remember that many rockets that aren't designed just so tend to flip in that area as well. Probably something to do with CoM changes due to fuel flow or some such combination of things. Snowball effect!

That could be.  I'm not positive, but the 10km area is also about where my fuel hits 50%... more or less...  perhaps the fuel flow CoM + small additional Kerbal mass + drag = back flip snowball effect?

[Corona688]

8 minutes ago, Just Jim said:

I also considered drag, but it occurred to me if it was drag issue, then wouldn't it be worse below 10km, in the really thick soup, so to speak, and not above it?

That depends heavily upon velocity.  Something really fast at 10K could experience more drag than something slow at sea level.

Also, when the atmosphere gets thin, the CoG problem will get worse, because your control surfaces will need to work harder to compensate.

Edited August 5, 2016 by Corona688

[Just Jim]

2 minutes ago, Corona688 said:

That depends heavily upon velocity.  Something really fast at 10K could experience more drag than something slow at sea level.

Good point, but I don't think that's it.  I kept it at about 300 m/s until 20km, and in the unmanned test flights it flew just fine with no flipping.

Edited August 5, 2016 by Just Jim

[Corona688]

Sorry, I sneak-edited.

Also, when the atmosphere gets thin, the CoG problem will get worse, because your control surfaces will need to work harder to compensate.

[MiniMatt]

The f9 - or whatever key bound to the aerodynamic overlay - key *may* (I haven't checked) offer some clarity - any big red arrows from their big green heads?

[Just Jim]

1 minute ago, MiniMatt said:

The f9 - or whatever key bound to the aerodynamic overlay - key *may* (I haven't checked) offer some clarity - any big red arrows from their big green heads?

Hmmmmm..... I'll have to try that.

[Corona688]

Why not mount the big radial parachutes lower to compensate?  With spaceplane symmetry, a la \o/, i.e. the 'R' key in VAB.

Edited August 5, 2016 by Corona688

[Just Jim]

1 minute ago, Corona688 said:

Why not mount the big radial parachutes lower to compensate?  With spaceplane symmetry, a la \o/, i.e. the 'R' key in VAB.

I just finished getting the screenshots I need (finally) but if I use it again I'll try that.  I may try it anyway, just to see if and how well it works. 

[wumpus]

That's a lot of mass for guys who barely top 1m.  I wonder how much of that is the RCS system.  Now we know why they do that funky waddle: too much gear weighing them down (and possibly simply fat kerbals as well).

[Corona688]

3 hours ago, wumpus said:

That's a lot of mass for guys who barely top 1m.  I wonder how much of that is the RCS system.  Now we know why they do that funky waddle: too much gear weighing them down (and possibly simply fat kerbals as well).

If they're carrying the same weight proportionally as a NASA astronaut, they'd only weigh 40 pounds / 18 kg.  Space Shuttle era suits weighed 300 pounds without the reaction pack(MMU), that was 300 more pounds!

Apollo suits were a little better at 200 pounds, but had no rockets.

Suddenly, I understand why they trained underwater.  Ye gods.

The 15 5 units of RCS fuel Kerbals carry weighs 20kg / 40 pounds by itself, though the game may not track that.

Edited August 5, 2016 by Corona688

[HvP]

I don't know who made this suggestion a while back, but I remember someone mentioning that the 4435 Narrow-Band Scanner has similar mass and dimensions as a Kerbal. So if you have the mod RCS Build Aid installed you can put the scanner in about the same place as your Kerbal will be and then tweak the weight distribution to balance the craft appropriately. RCS Build Aid can then show you in the VAB what the torque effect on your craft would be in flight (just remove the scanner when you're ready to fly.) As you might imagine it's also helps enormously with RCS balancing.

For your "throne" with the Kerbal seated on the top center; I've found that you need to very slightly move the command seat forward to center the Kerbal's head more over the vehicle's center of mass.

[Fearless Son]

Something I have found as a "workaround" for the issue of seated Kerbals adding mass is to shift the seats from being on top of a vehicle to being on the sides of it.  Admittedly, this is primarily something I do for rovers (keeps the center of mass low when the two seated Kerbals are beside the central axis instead of above it) but it could also work for rockets like this.  That way, the center of mass and center of drag all stay balanced across the center of thrust, assuming you fill both seats.  

[Snark]

Moving to Gameplay Questions.

[Geschosskopf]

On 8/5/2016 at 9:56 AM, Just Jim said:

So this brings up a question I've had for a long time... do Kerbals themselves have mass, and just how much? 

It depends.  If they're inside a command pod, then they have zero mass because the pod's mass doesn't change when they EVA.  However, when on EVA, they have a mass of 93.75kg or 0.09375 tons.  Then, when they go back inside, that mass vanishes as mysteriously as it appeared.  IOW, Kerbals, by their very existence, violate the Conservation of Mass.

 

On 8/5/2016 at 9:56 AM, Just Jim said:

But I'm not using MechJeb or Engineer or whatever those mods are that calculate numbers... so I have no real way to confirm this.

Does anyone know what the deal is with Kerbals and their mass, and how to factor it in when you're building something like this?

Well, a Kerbal weighs 0.09375 tons.  A lawn chair weighs 0.05 tons.  So a lawn chair occupied by an EVA Kerbal is 0.14375 tons.  So, when tweaking the design of the ship, PROVIDED it has a probe core so you can fly it without Kerbals, you can substitute some other part(s) of similar mass for either the Kerbal or the Kerbal-chair combination.  I recommend attaching the chairs and then using the offset tool to slide a Kerbal mass substitute under the chair, because if you substitute for both, it's hard to get the chair right where the substitute was.  And anything small is very sensitive to slight differences in CoM placement.

All that said, however, if you want to go rocket-riding, you also have to worry about asymmetric drag.  I really have no idea how to figure that out.