The drive is usually the surprise
Run the defaults: three outer motors at 42 dollars, a central one at 89, four igniters at 3.50, four dollars of consumables, a two dollar pad fee, 150 dollars of club and insurance spread over 20 flights a year, and a 90 mile round trip at 35 cents a mile split across three flights in the day. That comes to 253.00 for one press of the button, of which 215.00 is motors and 10.50 is the drive.
Change one thing — make it a two-flight day instead of three — and the drive share goes from 10.50 to 15.75. Make it a one-flight day and it is 31.50, which is three quarters of what a whole additional outer motor costs. The rule that falls out is unglamorous and reliable: the cheapest way to reduce cost per flight for most people is to fly more times per trip, not to buy cheaper motors.
Cost per newton-second is the honest comparison
Sticker prices between motor sizes are not comparable, because a big motor delivers more of the thing you are buying. Divide instead. The default cluster costs 229 dollars in motors, igniters and case amortisation, and delivers 3,120 newton-seconds, which is 7.3 cents per newton-second.
That number is what to carry when comparing a cluster against a single motor of similar total impulse, and it tends to favour the single one, because a cluster needs an igniter per motor and the fixed costs multiply. What a cluster buys instead is a thrust profile and an appearance that a single motor does not, and neither of those is a number this page produces.
The cluster sums, and where they stop being true
Total impulse is the sum of the individual impulses: three at 640 plus one at 1,200 is 3,120 newton-seconds. Total average thrust is the sum of the averages: three at 220 plus 400 is 1,060 newtons. Dividing one by the other gives 2.94 seconds, which is an effective burn time for the stack — useful, and not the burn time of anything in particular, since the outer motors burn for 2.91 seconds and the central one for 3.00.
Those sums are exactly right as totals and say nothing about the flight. Motors do not come up at the same instant. Two nominally identical motors do not produce identical curves. And a cluster whose motors are staggered in time has a thrust history that no pair of averages describes. The arithmetic here is for the budget and for the totals a rail exit calculation wants, not for a trajectory.
A motor that does not light
Set the count to 3 of 4 and the numbers change on one side only. The impulse drops to 2,480 newton-seconds, 20.5 percent short, and the average thrust to 840 newtons, 20.8 percent short. The cost does not change at all: you bought four motors and four igniters and the money left regardless.
The asymmetry problem is the one the page does not compute and mentions instead. Thrust that no longer acts along the centreline is a different situation from thrust that is merely smaller, and it is not one that a spreadsheet resolves. Whether a cluster is something to attempt at all, and under what conditions, belongs to your range rules and to whoever certified you.
What a season looks like
Twenty flights at 253.00 is 5,060 dollars for the year, of which 4,580 is motors and igniters and 210 is driving. Put differently, that year buys 62,400 newton-seconds of impulse. Whether that is a lot depends entirely on what you fly, but the figure is worth having in front of you once, because almost nobody knows theirs and almost everybody underestimates it.
Questions people ask
How much does one high-power rocket flight cost?
Only your own invoices can answer that, which is why every price on this page is a field. What the page does is make sure nothing is left out of the sum: the igniters, the share of the case, the consumables, the pad fee, the annual dues divided by how often you actually fly, and the drive divided by how many flights you get out of the trip. At the placeholder figures the answer is 253.00 for a four-motor cluster, and the interesting part is that 38 dollars of that is not motor money at all.
Is a cluster cheaper than one big motor?
Compare on cost per newton-second rather than on sticker price and you have a real answer. The default cluster buys impulse at 7.3 cents a newton-second including igniters and case amortisation. Put a single motor of similar total impulse through the same page — one motor, one igniter — and compare. Clusters usually lose that comparison, because the igniter cost multiplies with the motor count while the impulse only adds. They are flown for other reasons.
How do I work out the total impulse of a cluster?
Add the total impulse figures from the certification sheets of the motors you are flying. Three at 640 newton-seconds plus one at 1,200 is 3,120. Average thrust adds the same way, giving 1,060 newtons for the same stack. Dividing the two gives an effective burn time of 2.94 seconds. All three are correct as totals and none of them describe how the cluster actually behaves in time, because the motors do not come up together.
Should I count the reload case in the cost of a flight?
Spread over the flights you expect from it, yes, otherwise the first flight on a new case looks absurd and every flight afterwards looks free. The field takes the case price and the number of flights and divides. The honest difficulty is the second number, since a case lasts as long as it lasts, and if you have not worn one out you are guessing. Put in a low figure and a high one and see how much the answer moves — usually not much, which is itself useful to know.
Why does the flight cost change when I change the flights per trip?
Because the drive is a cost of the day, not of the flight, and it gets divided by however many times you press the button. At 90 miles and 35 cents a mile the trip costs 31.50 whatever happens; over three flights that is 10.50 each, over six it is 5.25, and over one it is the whole 31.50. For anyone who drives a long way to a launch, this line moves the per-flight cost more than any motor decision does.