Commercial Rockets

PSLV and GSLV — India, 1993

PSLV (Polar Satellite Launch Vehicle) is an odd one. It evolved from predecessors named SLV and ASLV, which were small solid-fuel rockets. It has a solid fuel first stage with a multisegment steel body, commonly augmented by six small strap-ons, then a hypergolic second stage, a solid fuel third stage which is narrower but has an enclosing ring to support the fairing, and finally, inside the fairing, a tiny twin-engined fourth burning hypergolics... but not the same hypergolic chemicals as the second stage: it uses monomethylhydrazine and mixed oxides of nitrogen, whereas the lower one works with the more usual combination of unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide. (Lately they’ve started mixing some hydrazine hydrate into the UDMH to make it more stable.) It’s as if they tried to develop and master as many different technologies as they could, all with one rocket. Even the solid stages differ: the upper one steers with a flexing nozzle, but the core booster steers by injecting liquid oxidizer into the sides. The stages also separate by different means, using explosives in some cases and mechanical latches in others.

The six strap-ons come in two sizes: the PSLV-G has short ones and the later PSLV-XL uses stretched ones with more thrust. The PSLV-CA (for “core alone”) has neither. When they are used, four of them ignite at launch time, and the other two not until 25 seconds later. In 2019 they added variants DL and QL, which use two and four side boosters respectively. This may mean the G-size boosters are now retired.

Despite this very idiosyncratic and irregular design, which seems to have been chosen to produce the greatest possible number of different ways to fail, the PSLV has a pretty good reliability record. It also set a record for the most separate satellites launched in a single flight, with 104 in early 2017. And it’s affordably priced.

The GSLV (for Geosynchronous) variant swaps out the solid third stage for a restartable hydrogen burner, adding another technology to the list. The little fourth stage is now optional, and I don’t think the option has yet been used. The GSLV uses much bigger strap-ons than the PSLV, and they burn liquid fuel — an inversion of the natural order for cores and strap-ons. These are loosely based on the hypergolic second stage, but longer and thinner. That stage uses an engine called the Vikas-4B, which started as a variant of the Viking engine used in older Ariane models. In the strap-ons, the engine is angled slightly away from vertical, and it has a sea-level sized bell; this variant is called the Vikas-2B. The four boosters don’t detach, but remain integrated with the first stage as it separates from the second... in fact, they carry the solid first stage for 40 seconds after it is exhausted.

The reliability record is not so good compared to the PSLV, despite the slightly less baroque design. It had to go through a major overhaul after a series of early failures. It has now been disused for a few years.

But the GSLV that’s based on the PSLV is old news. That’s the Mark II. Now here comes the GSLV Mark III, which is sometimes called the LVM3, but usually goes by the former name. Despite the familiar name, this is an all-new rocket. It’s quite a bit bigger than the old ones. Its core stage burns hypergolics in a pair of independently gimballed Vikas-2 engines, and it has two big fat solid boosters on the sides, giving it a silhouette similar to an Ariane 5. These boosters have steerable nozzle extensions. Unusually, the core stage does not ignite at liftoff, but near the time of booster separation. This lets its engines use larger bells, improving efficiency. The upper stage burns hydrogen, so once again we have a mix-and-match assortment of fuels, though this time it adds up to a much more conventional overall picture. As hydrogen burning upper stages go, this is a powerful one, and no further stage is needed. It seems less advanced than the Mark II upper stage, which had a staged-combustion cycle, while the new one is a gas generator... but that’s because the Mark II engine is a copy of a little Russian motor which they imported and used in the Mark I, while the big Mark III engine is all-Indian.

The rocket’s maiden suborbital test flight was used to test a heat shield for a future “Gaganyaan” crew capsule. They’re experimenting with a reusable rocket (to be called RLV), and also with a scramjet. The RLV would not only have a vertically-landing first stage, but a spaceplane second stage, and they are already doing drop tests to work on the automated landing. India is taking spaceflight very seriously. They want to show they can match anything offered by China. They hope to send a crew of three “gagannauts” into orbit by 2022.

Unfortunately the last couple of years have been tough. A failed lunar mission, followed by a major hit from the covid pandemic, have put the space agency ISRO far behind where it hoped to be. Maybe the next year can be a comeback.

PSLV-CA (no added boosters): mass 230 t, diam 2.8 m, thrust 2700 kN?, imp 2.3 km/s, type S, payload 2.5 t? (1.1%) [PSLV-XL 4.2t? (1.3%)], cost $6M/t?, record 50/0/3.
GSLV Mark II: mass 415 t, diam 2.8 m, thrust 4360 kN?, imp 2.6 km/s, type S+Gd, payload 5 t (1.2%), cost $9M/t, record 8/0/6.
[Show stages] (PSLV and GSLV)
Stage name PSOM (PSLV-G+) PSOM-XL (PSLV-XL) GS0/L40H (GSLV) PS1/GS1 PS2/GS2/L40 PS3 (PSLV) GS3/CUS15 (GSLV) * PS4/L2.6 *
Role (pos) count booster (S) ×0|6 booster (S) ×0|6 booster (S) ×4 core (1) upper (2) upper (3) upper (3) upper/kick (4) *
Diameter (m)   1.00   1.00   2.10   2.80   2.80   2.02   2.80   2.02
Liftoff mass (t) 11.1 14.7 48.2 161    42.8  8.4 17.6  3.6
Empty mass (t)  2.0  2.2  5.6 23    4.3  0.8  2.6  1.0
Fuel mass (t) ~2.7 ~3.8 ~15.7  ~41    ~14.2  ~2.3 ~2.5
Oxidizer mass (t) ~6.4 ~8.7 ~26.9  ~97    ~24.3  ~5.3 ~12.5 
Fuel type HTPB HTPB UDMH+HH * HTPB UDMH+HH * HTPB hydrogen MMH
Engine S9 S12 Vikas-2B S138 Vikas-4B S7 CE-7.5 L-2-5 ×2
Power cycle solid solid gas gen solid gas gen solid staged (ZF) pressure-fed?
Chamber pres. (bar) 59   59   75    8.4
Ox./fuel ratio   2.3?   2.3?   1.71   2.3?   1.71   2.3?   5.05
Thrust, vac max (kN) 719    765    4860     805    240    93   14.6
Thrust, SL initial (kN) 450?   678    2700?   
Spec. imp, vac (km/s)   2.60   2.60   2.80   2.60   2.96   2.89   4.45   3.02
Total imp, vac (t·km/s) 133    183    121    365    124    22   117     7.7
GSLV Mark III: mass 640 t, diam 4.0 m (width ~10.5), thrust 10300 kN?, imp 2.9 km/s, type Gd+S, payload 10 t (1.6%), cost $6M/t, record 3/1/0.
Stage name S200 L110 C25
Role (pos) count booster (S/1) ×2 core (2) * upper (3)
Diameter (m)   3.20   4.00   4.00
Liftoff mass (t) 237    125    33  
Empty mass (t) 31.2  8.9  5.1
Fuel mass (t) ~62    ~43    ~4.6
Oxidizer mass (t) ~144     ~73    ~23.3 
Fuel type HTPB UDMH+HH * hydrogen
Engine S200 Vikas-2 “X”? ×2 C25
Power cycle solid gas gen gas gen
Chamber pres. (bar) 57   59   60  
Ox./fuel ratio   2.3?   1.71   5.05
Thrust, vac max (kN) 5150     1678     200   
Thrust, SL initial (kN) 4330?    1513    
Spec. imp, vac (km/s)   2.60   2.90   4.40
Total imp, vac (t·km/s) 555    345    121