how fast do you get before switching to rockets when you fly a plane?

[lammatt]

since i dont airhog, the highest i get before the jets die is ~25km and i couldnt get much faster than ~1.2km/s surface speed before switching to rockets

i checked a ~80km orbit requires 2.2km/s orbital speed and my plane (usually Aeris4A) has only 1.3km/s dV with the rockets: limited by the oxidizers.

and i aint always able to get to orbit by the time the fuel runs out.

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i am thinking i should have gained more speed before switching to rocket

but i ran into the problem that at ~20km up high, the reentry effect kicks in at ~1.2km/s and my speed doesnt seem to go up any much.

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so i am curious about how fast do you guys get before switching to rockets?

Edited July 30, 2013 by lammatt

[littlebattler]

1.2-1.3k is perfectly good, 1-1.1 is doable. My suggestion is see if you can use more efficient rockets (nukes + aerospike/LV-45/LV-30/radials assist, 909s or aerospikes on their own). I know that with good fuel balance, two turbojets can carry twin nukes and an aerospike to 25km and reach 1.2km/s with good piloting (more difficult now since the new SAS is iffy) and reaches 72km orbit with some fuel left over if you turn off the aerospike at the right point (for me it's usually at around 1.8 km/s).

Of course, you can always solve the problem more easily by clipping intake parts together to boost the flight ceiling of your jet engines. I personally wouldn't do that as it takes away from the challenge of desigining a "proper" plane (as intended by the limitations of the parts), but the option is there and some people do like it.

[Merinsan]

Are you saying that you are only going to 2km in altitude?

I would suggest you aim for about 20km for a space plane.

I've hit 2km/s at that altitude (although it was an ugly plane). I expect at least 1.5km/s. I always try to burn off as much jet fuel as possible too, just to lighten the load.

[lammatt]

well... i mean... 20km...

see...i am chinese... and i aint good at math (which is very normal for chinese people; especially those who are science graduates like me... nah... just kidding but yes.. i counted the numbers wrong it should be 20)

Are you saying that you are only going to 2km in altitude?

I would suggest you aim for about 20km for a space plane.

I've hit 2km/s at that altitude (although it was an ugly plane). I expect at least 1.5km/s. I always try to burn off as much jet fuel as possible too, just to lighten the load.

btw... i suspect if you can really hit 1.2km/s at 2km in KSP

Edited July 30, 2013 by lammatt

[Swifty]

I fly at around 25km and get my craft going as fast as it can on jets i.e. It stops accelerating under jet power. I then pitch up a bit ~20 deg and get as higher climb rate as possible before flame out - then I switch to rockets

[Sma]

It may sound "backwords" but when you reach...maybe 18-20km in altitude, you can start throttling back to help save on fuel. So long as your speed is still increasing and your AP is still going up slowly then you should be ok. If you can maintain a small or no rise in altitude you should gain a ton of speed, and because you're jet engines are throttled back a bit you won't need as much air to run them. Then you can also squeeze a little more air into them by firing the rocket engines at the same time. THen like Swiftly said, once it stops accelerating pitch up and turn off the jets and close intakes (maybe assigned to a single action group) which in theory should decrease drag a bit.

[Merinsan]

well... i mean... 20km...

see...i am chinese... and i aint good at math (which is very normal for chinese people; especially those who are science graduates like me... nah... just kidding but yes.. i counted the numbers wrong it should be 20)

No problem, I was thinking you meant that. Otherwise I'd have been very impressed with the speed you achieved.

It's amazing how much extra speed you can pick up just by increasing your altitude 100m. That's why you want to keep going up nice and slow.

[arq]

At 25km you should be able to go much faster than 1200m/s. You should fly horizontal for longer to pick up speed on the jets. At 25km you should be able to get to 1300-1500m/s, in my experience.

I try to limit myself to 1 LVN because they are so darned heavy, and you usually don't need more than 1 since SSTOs tend to be lightweight. Another note, I have recently found aerospikes to be inferior to T30's for SSTO rockets. They are heavy so they have a rather bad TWR. The issue is that at 1500m/s at 25km, drag is *very* significant (especially because your jets usually have much more thrust than your rockets). The most important thing when switching to rockets is to gain altitude quickly to reduce drag. Any efforts to gain significant speed below roughly 35km are wasted fighting drag (every 5km reduces drag force by a factor of 2.7). Thus, engines with high thrust and good TWR (like the LV-T30) that can power you up to higher altitudes quickly are most useful for SSTOs. Above 10km, the ISP gets very close to the vacuum level, so the high atmospheric ISP of the aerospike (its main strength) is completely wasted in SSTO configurations where they are not used until 20km+.

For reference, my Minmus landing and return SSTO awhile back used 4 turbojets, 9 intakes, 2 aerospikes, and 1 LVN, though next time I do it I'll use something other than aerospikes.