Why Don’t you use directly N2 bottles to pressurize your tanks. You don’t need that much pressure, 4 or 5 – 200 bar bottles would fit, no?
We actually have a couple of detailed blogs on the subject and reasoning behind this design:
But the tl;dr is that we would need to bring approx. 67,000 standard liters of gaseous nitrogen on board for the flight, which equates to eleven 20 liter, 300bar COPV’s. That becomes a fairly large mass fraction of the whole rocket and probably the most expensive single system on Spica. The LN2 burner saves us a lot of weight and money in exchange for a bit more complexity.
Test like you want to fly, fly like you have tested.
The heat exchangers and hardware are lighter than more nitrogen bottles, which makes for a more effective rocket.
How about tap-off cycle? Or enginering to make it work would have almost no impact to performance?
That would complicate the engine’s design and manufacturing, which we would like to avoid.
How high would the Apogee of Spica be if you would for whatever reason stop the burn, let’s say for example one and a half seconds later than planned?
i would love to know that, because sometimes timing precision isn’t perfectly exact and i have no idea how big of an impact a small change in burn time has on apogee.
disclaimer: if you find spelling or grammar mistakes you can keep them, and it’s 2:13 A.M. here in switzerland right now so i’m a bit tired
The engine shutting off 1.5s later would not have a big effect on apogee, as you would still have utilized all the energy stored in the propellants. You, of course, would see a bigger (negative) impact if the engine shut off 1.5s early and did not use all of it’s propellants for some reason. But there you have a lot of variables to account for before coming to a clear answer.
We have members who volunteer in an organization that teaches kids coding and computer skills here in Denmark. So for one of their exercises they developed a fairly realistic Spica flight simulator where you can time your engine shut off, throttle the engine down before MECO or during MaxQ and see how all that affects G loads on the astronaut as well as the maximum apogee. The goal of the game is to keep the astronaut at “comfortable” G loads and maximize apogee. So if you have some spare time, you can experiment with these scenarios a bit, and hopefully it can give you a clearer understanding of the effect of these variables. You can find the simulator here:
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