The new SpaceX Falcon Heavy using Block 5 has almost 10% more maximum thrust at 2550 tons versus the first Falcon Heavy used in February of 2018.
Booster mate inside SpaceX's hangar at LC-39A ahead of Falcon Heavy’s static fire yesterday pic.twitter.com/G7ZPhOBkyj
— SpaceX (@SpaceX) April 6, 2019
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
— Elon Musk (@elonmusk) April 5, 2019
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“At low altitudes you want an ISP that is as low as possible.”
I assume you’re asserting this on the basis of power efficiency? That’s a pretty weird metric to use on a launcher. In general, efficiency is ghastly and isn’t something that a chemical rocket optimizes for.
I’ll buy that you want as much thrust as possible, but the most precious commodity on a rocket is reaction mass, and you can make the most thrust for the least mass by optimizing exit velocity (i.e., Isp). It’s not the most efficient use of the energy, but it is the most efficient use of the propellant.
Efficiency is a key metric for the electric propulsion folks on-orbit, because there electric power is often the scarce commodity. If you can make high mass flow * low exit velocity greater than low mass flow * high exit velocity for the same amount of power, then you’ll have higher thrust and higher specific energy imparted to the vehicle. This is the (somewhat dubious, IMO) sales pitch for VASIMR: Start at high mass flow when your specific energy is low, and trade mass flow for exit velocity as your specific energy increases.
But at launch, efficiency of chemical propulsion is the least of your worries.
Launch trajectories are a black art. If you have more thrust, you can loft a bit more for the same gravity drag, which then lets you pitch over more aggressively, because you’re then higher when you hit max-q, which lets you hit it faster.
Alternatively–and more importantly–you can launch a heavier payload for the same trajectory.
The extreme example is the MX missile’s steam cannon launch which, in effect, uses Earth as its reaction mass with a disappearingly small Isp.
The essence is it safes prop at low altitude and low kinetic energy by experiencing less gravity losses (you try to maximize the gains from centrifugal force compared to gravity), which translates into more payload into LEO (indeed, throttling back to keep G within tolerance). Max Dynamic pressure of atmosphere (and lateral G’s) usually are the limiting factor for civilian launchers. One result is there is a higher chance of recovering the middle stage, since it’ll have more prop in the tank, and can even decelerate more aggressively. (Since SpaceX builds with 40% safety margin (old info I recall), they have reserve). Or you can trade for a little bit more cargo.
The remark about ISP actually is incomplete. At low altitudes you want an ISP that is as low as possible. Or better, you want the ISP that is appropriate for the speed you are travelling at (in metric ISP is exit velocity/~ten ). Higher ISP is only needed at higher speeds, and ideally, to optimize fuel consumption, at zero altitude your ISP is close to zero (that’s how the physical equations work out). Since we do not have engines that do that, low altitude boosters operate in the lower 100-250 range with high TWR, while first stages at 300-450 and upper stages 360-542 (includes triprop). Thus: you want to optimize for TWR at zero altitude and for efficiency or the velocity of your rocket exhaust (in ISP) at higher kinetic speeds. Throttling high TWR engines gives both. I’ll skip nozzle theory for another day.
That gee only needs to be canceled with thrust for a short time. Once the rocket is above most of the atmosphere, thrust is made perpendicular to gravity. The engines have to be throttled back anyway for a period before, and after maxQ. More thrust means more throttling. and of course, the more gees you pull, the stronger(more mass) the structure has to be.
Like I said, more thrust is nice, but more delta vee, and higher ISP is what you really need.
Even at the same delta v, better thrust to weight ratio gives you more useable delta v, because the first 1 g of thrust is wasted just canceling gravity, it doesn’t add any speed. It’s only the thrust over 1 g that counts launching from Earth.
More thrust is nice, but unless it provides more deltaV, it’s not a big deal. If the increase in thrust is at the same fuel/oxidizer flow rate, it’s a huge deal.