What is an "Ideal Ascent Profile"?

[EdFred]

Or - to put it the other way around - the "ideal" ascent profiles only fit "ideal" vehicles. Yet another good point, although several of us have stressed that 'ideal' depends on design.

On the other hand, if I want to complain about all the "boring" advice when my vehicle has a TWR of 0.1 at launch I shouldn't be surprised that no-one seems to help me get off the ground. I'm not saying that's your case, but your design is, erm, stressing the boundaries, shall we say?

You mean 15,000dV won't be enough to get you to orbit?

[Pecan]

Yeah exactly - if the thing won't lift-off no advice in the world is going to help so I'd just have to live with the limits of my design.

NASA make paperweights too, right?

[EdFred]

Yeah exactly - if the thing won't lift-off no advice in the world is going to help so I'd just have to live with the limits of my design.

NASA make paperweights too, right?

Yeah, I've run into that with minimalist designs. "Oh, this rocket is way more efficient than I expected." Launch. Nothing happens. "WTF?" Grumbling ensues when I realize that I left KER on Duna/Mun/some other low gravity body in the VAB.

[Laie]

You mean 15,000dV won't be enough to get you to orbit?

dV isn't everything. If it can't get off the ground... (shrugs). But we've discussed that already. Besides, we're talking about an ideal ascent (and seem to silently assume that "ideal" means the most energy-efficient). That's not the only approach and may not necessarily be the cheapest, though it often is.

As to your rocket, Alshains' general approach should still work: a) get to a 30-40km apoapsis as quickly as TWR and terminal velocity allows, then keep going with as much thrust as necessary. Only that your rocket can't provide quite as much thrust as would be desirable, for a while, after the first stage runs out. It's still enough to keep you going and eventually make orbit, though, so what.

In order to run a more ideal ascent, you should replace the central Skipper with a Mainsail. Heavier and worse ISP, but also less gravity losses during the first three minutes. It should prove to be slightly more efficient overall (also slightly more expensive; and as the previous design seems to be good enough, why bother).

[cantab]

With low-TWR upper stages I've found I need a steeper-than-normal ascent profile. I believe it's because a low TWR will waste more thrust in percentage terms overcoming drag, so it needs to get higher to reduce the drag losses.

[EdFred]

With low-TWR upper stages I've found I need a steeper-than-normal ascent profile. I believe it's because a low TWR will waste more thrust in percentage terms overcoming drag, so it needs to get higher to reduce the drag losses.

I'm pretty sure it has less to do with drag but that when you're experiencing 40kN of gravitational force and your rocket only provides 40kN of net thrust, anything besides pointing straight away from the planet's center results in a vertical deceleration. Of course when you've generated enough altitude, you actually want to fall, since that's what an orbit is. Of course you want to do that with about 2275m/s of horizontal velocity. If you've got a net 1.0TWR ratio, and you're pitched at 45º on ascent, you're only getting a vertical TWR of about 0.7, about 0.7 in the horizontal, and losing vertical velocity. This vertical velocity loss can be overcome at 45º by having at least a net 1.414 TWR, and a net 2.0TWR when pitched over to 30º. Of course, at some point you don't want any more vertical velocity.

[Laie]

With low-TWR upper stages I've found I need a steeper-than-normal ascent profile. I believe it's because a low TWR will waste more thrust in percentage terms overcoming drag, so it needs to get higher to reduce the drag losses.

Exactly. The steep early ascent buys you time at high altitudes, where drag (almost) doesn't matter and where your put-put upper stage can build up enough speed.

[mhoram]

Once I had a closer look into ideal ascent paths for a ship from Kasuha that had a very low TWL-upper stage.

The optimal ascent strategy for reaching a 75km orbit was to first get to an altitude of ~95km so that it had enough time to circularize at 75km.

So I agree that steep ascents are advisable for low-TWR-late-stages.

[sarbian]

The optimal ascent strategy for reaching a 75km orbit was to first get to an altitude of ~95km so that it had enough time to circularize at 75km.

And some wonders why there is a "more realism" movement...

[cantab]

You could make a rocket that required the same approach in RO if you wanted. You can argue that it's a kooky design that no real engineer would let fly, but there need be nothing unphysical involved.

[sarbian]

The stock drag makes those vertical ascent more optimal for nearly anything. To need that in RO you have to use stupid designs.

The problem with stock aero is that it is just counter intuitive. No need to go as far as FAR to fix that.

[Laie]

So I agree that steep ascents are advisable for low-TWR-late-stages.

Also depends on how late we are talking about. If that upper stage in EdFred's sample rocket could afford to come online only a few moments later, it's TWR would be wholly sufficient. Plenty, even.

[EdFred]

Also depends on how late we are talking about. If that upper stage in EdFred's sample rocket could afford to come online only a few moments later, it's TWR would be wholly sufficient. Plenty, even.

It's sufficient as it is. I have messed with different manual (and MJ programmed) ascent profiles with that, and the bottom stage can drop below 25km, and still obtain a 80km orbit with a little go juice left in the orange tank, if done correctly. Depending where the stage is dropped and how you continue the ascent I've seen anywhere from under 50dV left, to 300dV left. I've also not made it to orbit when I pitched over a bit too early.