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# Delta v for mono props

## Question

Is there a way to calculate the amount of delta v for a monopropellant engine? Trying to work out if my satellite will have enough delta v to position itself

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If you are talking about the RCS thrusters, they have a vacuum specific impulse of 240s. So you can do the math ∆v = ISP x 9.81 m/s2 x ln(m0/m1

The only monoprop engine is the O-10 "Puff" MonoPropellant Fuel Engine and it has an ISP of 250s

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1 hour ago, Echo__3 said:

If you are talking about the RCS thrusters, they have a vacuum specific impulse of 240s. So you can do the math ∆v = ISP x 9.81 m/s2 x ln(m0/m1

The only monoprop engine is the O-10 "Puff" MonoPropellant Fuel Engine and it has an ISP of 250s

Yes, sorry rcs engines.

So is m0 without fuel and m1 with full fuel? What's Ln?

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ln is the natural logarithm.

Change in velocity equals your specific impulse of 240 seconds times 9.81 meters per second per second times the natural logarithm of your wet mass divided by your dry mass.

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12 hours ago, Echo__3 said:

ln is the natural logarithm.

Change in velocity equals your specific impulse of 240 seconds times 9.81 meters per second per second times the natural logarithm of your wet mass divided by your dry mass.

Right, so...if anyone fancies checking my maths.

Satellite is made up as follows...

1 x probodobodyne HECS

3 x stratus V roundified monoprop tanks

3 x place anywhere 7 linear rcs ports

3 x OX-4L 1x6 photovoltaic panels

1 x Z-200 battery

1 x RA-2 relay antenna

I worked out a delta v of 10195

Am I as bad at math as I think I am?

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@ConArt70 that number seems excessively high. What’s the wet and dry mass of this probe?

My calculations say ISP*g is 240s*9.807m/s/s = 2353.7m/s, so to get over 10,000m/s of delta-V, ln(m0/m1) would have to be well above 4; the inverse of ln(x) is ex and e ~= 2.72 so if ln(x) = 4, x = e4 which is 54.6! KSP fuel tanks don’t have mass ratios nearly that good- I don’t think any of them are even in double figures- so I suspect you’ve done the calculation backwards somewhere, but without knowing your m0 and m1 (and without having KSP open in front of me to replicate the design) I can only guess.

edit- after reading your previous comment, I think you have m0 and m1 mixed up- m0 is wet mass with full fuel, m1 is dry mass with zero fuel.

Edited by jimmymcgoochie
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14 hours ago, ConArt70 said:

Right, so...if anyone fancies checking my maths.

Satellite is made up as follows...

1 x probodobodyne HECS

3 x stratus V roundified monoprop tanks

3 x place anywhere 7 linear rcs ports

3 x OX-4L 1x6 photovoltaic panels

1 x Z-200 battery

1 x RA-2 relay antenna

I worked out a delta v of 10195

Am I as bad at math as I think I am?

This is what I get :-

HECS                                                     Wet = 0.1T        Dry = 0.1T

Stratus V Roundified  x 3          Wet = 0.3T         Dry = 0.06T

Placeanywhere RCS x 3            Wet=0.06T         Dry=0.06T

OX-4L 1x6 Photovoltaic x 3   Wet=0.0525T   Dry=0.0525T

Z-200 Battery                                 Wet=0.01T        Dry=0.01T

RA-2 Relay antenna                    Wet=0.15T         Dry=0.15T

Totals                                                   Wet=0.6725T   Dry=0.4325T

0.6725/0.4325=1.555

ln(1.555)=0.4415

240(ISP vac) x 9.81 x 0.4415 = 1039.5 m/s

Edited by AstroG
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1 hour ago, ConArt70 said:

I worked out a delta v of 10195

A tour of the entire system just using rcs?! Would make a good one for the challenge thread actually

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7 hours ago, FruitGoose said:

A tour of the entire system just using rcs?! Would make a good one for the challenge thread actually

Stock RCS would be challenging, but I know of a nice monoprop arcjet thruster with an ISP of 1300...

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22 hours ago, jimmymcgoochie said:

@ConArt70 that number seems excessively high. What’s the wet and dry mass of this probe?

My calculations say ISP*g is 240s*9.807m/s/s = 2353.7m/s, so to get over 10,000m/s of delta-V, ln(m0/m1) would have to be well above 4; the inverse of ln(x) is ex and e ~= 2.72 so if ln(x) = 4, x = e4 which is 54.6! KSP fuel tanks don’t have mass ratios nearly that good- I don’t think any of them are even in double figures- so I suspect you’ve done the calculation backwards somewhere, but without knowing your m0 and m1 (and without having KSP open in front of me to replicate the design) I can only guess.

edit- after reading your previous comment, I think you have m0 and m1 mixed up- m0 is wet mass with full fuel, m1 is dry mass with zero fuel.

Ah, no, I got the wet/dry the correct way around. What I did do wrong was I squared 9.81. I assumed you had to do this as part of the equation

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On 4/1/2021 at 2:18 PM, FruitGoose said:

A tour of the entire system just using rcs?! Would make a good one for the challenge thread actually

Just to clarify somewhat about why I was trying to work this out. I didn't have any small engines unlocked yet and so was trying to figure out if the relay satellites I were about to place in synchronous orbits had enough delta v to circularise using rcs thrusters.

Interesting idea there though. I'd be interested in seeing someone attempt it!

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So, another question relating to this problem.

The satellite in question uses three rcs thrusters. The thruster has an ISP of 240. Does that mean I need to multiply 240 by 3 or just still use 240 in the equation?

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Just use 240s in the 'rocket equation

If you want a way to remember that, Isp is an efficiency, so three thrusters have the same efficiency as one.  The number means you get 240 kg-force for every 1 kg/s of propellant-use-rate.

With three single-jet RCS ports on that probe, you will need to orient it so that one probe is pushing you prograde, in order to do your circularization.  Also, the port should be centered well enough that it does not tend to turn your probe, any more strongly than your reaction-wheel torque can counter.

It is likely to work for you (I have done similar) but as you said, it will be interesting to attempt.

Edited by OHara
typos
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27 minutes ago, OHara said:

Just use 240s in the 'rocket equation

If you want a way to remember that, Isp is an efficiency, so three thrusters have the same efficiency as one.  The number means you get 240 kg-force for every 1 kg/s of propellant-use-rate.

With three single-jet RCS ports on that probe, you will need to orient it so that one probe is pushing you prograde, in order to do your circularization.  Also, the port should be centered well enough that it does not tend to turn your probe, any more strongly than your reaction-wheel torque can counter.

It is likely to work for you (I have done similar) but as you said, it will be interesting to attempt.

Yeah, so basically I have three rcs place anywhere ports on the bottom of the craft, essentially replacing what would normally be a liquid fuel engine.

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