# Calculating total burn time for a rocket under constant acceleration with two propellant consumption rates

I tried posting this question on the Physics side of StackExchange but didn't get any response after a week, presumably because it was lost within the broadness of physics in general, so I'm hoping that the focus on space-related topics here might garner more attention. This is a math-centric question though, so if there are more suitable places to ask, I'm open for suggestions.

I'm trying to create a simplified model of hypothetical fusion-powered thrusters for a sci-fi setting (on Excel), such that upon entering ship mass and operating parameters, all the common rocket performance metrics will be calculated as outputs. I'm aiming for a degree of realism, but being simplified, the model as it is now will neglect a lot of the complexities behind the engineering and nuclear physics of the ship design itself.

I'll provide all the work I have done further below, but I want to elaborate on my question first to provide context. Essentially, I'm having trouble figuring out how to calculate the total time a constant acceleration thruster can burn because I have two types of propellant being consumed at different rates: the fusion fuel, and inert propellant (i.e. water). This is because I wanted a high exhaust velocity thruster that could inject extra mass into the exhaust to increase thrust/accel when needed. I have kept the fusion fuel's mass flow rate constant to maintain a constant thrust power at a fixed specific power, so I know that exhaust velocity and thrust will go down as the mass of the ship decreases in order to maintain the target constant acceleration.

With a deltaV calculated, I assumed that the total burn time was just deltaV divided by the acceleration, as it should give me the time it takes to reach that velocity, and by definition of deltaV, should be when it has expended its full propellant load. However, a spreadsheet iteration check proved otherwise, and after a couple days of fruitless thinking later, have led me here.

Anyway, here are my numbers for the ship.

Pure fusion means the ship is only utilizing the fusion products as reaction mass. Therefore the additional propellant is untouched and is treated as essentially the ship's dry mass. The following calcs were done with that in mind.

Overall, I didn't see any issue with this set of results. Since I'm not varying the fusion fuel's mass flow rate, the thrust power remains the same, and by extension the Ve and thrust. As this entails a constant thrust case, the acceleration should increase to account for the ship getting lighter, which I am pleased to see being reflected here. The two time calcs for expending all the fusion fuel also match as expected.

Then comes the constant acceleration case. I kept the same thrust power to emulate the scenario of simply injecting more reaction mass without changing the fusion mdot. Here, I figured that since acceleration is known (being chosen by the pilot), I could calculate the variable thrust instead, and get Ve from there, along with everything else.

Overall, a lot of things still seem to be working. Exhaust velocity, thrust, additional prop flow rate, and specific impulse follow the expected trends. But the two highlighted values of delta V and the burn time are ones I'm suspicious about, since when I tried iterating on a spreadsheet, that burn time was at a point when the ship still had enough of both types of propellant to use. Delta V started off as the calculated value at t=0, but confusingly started to increase for a bit before going down as expected. It did approach zero around when the ship's total mass equaled its dry mass, but the inert propellant mass had gone negative, so that doesn't make realistic sense. Ve, Trust, and Fmdot maintained their proper trends.

To wrap it up, I suspect that something isn't being properly reflected in my calculations, probably the change in mass over time, but technically the ideal rocket equation is independent of time. I also think that the mass ratio doesn't distinguish between the two propellants, but I'm not sure if that matters, or how I can account for it.

All I can say is that for a constant Fmdot in a constant accel case, the propellant flow rate is decreasing with time. I'd like to be able to calculate when the propellant is expended, as there are many resulting scenarios I want to consider:

1. Is there an analytic solution to figuring out how much fusion fuel is left when the inert prop is depleted?

2. Is there some fundamental relation between the masses of the two propellants and ship (whether it is at a const thrust power, accel, or some other condition) such that I can find a set of parameters to deplete both around the same time?

3. And while I'd rather not have transient Fmdots and thrust powers, but as an extension of #2, is there a way to achieve the simultaneous depletion by steadily increasing the Fmdot to bump up the thrust power, reducing prop MFR so that both are used up at the same time?

I hope that the solution is simple and I'm just overlooking it, or misunderstanding something. If it's not, then I hope that the smarter folks out here might be able to lend me a hand. Resolving #1 would be more than enough for me, as the depletion of the inert propellant simplifies the problem to conventional case of a single propellant. However, if anyone could resolve #2/3 either analytically or numerically, that would be tremendously appreciated.

And finally a thank you to those patient enough to read through all of this in the first place.

Edit: Organic Marble pointed out that the "constant accel" condition may be inaccurate, as vehicle mass has to be assumed first in order to derive the propellant MFR as a final output, whereas engines should be calculating thrust from the MFR as an input. Further elaborations on this matter is welcome.

• I think you have to integrate over the burn rather than just use algebra. It's been way too long since I've dealt with that kind of math to come up with an answer quickly, though. Jul 18, 2021 at 16:09
• How do you calculate the thrust produced by the engine for a given "Add Prop MFR"? For example, what thrust does the engine produce at an "Add Prop MFR" of 20.0? Jul 18, 2021 at 19:58
• @OrganicMarble the inert propellant mass flow rate is calculated from the thrust divided over the the exhaust velocity, subtracting the mass contribution of the fusion fuel. The constant accel table calculates in this order of relation for the variables: Pick a desired accel -> using constant thrust power, calc thrust for the mass at that time -> from thrust, calculate exhaust velocity -> with thrust and Ve, calc prop MFR. So numerically experimenting with the sheet, a 20 kg/s add prop mfr (initial) equates to around 3.7Gs, or 36m/s^2 Jul 18, 2021 at 20:35
• @OrganicMarble which is 10900 kN of thrust. So basically the MFR is the last thing to derive from the system starting from the acceleration as your arbitrary input. Jul 18, 2021 at 20:43
• In the real world, the thrust from an engine would be set by its throttle (e.g. the amount of fuel supplied to the engine). If you can't calculate the thrust from a propellant flow, I'm inclined to call this a completely fictional (although interesting) engine and probably not a good fit for this site. Jul 18, 2021 at 20:43

I noticed between the info in the question, additional info in comments, and an assumption of 0.0 delta thrust at 0.0 flowrate, there was enough information to draw a exponential curve for the throttle.

Delta Thrust (N) Inert Prop Flowrate (kg/s)
0 0
4146140 4.18
10141140 20.0
13956140 37.6

Using this throttle characteristic curve, I wrote a simple simulation. At each time step, the "engine controller" calculates the throttle setting (i.e. mass flowrate) needed to provide enough delta thrust to achieve the desired constant acceleration.

The burn duration to use up all the inert prop is close to that stated in the question. So is the total delta-v, but that's a softball, since it's at constant acceleration.

For the parameters I calculated, the only significant difference I saw was in the inert prop mass flowrate at cutoff - I had almost twice what you did. This could be because of inaccuracies in fitting the throttle characteristic curve, or something else.

  Cutoff Parameters

Time:   8758.00000      sec

Performance

Inert mdot      7.74639273      kg/s
Thrust          7306489.00      N
Acceleration    49.0025787      m/s^2
Velocity        429159.062      m/s


Is there an analytic solution to figuring out how much fusion fuel is left when the inert prop is depleted?

Yes (well, sorta..). Since the fusion fuel flowrate is constant, you just multiply that flowrate by the time to depletion of the inert propellant, and subtract the result from the starting fuel mass.

To partially answer #2, you can achieve simultaneous depletion by setting a different (lower) constant acceleration.

   Cutoff Parameters

Time:   498257.000      sec

Performance

Inert mdot      3.23462412E-02  kg/s
Thrust          1247464.75      N
Acceleration    12.4745159      m/s^2
Velocity        6226965.00      m/s


For #3, those answers can be derived by playing around with the sim.

Simulation equations

$$T_d = (A_d M_t) - T_f$$ $$\dot m_i = (\frac {T_d} {K_1})^{K_2}$$ $$M_i = M_i - \dot m_i\Delta t$$ $$M_f = M_f - \dot m_f\Delta t$$ $$M_t = M_i + M_f + M_s$$ $$T_t = T_d + T_f$$ $$A_a = \frac {T_t} { M_t}$$ $$V = V + A_a\Delta t$$

• $$A_a$$ Actual acceleration
• $$A_d$$ Desired acceleration
• $$K_1$$, $$K_2$$ Throttle characteristic coefficients from curvefit
• $$M_f$$ Mass of fusion propellant
• $$M_i$$ Mass of inert propellant
• $$M_s$$ Mass of structure (aka dry mass)
• $$M_t$$ Total vehicle mass
• $$\dot m_f$$ Flowrate of fusion propellant
• $$\dot m_i$$ Flowrate of inert propellant
• $$T_d$$ Desired thrust from inert propellant
• $$T_f$$ Thrust from fusion propellant
• $$T_t$$ Total thrust
• $$V$$ Velocity
• $$\Delta t$$ Simulation time step
• Thank you for revisiting the question with data! It's tremendously helpful to have someone sanity check my numbers. Given that masses (dry and wet) can vary on a ship to ship basis, do you think it would be useful to develop for each ship a "characteristic throttle function", relating the delThrust and prop MFR? Perhaps with a few more steps to get a better R^2 value for the fitting exponential equation? I think that with an equation and coefficients, I can get the spreadsheet to produce the same data from your timeplots in tabulated form as to facilitate later calculations. Jul 20, 2021 at 5:24
• @Genso some more data points might help the throttle curve fit, especially at low flowrates where the curve is changing a lot. I forgot to mention that I also ran the "fusion-only" scenario in my "sim" and it matched what you had in the question 100%. Jul 20, 2021 at 13:43
• Thanks again for checking my numbers. I believe that with the plots you provided as reference, I'll be able to create my own for the various things I want to test. My final request before accepting your answer would be if you could provide the spreadsheet you used, or the data points in a text file/pastebin so I can use them as a starting point. Jul 22, 2021 at 4:49
• @Genso paste for the simo depletion case pastebin.com/i5pAY91W Jul 22, 2021 at 22:28
• @Genso paste for the 49 m/s^2 case pastebin.com/6wmL9rj5 Jul 22, 2021 at 22:29